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Towards an Ultra-High-Speed Combustion Pyrometer

Towards an Ultra-High-Speed Combustion Pyrometer International Journal of Turbomachinery Propulsion and Power Article Alberto Sposito, Dave Lowe and Gavin Sutton * National Physical Laboratory (NPL), Hampton Road, Teddington TW11 0LW, UK; sposito.alberto@gmail.com (A.S.); dave.lowe@npl.co.uk (D.L.) * Correspondence: gavin.sutton@npl.co.uk y This paper was presented at the 9th EVI-GTI International Gas Turbine Instrumentation Conference, Graz, Austria on 20–21 November 2019. Received: 2 March 2020; Accepted: 10 December 2020; Published: 15 December 2020 Abstract: Measuring reliably the correct temperature of a sooty flame in an internal combustion engine is important to optimise its eciency; however, conventional contact thermometers, such as thermocouples, are not adequate in this context, due to drift, temperature limitation (2100 K) and slow response time (~10 ms). In this paper, we report on the progress towards the development of a novel ultra-high-speed combustion pyrometer, based on collection of thermal radiation via an optical fibre, traceably calibrated to the International Temperature Scale of 1990 (ITS-90) over the temperature range T = (1073–2873) K, with residuals <1%, and capable of measuring at a sampling rate of 250 kHz. Keywords: combustion; temperature; pyrometer; optical sensor 1. Introduction Traceable, reliable measurement of combustion temperature is important because it can improve the understanding of the combustion process and provide a mechanism for the optimisation of engine power, fuel consumption and emissions [1]. These measurements are performed under highly dynamic conditions, with temperature changes of up to ~3300 K occurring on a millisecond timescale. Conventional temperature sensors based on contact thermometry (e.g., thermocouples) are inadequate in this context, due to their slow response time (~10 ms), temperature limitation (2100 K), drift and perturbation of the combustion process. To address this challenge, with particular reference to internal combustion and diesel engines, we are developing a novel ultra-high-speed combustion pyrometer, within the framework of the European joint research project DynPT—Development of measurement and calibration techniques for dynamic pressures and temperatures, part of the European Metrology Programme for Innovation and Research (EMPIR) [2]. 2. System Design and Theoretical Model A schematic of the thermometer system design is shown in Figure 1. It consists of: A sensor: a 2 m long gold-coated multi-mode (MM) step-index fibre, with 400 m core diameter, numerical aperture NA = 0.22, stainless-steel monocoil sheathing, a sub-miniature (SMA) connector on one end (hot front end) and a fibre-channel (FC) connector on the other end (cold back end)—for testing purposes, this was placed inside a ~1.7 m long stainless-steel tube (outer diameter: 20 mm, inner diameter: 16 mm), with the SMA connector protected by a recessed sapphire window; sensor and packaging can be tailored to the final application and installation requirements (e.g., addition of a collimating lens). An extension lead fibre: a lightly-armoured 10 m long MM step-index fibre patch-cord, with 600m core diameter, NA = 0.22, dual acrylate coating, 3 mm diameter polyvinyl chloride (PVC) sleeve and FC connectors on both ends—this connects the sensor (on the FC connector) to the interrogator. Int. J. Turbomach. Propuls. Power 2020, 5, 31; doi:10.3390/ijtpp5040031 www.mdpi.com/journal/ijtpp Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 2 of 15 • An extension lead fibre: a lightly-armoured 10 m long MM step-index fibre patch-cord, with 600 Int. J. Turbomach. Propuls. Power 2020, 5, 31 2 of 16 µm core diameter, NA = 0.22, dual acrylate coating, 3 mm diameter polyvinyl chloride (PVC) sleeve and FC connectors on both ends—this connects the sensor (on the FC connector) to the A passive optoelectronic interrogator, assembled in-house and consisting mainly of: interrogator. • A passive optoelectronic interrogator, assembled in-house and consisting mainly of: # a custom-made 1  3 mm step-index fibre coupler/splitter with 600 m core diameter, NA = 0.22 and FC connectors on all ports; o a custom-made 1 × 3 mm step-index fibre coupler/splitter with 600 µm core diameter, NA # = 0. thr 22 ee an photodetector d FC connectors o assemblies, n all port using s; o -the-shelf components, for measuring optical o three photodetector asse thermal radiation at 3 dimblies, using erent wavelengths: off-the-sh  =elf components, for me 850 nm,  = 1050 nm and asur  ing optical = 1300 nm; 1 2 3 thermal radiation at 3 different wavelengths: λ1 = 850 nm, λ2 = 1050 nm and λ3 = 1300 nm; # a power supply unit to power the photodetectors. o a power supply unit to power the photodetectors. A National Instrument (NI) data acquisition (DAQ) system, with maximum sampling rate • fA Nat = ion 1 MHz, al Instrument connected (NI) to dthe ata ac optoelectr quisition onic (DAQ interr ) sy ogator stem, w via ithBNC maxim cables um sand ampto ling a r Personal ate fMAX MAX Computer = 1 MHz, connected to the optoel (PC) via a USB cable. ectronic interrogator via BNC cables and to a Personal Computer (PC) via a USB cable. Figure 1. Schematic of the system. Figure 1. Schematic of the system. Fibr Fibres w es with ith large large core core d diameter iameter an and d lar large ge NA NA w wer eree c chosen hosen t to o m maximise aximise collection of optic collection of optical al thermal thermal radia radiation; tion; t the he gold gold (Au) co (Au) coating ating all allows ows t the he fi fibr bre to e to withstand withstand high high t temperatur emperatures, up es, up to ~1000 to ~1000 K K, , although although t the he cor core di e diameter ameter of of Au-coated Au-coated fibre fibress is is limited limited t to o 4 400 00 µm. m. The The wave wavelengths lengths of the of the photodetector assemblies photodetector assemblies we wer re chosen b e chosen a based sed on on previo previous us experien experience ce to to avoid avoid spspectral ectral feat featur ureses (e(emission mission an and d ab absorption sorption lilines) nes) from fromtthe he combust combustion ion by-product by-products s and and the the components components of the pyrot of the pyrotechnic echnic ch chargesarges (see (see figures fi below gure,s below taken from , ta earlier ken fr spectr om e oscopic arlier spect experiments), roscopic as experi well ments), a as to test s wel thel as to assumption test the assumpti that the measur on that the measur ed combustion ed combustion p process behaves rocess behave like a blackbody s like a (emissivity blackbody (e " miss = 1.0)—good ivity ε = 1 agr .0)eement —good amongst agreement a the temperatur mongst the te es estimated mperature at s e di s timated erent wavelengths at different can wavelen be used gths to can be confirm use that d tothe confirm th blackbody at th condition e blackbois dy cond met. ition is met. Figur Figure e 2 2a sho a shows ws the emission spectrum the emission spectrum captur captured ed wit with h a a Si Si spectro spectrometer meter, where , where the fo the following llowing featur features were es were identified: identified: A. 589 nm—Sodium (Na) emission lines; A. 589 nm—Sodium (Na) emission lines; B. 619 nm—CaOH emission lines; B. 619 nm—CaOH emission lines; C. 693 nm—Potassium (K) emission lines; C. 693 nm—Potassium (K) emission lines; D. 767 nm—K emission and absorption lines; D. 767 nm—K emission and absorption lines; E. 960 nm—Uncertain of assignment. Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 3 of 15 Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 3 of 15 Int. J. Turbomach. Propuls. Power 2020, 5, 31 3 of 16 E. 960 nm—Uncertain of assignment. E. 960 nm—Uncertain of assignment. Figure 2. Pyrotechnic emission spectrum from (a) Si spectrometer, (b) InGaAs spectrometer, ~36 ms after ignition. The coloured vertical lines identify the chosen wavelength: 850 (blue line in (a)), 1050 Figure 2. Pyrotechnic emission spectrum from (a) Si spectrometer, (b) InGaAs spectrometer, ~36 ms Figure 2. Pyrotechnic emission spectrum from (a) Si spectrometer, (b) InGaAs spectrometer, ~36 ms after (green line in (b)) and 1300 nm (red line in (b)). after ignition. The coloured vertical lines identify the chosen wavelength: 850 (blue line in (a)), 1050 ignition. The coloured vertical lines identify the chosen wavelength: 850 (blue line in (a)), 1050 (green (green line in (b)) and 1300 nm (red line in (b)). line in (b)) and 1300 nm (red line in (b)). Figure 2a also shows the blackbody spectrum from a tungsten calibration lamp (with a temperature of 3165 K) overlapped to the measured spectrum. The agreement between the shape of Figure 2a also shows the blackbody spectrum from a tungsten calibration lamp (with a Figure 2a also shows the blackbody spectrum from a tungsten calibration lamp (with a temperature the two spectra suggests that the blackbody assumption for a fireball is a valid hypothesis. temperature of 3165 K) overlapped to the measured spectrum. The agreement between the shape of of 3165 K) overlapped to the measured spectrum. The agreement between the shape of the two spectra Figure 2b shows the emission spectrum captured with an InGaAs spectrometer, where the the two spectra suggests that the blackbody assumption for a fireball is a valid hypothesis. suggests that the blackbody assumption for a fireball is a valid hypothesis. following features were identified: Figure 2b shows the emission spectrum captured with an InGaAs spectrometer, where the Figure 2b shows the emission spectrum captured with an InGaAs spectrometer, where the following features were identified: following features were identified: A. 1104 nm—K emission lines; A. B. A. 11 11 01104 6 49 nm—K nm—K nm—K emiss emiss emission ion line ion line lines; ss ; ; B. 1169 nm—K emission lines; B. C. 11 Broad 69 nm—K OH ab emiss sorption in ion line the fibre; s; C. Broad OH absorption in the fibre; C. D. Broad 1243 a OH nd 12 ab 52 sorption in nm—K emissi the fibre; on lines; D. 1243 and 1252 nm—K emission lines; D. E. 1Broad 243 an OH d 12ab 52sorption in nm—K emissi the fibre; on lines; E. Broad OH absorption in the fibre; E. F. Broad 1517 nm—K OH ab emiss sorption in ion line the fibre; s. F. 1517 nm—K emission lines. F. 1517 nm—K emission lines. As photodetectors with variable gain G were used, a simple theoretical model was developed to As photodetectors with variable gain G were used, a simple theoretical model was developed to estimate the optical power measured by each photodetector and how their voltage signals change As photodetectors with variable gain G were used, a simple theoretical model was developed to estimate the optical power measured by each photodetector and how their voltage signals change with with G. Their bandwidth B also decreases with increasing G, adjustable in 10 dB steps from 0 to 70 estimate the optical power measured by each photodetector and how their voltage signals change G. Their bandwidth B also decreases with increasing G, adjustable in 10 dB steps from 0 to 70 dB. dB. with G. Their bandwidth B also decreases with increasing G, adjustable in 10 dB steps from 0 to 70 First of all, the blackbody radiation power coupled into the core of the optical fibre (see geometry First of all, the blackbody radiation power coupled into the core of the optical fibre (see geometry dB. sketched in Figure 3) was calculated, assuming an emissivity " = 1 and optical transmission of the fibre sketched in Figure 3) was calculated, assuming an emissivity ε = 1 and optical transmission of the First of all, the blackbody radiation power coupled into the core of the optical fibre (see geometry over the range  = (0.3–2.4) m as specified in the Au-coated fibre datasheet. fibre over the range λ = (0.3–2.4) µm as specified in the Au-coated fibre datasheet. sketched in Figure 3) was calculated, assuming an emissivity ε = 1 and optical transmission of the fibre over the range λ = (0.3–2.4) µm as specified in the Au-coated fibre datasheet. Figure 3. Geometry of the end of the sensor (Au-coated fibre): d = fibre core diameter; θ = maximum Figure 3. Geometry of the end of the sensor (Au-coated fibre): d = fibre core diameter;  = maximum acceptance half-angle. acceptance half-angle. Figure 3. Geometry of the end of the sensor (Au-coated fibre): d = fibre core diameter; θ = maximum acceptance half-angle. Neglecting the Fresnel reflection losses from the end-facet of the fibre and from the sapphire Neglecting the Fresnel reflection losses from the end-facet of the fibre and from the sapphire window, the total blackbody radiation power coupled into the fibre core over the full blackbody window, the total blackbody radiation power coupled into the fibre core over the full blackbody Neglecting the Fresnel reflection losses from the end-facet of the fibre and from the sapphire radiation spectrum (i.e., all wavelengths) is: radiation spectrum (i.e., all wavelengths) is: window, the total blackbody radiation power coupled into the fibre core over the full blackbody radiation spectrum (i.e., all wavelengths) is: 4 Pin, TOT(T) = ΩAεσT /π Pin, TOT(T) = ΩAεσT /π where: where: Int. J. Turbomach. Propuls. Power 2020, 5, 31 4 of 16 P (T) = WA"T / in, TOT where: T is the blackbody temperature in K; 8 2 4 = 5.67 10 Wm K is the Stefan–Boltzmann constant; 2 7 2 A = d /4 = 1.25664 10 m is the fibre core area; W = tan () is the maximum solid acceptance angle of the Au-coated fibre, with  the maximum acceptance half-angle of the Au-coated fibre, which is related to the NA of the fibre as: NA = nsin() = 0.22 As the refractive index of air is n 1, the maximum solid acceptance angle can be re-written as: W = tan (arcsin(NA)) 0.16 sr Hence, the total blackbody radiation power coupled into the optical fibre is: 16 4 P (T) 3.624 10 T W in, TOT The fractional power coupled into the optical fibre over the wavelength range  = (0.3–2.4) m, P (T), can be calculated numerically or through tabulated values, considering the in wavelength-temperature products. With such a model, approximately 10 mW of optical thermal radiation is coupled into the optical fibre for T  2500 K; however, for T = 300 K: P (T) < 10 pW. in To calculate the optical power measured by each photodetector and the associated voltage signals, the losses in the optical transmission line from the sensor head to the detectors need to be considered. To estimate the signals accurately, the following contributions need to be taken into account: 1. The transmission factor of the sapphire window placed in front of the fibre end-facet, due to Fresnel reflection losses (7% at each interface/surface): t = 0.93 0.93 = 0.8649. 2. The transmission factor at the end-facet of the Au-coated fibre, due to Fresnel reflection losses: t = 0.96. 3. Transmission losses of 12 m of fibre (2 m sensor + 10 m of extension lead fibre)—considering that typical losses for large-core multi-mode fibre are of the order of 10 dB/km or less at  = (0.6–1.6)m: t =0.12 dB 0.973. fibre 4. Losses due to optical connectors (3), typically of the order of 0.3 dB each—i.e., a transmission factor t = 0.933 connector 5. The splitting ratio of the 1 3 optical coupler/splitter: t  0.333. splitter 6. The optical transmission (t ) of the bandpass filters in the photodetector assemblies—it filter is worth noting that the filters used have di erent values of optical transmission peak and Full-Width-at-Half Maximum (FWHM): # t = 70%; 850 nm # t = 45%; 1050 nm # t = 40%; 1300 nm # FWHM = 40 nm 8 nm; 850 nm # FWHM = 10 nm 2 nm; 1050 nm # FWHM = 30 nm 6 nm. 1300 nm Hence, the optical power incident on the photodetectors can be calculated as: Int. J. Turbomach. Propuls. Power 2020, 5, 31 5 of 16 P (, T) = t t t t (t ) t ()P (, T)  0.22t ()P (, T) (1) connector i 0 1 fibre splitter filter in filter in Finally, to calculate the voltage signal, we also need to consider the photodetector transimpedance gain G [V/A] and responsivity R( ) [A/W], which is a function of wavelength; hence: i i Z Z Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 5 of 15 V = G R ()P (, T)d = 0.22G t ()R ()P (, T)d (2) i i i i i filter i in 𝑃 (𝜆, 𝑇 ) = 𝐴 𝛺𝐿 (𝜆, 𝑇) where: P (, T) = AWL (, T) in b with: with: 2𝑐 1 2𝑐 ( ) 2c 1 2c 𝐿 𝜆, 𝑇 = ≈ 𝑒 1 1 c /T ⁄ 2 L (, T) =  e 𝜆 𝑒 −1 𝜆 5 5 c /T e 1 −16 −2 −1 where c1 and c2 are the first and second radiation constants equal to 0.59552197 × 10 W·m ·sr and 16 2 1 where c and c are the first and second radiation constants equal to 0.59552197 10 Wm sr and 1 2 −2 1.438769 × 10 m·K, respectively, and the expression after the ≈ symbol is the Wien approximation 1.438769 10 mK, respectively, and the expression after the symbol is the Wien approximation valid for c2 >> λT. valid for c >> T. Equations (1) and (2) were evaluated at different temperatures, with the results compared with Equations (1) and (2) were evaluated at di erent temperatures, with the results compared with preliminary experimental data. These showed a lower signal than expected (by a factor of ~2), likely preliminary experimental data. These showed a lower signal than expected (by a factor of ~2), likely due due to extra connection losses. This was fed back into the model, which gave the results shown in to extra connection losses. This was fed back into the model, which gave the results shown in Figure 4 Figure 4 in terms of optical power incident onto the photodetectors—noise floors are shown only for: in terms of optical power incident onto the photodetectors—noise floors are shown only for: G = 0 dB, G = 0 dB, B = 12 MHz (highest noise floor); G = 20 dB, B = 1 MHz and G = 70 dB, B = 3 kHz (lowest B = 12 MHz (highest noise floor); G = 20 dB, B = 1 MHz and G = 70 dB, B = 3 kHz (lowest noise floor). noise floor). Figure 4. Blackbody radiation power incident onto photodetectors as a function of blackbody Figure 4. Blackbody radiation power incident onto photodetectors as a function of blackbody temperature. temperature. Noise floor was estimated for each photodetector as: Noise floor was estimated for each photodetector as: 1/2 P = NEP B  R /R( ) n, i / MAX i 𝑃 =𝑁𝑃𝐸 𝐵 𝑅 /𝑅(𝜆 ) where NEP is the Noise Equivalent Power and R is the peak responsivity—both provided in the where NEP is the Noise Equivalent Power and RMAX is the peak responsivity—both provided in the MAX photodetector datasheets. photodetector datasheets. Figure 4 shows that, in any case, the instrument should be capable of measuring temperatures Figure 4 shows that, in any case, the instrument should be capable of measuring temperatures T T > 1600 K at all wavelengths, with photodetectors set at G = 0 dB and B = 12 MHz. However, > 1600 K at all wavelengths, with photodetectors set at G = 0 dB and B = 12 MHz. However, considering that the maximum sampling rate of the DAQ system is f = 1 MHz, a gain setting of considering that the maximum sampling rate of the DAQ system is fMAX = 1 MHz, a gain setting of G MAX = 20 dB (B = 1 MHz) would allow measurement of temperatures as low as ~1000 K with the 1300 nm photodetector (but not at λ1 = 850 nm and λ2 = 1050 nm), with no penalty in terms of speed. The minimum and maximum temperatures measurable by the instrument are dictated, respectively, by the noise level (experimentally measured as ~1 mV for most values of G) and the saturation level (~10 V) of the photodetectors. To find the photodetector settings that optimise the measurable temperature range, Equation (2) was evaluated at different temperatures for different G and B settings of the photodetectors. Voltage signals generated by the three photodetectors were plotted versus temperature for all values of G and B. Figure 5 shows two of these plots for representative values of G and B. Int. J. Turbomach. Propuls. Power 2020, 5, 31 6 of 16 Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 6 of 15 G = 20 dB (B = 1 MHz) would allow measurement of temperatures as low as ~1000 K with the 1300 nm • With a gain of G = 20 dB (B = 1 MHz—Figure 5a), the instrument can measure a minimum photodetector (but not at  = 850 nm and  = 1050 nm), with no penalty in terms of speed. temperature of ~1150 K at 1 a single wavelengt 2 h (λ3 = 1300 nm) or ~1400 K at all three wavelengths. The minimum and maximum temperatures measurable by the instrument are dictated, respectively, • With a gain of G = 30 dB (B = 260 kHz), the minimum measurable temperature can be brought by the noise level (experimentally measured as ~1 mV for most values of G) and the saturation level down to ~1025 K for single-wavelength measurement (λ3 = 1300 nm) and ~1275 K at all three (~10 V) of the photodetectors. To find the photodetector settings that optimise the measurable wavelengths, but at cost of reduced sampling speed (f ≤ B = 260 kHz), while still avoiding temperature range, Equation (2) was evaluated at di erent temperatures for di erent G and B settings saturation at 3300 K, our maximum temperature of interest. of the photodetectors. Voltage signals generated by the three photodetectors were plotted versus • With a gain of G ≥ 40 dB (B = 90 kHz—Figure 5b), the photodetectors would start saturating at temperature for all values of G and B. Figure 5 shows two of these plots for representative values of G TMAX < 3300 K and their bandwidth would decrease significantly, down to B = 3 kHz at G = 70 and B. dB. Figure 5. Modelled signals from photodetectors versus blackbody temperature at di erent G and B Figure 5. Modelled signals from photodetectors versus blackbody temperature at different G and B settings: (a) G = 20 dB and B = 1 MHz, (b) G = 40 dB and B = 90 kHz. settings: (a) G = 20 dB and B = 1 MHz, (b) G = 40 dB and B = 90 kHz. With a gain of G = 20 dB (B = 1 MHz—Figure 5a), the instrument can measure a minimum Hence, the optimum photodetector gain is G = 30 dB, which allows temperatures above 1025 K temperature of ~1150 K at a single wavelength ( = 1300 nm) or ~1400 K at all three wavelengths. to be measured for λ3 = 1300 nm, or temperatures above 1275 K to be measured for all wavelengths, With a gain of G = 30 dB (B = 260 kHz), the minimum measurable temperature can be brought with a maximum sampling rate f ≤ B = 260 kHz. down to ~1025 K for single-wavelength measurement ( = 1300 nm) and ~1275 K at all three Int. J. Turbomach. Propuls. Power 2020, 5, 31 7 of 16 wavelengths, but at cost of reduced sampling speed (f  B = 260 kHz), while still avoiding saturation at 3300 K, our maximum temperature of interest. With a gain of G 40 dB (B = 90 kHz—Figure 5b), the photodetectors would start saturating at T < 3300 K and their bandwidth would decrease significantly, down to B = 3 kHz at G = 70 dB. MAX Hence, the optimum photodetector gain is G = 30 dB, which allows temperatures above 1025 K to be measured for  = 1300 nm, or temperatures above 1275 K to be measured for all wavelengths, with a maximum sampling rate f  B = 260 kHz. Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 7 of 15 3. Instrument Calibration 3. Instrument Calibration 3.1. Test Rig 3.1. Test Rig The instrument was calibrated using a Thermo Gauge blackbody radiation furnace and a The instrument was calibrated using a Thermo Gauge blackbody radiation furnace and a KE- KE-Technologie GmbH LP3 linear pyrometer calibrated traceably to the ITS-90 [3], with the Technologie GmbH LP3 linear pyrometer calibrated traceably to the ITS-90 [3], with the stainless- stainless-steel tube of the packaged sensor filled with sand to avoid overheating the Au-coated steel tube of the packaged sensor filled with sand to avoid overheating the Au-coated fibre that could fibre that could be irreversibly damaged. A photograph of part of the test rig is shown in Figure 6: be irreversibly damaged. A photograph of part of the test rig is shown in Figure 6: the hot Thermo the hot Thermo Gauge blackbody furnace and temperature sensor are visible in the background and Gauge blackbody furnace and temperature sensor are visible in the background and foreground, foreground, respectively. The latter is placed on a stainless-steel V-groove mounted on an optical respectively. The latter is placed on a stainless-steel V-groove mounted on an optical breadboard and breadboard and safely held in place by removable brackets bolted to the breadboard. This breadboard is safely held in place by removable brackets bolted to the breadboard. This breadboard is installed on installed on a motorised stage, controlled by a computer, for horizontal and vertical alignment. The LP3 a motorised stage, controlled by a computer, for horizontal and vertical alignment. The LP3 (not (not visible in Figure 6) is mounted on the same framework, so that it can be easily moved in front of visible in Figure 6) is mounted on the same framework, so that it can be easily moved in front of the the blackbody, in place of the sensor, to measure the temperature at each setpoint of the calibration. blackbody, in place of the sensor, to measure the temperature at each setpoint of the calibration. Figure 6. Photograph of the calibration furnace: the instrument sensor, housed in a steel tube, is sitting Figure 6. Photograph of the calibration furnace: the instrument sensor, housed in a steel tube, is sitting on the V-groove placed in front of blackbody furnace, ready to be manually moved in and out of it on the V-groove placed in front of blackbody furnace, ready to be manually moved in and out of it for for dynamic calibration at a set temperature. dynamic calibration at a set temperature. Data from the instrument were acquired using a NI LabVIEW program written in-house and Data from the instrument were acquired using a NI LabVIEW program written in-house and executed on the PC that is part of the system, whereas the blackbody furnace was controlled with a executed on the PC that is part of the system, whereas the blackbody furnace was controlled with a separate desktop computer that also controls the motorised framework. separate desktop computer that also controls the motorised framework. 3.2. Test Method The instrument was configured with the photodetectors set with optimum gain G = 30 dB (B = 260 kHz) and the sampling rate set at f = 250 kHz. At the beginning of the calibration, the voltage offset from the three photodetectors was measured once, to zero the photodetectors. The instrument was calibrated in the temperature range T = (1073–2873) K, in steps of ∆T = 200 K, according to the following procedure: 1. The blackbody furnace was set at the required temperature set-point. 2. The temperature of the blackbody cavity was monitored using the LP3. 3. Once the blackbody temperature reached stability, a measurement was taken from the LP3, by measuring the average and standard deviation over ~30 s (the LP3 is sampled at 1 Hz). 4. The LP3 was moved out of the way and the sensor moved into place, so that it was in line with and parallel to the long axis of the blackbody, as shown in Figure 6. Int. J. Turbomach. Propuls. Power 2020, 5, 31 8 of 16 3.2. Test Method The instrument was configured with the photodetectors set with optimum gain G = 30 dB (B = 260 kHz) and the sampling rate set at f = 250 kHz. At the beginning of the calibration, the voltage o set from the three photodetectors was measured once, to zero the photodetectors. The instrument was calibrated in the temperature range T = (1073–2873) K, in steps of DT = 200 K, according to the following procedure: 1. The blackbody furnace was set at the required temperature set-point. 2. The temperature of the blackbody cavity was monitored using the LP3. 3. Once the blackbody temperature reached stability, a measurement was taken from the LP3, by measuring the average and standard deviation over ~30 s (the LP3 is sampled at 1 Hz). 4. The LP3 was moved out of the way and the sensor moved into place, so that it was in line with and parallel to the long axis of the blackbody, as shown in Figure 6. 5. Data acquisition and logging were started on the instrument. 6. Manually, the sensor was quickly moved into and out of the blackbody (within a few seconds). 7. Two measurements were made at each set-point temperature. 3.3. Test Data Analysis Method The raw voltage signals from the three photodetectors of the optoelectronic interrogator were analysed to find the optimum calibration point in each signal. This is explained in Figure 7, showing typical measurement traces—the signal from the 1050 nm photodetector was the lowest, because of the combined e ect of the responsivity of the photodetector and the transmission and the bandwidth of the optical bandpass filter. Considering Figure 7b: t < 1.3 s: the blackbody cavity had a temperature gradient along the cavity wall and across its rear surface, it was hotter to the outside, and this was seen as the sensor approached: the radiance signal increased as the field of view of the sensor was initially filled. t (1.3–1.6) s: the signal fell as the sensor progressively saw more of the cooler central section of the back wall. t  (1.6–1.77) s: there was a period when the blackbody temperature fell due to heat lost to the cold sensor. t (1.77–1.92) s: as the sensor was withdrawn, the hotter regions of the blackbody cavity were seen again, so that the signal increased. t > 1.92 s: the signal decreased, as the sensor was withdrawn from the blackbody cavity. The maximum in the signal during sensor removal was lower than during insertion. This is consistent with the cooling of the blackbody cavity. The voltages recorded for calibration were chosen at the inflection point of each signal, highlighted by the blue circle, as it corresponds to the point when the field of view of the fibre is filled with thermal radiation from the back wall, before any further cooling caused by the sensor. Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 8 of 15 5. Data acquisition and logging were started on the instrument. 6. Manually, the sensor was quickly moved into and out of the blackbody (within a few seconds). 7. Two measurements were made at each set-point temperature. 3.3. Test Data Analysis Method The raw voltage signals from the three photodetectors of the optoelectronic interrogator were analysed to find the optimum calibration point in each signal. This is explained in Figure 7, showing typical measurement traces—the signal from the 1050 nm photodetector was the lowest, because of Int. J. Turbomach. Propuls. Power 2020, 5, 31 9 of 16 the combined effect of the responsivity of the photodetector and the transmission and the bandwidth of the optical bandpass filter. Figure 7. Typical calibration measurement traces: the black line in the middle of chart (a) identifies Figure 7. Typical calibration measurement traces: the black line in the middle of chart (a) identifies the the points used for calibration; (b) is a close-up on the 1300 nm signal and its calibration point. points used for calibration; (b) is a close-up on the 1300 nm signal and its calibration point. Considering Figure 7b: Calibration was performed by fitting experimental data to the Planckian version of the Sakuma–Hattori equation with three adjustable parameters A , B and c for each wavelength  [4]: • t < 1.3 s: the blackbody cavity had a temperature gradient i along i the cav i ity wall and across itis rear surface, it was hotter to the outside, and this was seen as the sensor approached: the radiance signal increased as the field of view of the sensor was initially filled. V = (3) c /( T+B ) i i i e 1 At each calibration point, the two voltage measurements for each wavelength/photodetector were averaged; each average was then converted into temperature using the inverse function of Equation (3): c 1 T =   (4) ln + 1 Optimum values of the adjustable parameters were found using the Generalized Reduced Gradient (GRG) solving method for smooth non-linear problems, to minimise the sum of the squares of temperature di erences with the LP3. Int. J. Turbomach. Propuls. Power 2020, 5, 31 10 of 16 3.4. Calibration Test Results The average signal at each setpoint was measured for each wavelength/photodetector and plotted versus the set-point temperature measured from the LP3 linear pyrometer. Figure 8 shows good agreement between experimental data and the theoretical model at G = 30 dB, as used for the calibration, with the signal from the 1300 nm photodetector higher than predicted, most likely due to overestimated losses, as a single figure was used for all three wavelengths. Figure 8 also shows that the instrument can measure a temperature as low as 1073 K at  = 1300 nm or 1273 K at all three wavelengths—these minimum temperatures match with those expected from the theoretical model. Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 10 of 15 Figure 8. Experimental data compared with theoretical model. Figure 8. Experimental data compared with theoretical model. Using the inverse Planck function—i.e. Equation (4)—average voltage measurements for each Using the inverse Planck function—i.e. Equation (4)—average voltage measurements for each wavelength at each set-point were converted into temperatures and the adjustable parameters were wavelength at each set-point were converted into temperatures and the adjustable parameters were optimised to minimise the sum of the squares of calibration residuals. The optimum calibration optimised to minimise the sum of the squares of calibration residuals. The optimum calibration coefficients are shown in Table 1 (Bi coefficients are not included, because they were found to be equal coecients are shown in Table 1 (B coecients are not included, because they were found to be equal to zero) and the residuals are shown in Figure 9, showing relative temperature differences within to zero) and the residuals are shown in Figure 9, showing relative temperature di erences within1% ±1% (absolute differences are within ±15 K). (absolute di erences are within15 K). Table 1. Optimum values of the adjustable calibration coefficients, minimising sum of squares of Table 1. Optimum values of the adjustable calibration coecients, minimising sum of squares of relative errors. relative errors. λi [nm] Ai [V] ci [µm K] [nm] A [V] c [m K] i i i 850 527.6 14,082.2 850 527.6 14,082.2 1050 53.0 14,431.8 1050 53.0 14,431.8 1300 91.7 14,307.3 1300 91.7 14,307.3 Figure 9. Calibration residuals. Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 10 of 15 Figure 8. Experimental data compared with theoretical model. Using the inverse Planck function—i.e. Equation (4)—average voltage measurements for each wavelength at each set-point were converted into temperatures and the adjustable parameters were optimised to minimise the sum of the squares of calibration residuals. The optimum calibration coefficients are shown in Table 1 (Bi coefficients are not included, because they were found to be equal to zero) and the residuals are shown in Figure 9, showing relative temperature differences within ±1% (absolute differences are within ±15 K). Table 1. Optimum values of the adjustable calibration coefficients, minimising sum of squares of relative errors. λi [nm] Ai [V] ci [µm K] 850 527.6 14,082.2 Int. J. Turbomach. Propuls. Power 2020, 5, 31 11 of 16 1050 53.0 14,431.8 1300 91.7 14,307.3 Figure 9. Calibration residuals. Figure 9. Calibration residuals. Having calibrated the sensor, the maximum measurable temperatures can be estimated by extrapolation of the Planck function in Equation (3), until the photodetector saturation level (V = 10 V) is reached or, more accurately, by replacing this value in the inverse Planck MAX function—i.e., Equation (4). In a similar way, minimum measurable temperatures were estimated by replacing the noise level (V  1 mV) in the inverse Planck function. Table 2 shows minimum and noise maximum measurable temperatures. Table 2. Minimum and maximum measurable temperatures. [nm] T [K] T [K] i MIN MAX 850 1260 4160 1050 1260 7470 1300 960 4740 4. Dynamic Tests 4.1. Test Rig To demonstrate the speed of the instrument, dynamic tests were performed using theatrical flash charges [5] in the pyrotechnic facility at NPL (National Physical Laboratory). This consists of a vented enclosure where pyrotechnic charges, placed on a stage, are remotely triggered with a controller that is connected and synchronised with the instrument. The sensor is mounted such that its front end protrudes into the enclosure with its tip ~15 cm above and ~5 cm away from the centre of the charge. The optimum position of the sensor is based on experience from previous tests, when we also conducted absorption/transmission experiments, from which no optical transmission was observed during the explosion, thus suggesting that the fireball is opaque and supporting our blackbody assumption, and an initial absorption coecient = 0.25 cm was estimated at  850 nm. 4.2. Test Method Two sets of explosion tests were performed: a preliminary set of 3 tests with medium pyrotechnic charges and another set of 3 tests with large pyrotechnic charges. In all cases, the photodetector gain was set at G = 30 dB, as the instrument was calibrated only with this setting. Sampling rate and number Int. J. Turbomach. Propuls. Power 2020, 5, 31 12 of 16 of samples were set, respectively, at f = 50 kHz and N = 50,000 (giving an acquisition time t = N/f = 1 s) for the first 4 tests and then at f = 250 kHz and N = 25,000 (giving an acquisition time t = N/f = 0.1 s) for the last 2 tests. Sampling rate f and number of samples N were initially chosen based on experience from previous explosion tests, to collect enough data at a high speed but without having to record excessive data. N and f were changed in the last two tests, based on observations from the previous test, again to avoid recording data where no signal was present, but also to capture finer details and test the maximum sampling speed. 4.3. Test Results The preliminary set of tests with medium pyrotechnic charges (shown in Figure 10) demonstrated that f = 50 kHz was sucient to measure the rapid temperature rise and decay and identify signal structure in between. Variability in temperature evolution was observed from test to test—this was to be expected as no two charges are the same. Nevertheless, there was a good correlation among all traces for a given test, although the temperature agreement was poor—in particular, the temperature estimated from the signal at  = 1300 nm was significantly lower than the other two, by up to ~600 K. This suggested that the e ective emissivity at the longest wavelength was significantly less than unity—i.e., the blackbody condition necessary for successful thermometry is not met. On the contrary, the set of tests with large pyrotechnic charges shown in Figure 11 produced more consistent results and better agreement among temperatures measured at di erent wavelengths, meaning that the blackbody Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 12 of 15 condition (" = 1) is more closely met than with medium charges. In explosion tests with large charges, the maximum temperatures estimated at di erent wavelengths agree with each other within up to charges. In explosion tests with large charges, the maximum temperatures estimated at different ~137 K or ~4.5%. wavelengths agree with each other within up to ~137 K or ~4.5%. Figure 10. Time trend of temperatures for pyrotechnic tests with medium charges—temporal offset Figure 10. Time trend of temperatures for pyrotechnic tests with medium charges—temporal o set introduced for clarity. introduced for clarity. Figure 11 shows again that a sampling rate of f = 50 kHz was still fast enough to capture events from large charges, despite shorter pulse duration (<20 ms versus ~200 ms), sharper rise time (<1 ms versus ~10 ms) and faster decay times (~10 ms versus ~100 ms) than medium charges. The sampling rate was increased to f = 250 kHz in the last two trials to test the maximum sampling frequency allowed by the gain set in the photodetectors (G = 30 dB). A temperature rise of up to ~3.25 K/µs was estimated for explosions of large charges. Figure 11. Time trend of temperatures for pyrotechnic tests with large charges—temporal offset introduced for clarity. Figure 12 shows that large pyrotechnic charges produced not only more consistent results, but also higher peak temperatures than medium charges, by ~700 K. It is also worth observing that the Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 12 of 15 charges. In explosion tests with large charges, the maximum temperatures estimated at different wavelengths agree with each other within up to ~137 K or ~4.5%. Figure 10. Time trend of temperatures for pyrotechnic tests with medium charges—temporal offset introduced for clarity. Figure 11 shows again that a sampling rate of f = 50 kHz was still fast enough to capture events from large charges, despite shorter pulse duration (<20 ms versus ~200 ms), sharper rise time (<1 ms versus ~10 ms) and faster decay times (~10 ms versus ~100 ms) than medium charges. The sampling rate was increased to f = 250 kHz in the last two trials to test the maximum sampling frequency Int. J. Turbomach. Propuls. Power 2020, 5, 31 13 of 16 allowed by the gain set in the photodetectors (G = 30 dB). A temperature rise of up to ~3.25 K/µs was estimated for explosions of large charges. Figure 11. Time trend of temperatures for pyrotechnic tests with large charges—temporal offset Figure 11. Time trend of temperatures for pyrotechnic tests with large charges—temporal o set introduced for clarity. introduced for clarity. Figure 12 shows that large pyrotechnic charges produced not only more consistent results, but Figure 11 shows again that a sampling rate of f = 50 kHz was still fast enough to capture events also higher peak temperatures than medium charges, by ~700 K. It is also worth observing that the from large charges, despite shorter pulse duration (<20 ms versus ~200 ms), sharper rise time (<1 ms versus ~10 ms) and faster decay times (~10 ms versus ~100 ms) than medium charges. The sampling rate was increased to f = 250 kHz in the last two trials to test the maximum sampling frequency allowed by the gain set in the photodetectors (G = 30 dB). A temperature rise of up to ~3.25 K/s was estimated for explosions of large charges. Figure 12 shows that large pyrotechnic charges produced not only more consistent results, but also Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 13 of 15 higher peak temperatures than medium charges, by ~700 K. It is also worth observing that the temperature measured at  = 850 nm was always the highest, whereas the temperature measured at temperature measured at λ1 = 850 nm was always the highest, whereas the temperature measured at = 1300 nm was always the lowest. λ3 = 1300 nm was always the lowest. Figure 12. Plot of maximum temperatures versus test number. The inset on the upper left shows some Figure 12. Plot of maximum temperatures versus test number. The inset on the upper left shows some statistics: Mean TMAX and σ (TMAX) are, respectively, the average and the standard deviation of peak statistics: Mean T and  (T ) are, respectively, the average and the standard deviation of peak MAX MAX temperatures measured at different λ. temperatures measured at di erent . In any case, having calibrated our instrument with a blackbody cavity, it is possible to state that the fireball will have reached at least the highest measured temperature, regardless of the emissivity and of the blackbody assumption. In fact, if the blackbody assumption is made (ε = 1), but the true emissivity is ε < 1 and constant with wavelength (i.e., the fireball is not a blackbody, but a grey body), then the difference between the true temperature T and the measured temperature, also called colour temperature, Tc, can be written as: 1 1 𝜆 = + ln (𝜀 ) 𝑇 𝑇 𝑐 From this expression, an error in emissivity of Δ𝜀 will lead to an error in the inferred temperature: 𝜆𝑇 Δ𝑇 = − Δ𝜀 where: Δ𝑇 = 𝑇 −𝑇 and Δ𝜀 = 1 − ε . From the expression above, it is clear that the temperature error is temperature- and wavelength- dependent and that a grey body would not provide identical temperature readings at different wavelengths (as in the blackbody case), as shown also in Figure 13, where temperature error is plotted versus wavelength at three given emissivities at T = 3000 K, and in Figure 14, where temperature error is plotted versus true temperature at the three wavelengths used for ε = 0.8. It is worth observing that, for a given emissivity, the error is smaller for shorter wavelength, which agrees with the experimental findings (see Figure 12). Int. J. Turbomach. Propuls. Power 2020, 5, 31 14 of 16 In any case, having calibrated our instrument with a blackbody cavity, it is possible to state that the fireball will have reached at least the highest measured temperature, regardless of the emissivity and of the blackbody assumption. In fact, if the blackbody assumption is made (" = 1), but the true emissivity is " < 1 and constant with wavelength (i.e., the fireball is not a blackbody, but a grey body), then the di erence between the true temperature T and the measured temperature, also called colour temperature, T , can be written as: 1 1 ( ) = + ln " T T c c 2 From this expression, an error in emissivity of D" will lead to an error in the inferred temperature: DT = D" where: DT = T T and D" = 1 ". From the expression above, it is clear that the temperature error is temperature- and wavelength-dependent and that a grey body would not provide identical temperature readings at di erent wavelengths (as in the blackbody case), as shown also in Figure 13, where temperature error is plotted versus wavelength at three given emissivities at T = 3000 K, and in Figure 14, where temperature error is plotted versus true temperature at the three wavelengths used for " = 0.8. It is worth observing that, for a given emissivity, the error is smaller for shorter wavelength, which agrees with the experimental findings (see Figure 12). Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 14 of 15 Figure 13. Plot of temperature error versus wavelength at T = 3000 K for different values of emissivity. Figure 13. Plot of temperature error versus wavelength at T = 3000 K for di erent values of emissivity. Figure 14. Plot of temperature error versus temperature for ε = 0.8 and for the three chosen wavelengths. 5. Conclusions In summary, a novel ultra-high-speed combustion pyrometer, based on collection of thermal radiation via an optical fibre, was successfully designed, developed and tested. The instrument was traceably calibrated to the ITS-90 over the temperature range T = (1073–2873) K with residuals <1%. Dynamic tests with pyrotechnic charges demonstrated that the instrument can measure rapid (sub- ms) events, due to its high sampling rate (up to 250 kHz): a temperature rise of up to ~3.25 K/µs was estimated for explosions of large pyrotechnic charges. The accuracy of the temperature measurements can be assessed by considering the extent of agreement between readings at the three wavelengths—a self-diagnostic feature that is a critical strength of the technique. However, even when agreement between temperatures is poor, we can say, with a high level of confidence, that the fireball temperature is at least that reported by the reading at 850 nm. In future, the instrument will be tested in a maritime test engine. Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 14 of 15 Int. J. Turbomach. Propuls. Power 2020, 5, 31 15 of 16 Figure 13. Plot of temperature error versus wavelength at T = 3000 K for different values of emissivity. Figure 14. Plot of temperature error versus temperature for ε = 0.8 and for the three chosen Figure 14. Plot of temperature error versus temperature for " = 0.8 and for the three chosen wavelengths. wavelengths. 5. Conclusions 5. Conclusions In summary, a novel ultra-high-speed combustion pyrometer, based on collection of thermal radiation In summar via any, optical a novel fibr ultr e, a-high-speed c was successfully ombustion designed, pyrometer, based on collec developed and tested. The tion of therm instrument al radiation via an optical fibre, was successfully designed, developed and tested. The instrument was was traceably calibrated to the ITS-90 over the temperature range T = (1073–2873) K with residuals < traceab 1%. Dynamic ly calibrated to the ITS-90 over t tests with pyrotechnic he temperature ran charges demonstrated ge T = (1 that 073the –2873) instr K ument with residua can measur ls <1%. e Dynamic tests with pyrotechnic charges demonstrated that the instrument can measure rapid (sub- rapid (sub-ms) events, due to its high sampling rate (up to 250 kHz): a temperature rise of up to ~3.25 ms) events, due to i K/s was estimated ts high for saexplosions mpling rate ( of u lar p to ge pyr 250 otechnic kHz): a t char emp ges. eratThe ure ris accuracy e of upof tothe ~3.2 temperatur 5 K/µs wae s estimated for explosions of large pyrotechnic charges. The accuracy of the temperature measurements can be assessed by considering the extent of agreement between readings at the three wavelengths—a measurements can be self-diagnostic assessed by con featuresthat idering the ex is a critical te str nt of ag engthreeme of then technique. t between readin However gs ,at the even when three wavelengths—a self-diagnostic feature that is a critical strength of the technique. However, even agreement between temperatures is poor, we can say, with a high level of confidence, that the fireball when agre temperatur em e is ent between temperature at least that reported by s the is poor, we c reading ata 850 n say nm. , wit Inhfutur a high leve e, the instr l of ument confidenc wille, t behtested at the fireball temperature is at least that reported by the reading at 850 nm. In future, the instrument will in a maritime test engine. be tested in a maritime test engine. Author Contributions: Conceptualization, G.S.; methodology, A.S. and G.S.; software, A.S., D.L. and G.S.; validation, A.S.; formal analysis, A.S.; investigation, A.S. and D.L.; resources, A.S., D.L. and G.S.; data curation, A.S.; writing—original draft preparation, A.S.; writing—review and editing, D.L. and G.S.; visualization, A.S.; supervision, G.S.; project administration, A.S. and G.S.; funding acquisition, G.S. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded through the European Metrology research Programme (EMRP) Project 17IND07 DynPT. The EMRP is jointly funded by EMRP participating countries within EURAMET and the European Union. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. References 1. Teichmann, R.; Wimmer, A.; Schwarz, C.; Winklhofer, E. Combustion Diagnostic. In Combustion Engines Development—Mixture Formation, Combustion, Emissions and Simulation; Merker, G.P., Schwarz, C., Teichmann, R., Eds.; Springer: Berlin/Heidelberg, Germany, 2012; p. 115. 2. Saxholm, S.; Högström, R.; Sarraf, C.; Sutton, G.; Wynands, R.; Arrhén, F.; Jönsson, G.; Durgut, Y.; Peruzzi, A.; Fateev, A. Development of measurement and calibration techniques for dynamic pressures and temperatures (DynPT): Background and objectives of the 17IND07 DynPT project in the European Metrology Programme for Innovation and Research (EMPIR). J. Phys. Conf. Ser. 2018, 1065, 162015. [CrossRef] Int. J. Turbomach. Propuls. Power 2020, 5, 31 16 of 16 3. Preston-Thomas, H. The International Temperature Scale of 1990 (ITS-90). Metrologia 1990, 27, 3–10. [CrossRef] 4. Saunders, P.; White, D.R. Physical basis of interpolation equations for radiation thermometry. Metrologia 2003, 40, 195–203. [CrossRef] 5. Safety Datasheet of LeMaitre Ltd. Theatrical Flashes. Available online: https://www.lemaitreltd.com/ products/pyrotechnics/pyroflash/theatrical-flashes-stars/theatrical-flashes/sds-flashes-robotics-spds/ (accessed on 22 November 2020). Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional aliations. © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY-NC-ND) license (http://creativecommons.org/licenses/by-nc-nd/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png International Journal of Turbomachinery, Propulsion and Power Multidisciplinary Digital Publishing Institute

Towards an Ultra-High-Speed Combustion Pyrometer

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International Journal of Turbomachinery Propulsion and Power Article Alberto Sposito, Dave Lowe and Gavin Sutton * National Physical Laboratory (NPL), Hampton Road, Teddington TW11 0LW, UK; sposito.alberto@gmail.com (A.S.); dave.lowe@npl.co.uk (D.L.) * Correspondence: gavin.sutton@npl.co.uk y This paper was presented at the 9th EVI-GTI International Gas Turbine Instrumentation Conference, Graz, Austria on 20–21 November 2019. Received: 2 March 2020; Accepted: 10 December 2020; Published: 15 December 2020 Abstract: Measuring reliably the correct temperature of a sooty flame in an internal combustion engine is important to optimise its eciency; however, conventional contact thermometers, such as thermocouples, are not adequate in this context, due to drift, temperature limitation (2100 K) and slow response time (~10 ms). In this paper, we report on the progress towards the development of a novel ultra-high-speed combustion pyrometer, based on collection of thermal radiation via an optical fibre, traceably calibrated to the International Temperature Scale of 1990 (ITS-90) over the temperature range T = (1073–2873) K, with residuals <1%, and capable of measuring at a sampling rate of 250 kHz. Keywords: combustion; temperature; pyrometer; optical sensor 1. Introduction Traceable, reliable measurement of combustion temperature is important because it can improve the understanding of the combustion process and provide a mechanism for the optimisation of engine power, fuel consumption and emissions [1]. These measurements are performed under highly dynamic conditions, with temperature changes of up to ~3300 K occurring on a millisecond timescale. Conventional temperature sensors based on contact thermometry (e.g., thermocouples) are inadequate in this context, due to their slow response time (~10 ms), temperature limitation (2100 K), drift and perturbation of the combustion process. To address this challenge, with particular reference to internal combustion and diesel engines, we are developing a novel ultra-high-speed combustion pyrometer, within the framework of the European joint research project DynPT—Development of measurement and calibration techniques for dynamic pressures and temperatures, part of the European Metrology Programme for Innovation and Research (EMPIR) [2]. 2. System Design and Theoretical Model A schematic of the thermometer system design is shown in Figure 1. It consists of: A sensor: a 2 m long gold-coated multi-mode (MM) step-index fibre, with 400 m core diameter, numerical aperture NA = 0.22, stainless-steel monocoil sheathing, a sub-miniature (SMA) connector on one end (hot front end) and a fibre-channel (FC) connector on the other end (cold back end)—for testing purposes, this was placed inside a ~1.7 m long stainless-steel tube (outer diameter: 20 mm, inner diameter: 16 mm), with the SMA connector protected by a recessed sapphire window; sensor and packaging can be tailored to the final application and installation requirements (e.g., addition of a collimating lens). An extension lead fibre: a lightly-armoured 10 m long MM step-index fibre patch-cord, with 600m core diameter, NA = 0.22, dual acrylate coating, 3 mm diameter polyvinyl chloride (PVC) sleeve and FC connectors on both ends—this connects the sensor (on the FC connector) to the interrogator. Int. J. Turbomach. Propuls. Power 2020, 5, 31; doi:10.3390/ijtpp5040031 www.mdpi.com/journal/ijtpp Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 2 of 15 • An extension lead fibre: a lightly-armoured 10 m long MM step-index fibre patch-cord, with 600 Int. J. Turbomach. Propuls. Power 2020, 5, 31 2 of 16 µm core diameter, NA = 0.22, dual acrylate coating, 3 mm diameter polyvinyl chloride (PVC) sleeve and FC connectors on both ends—this connects the sensor (on the FC connector) to the A passive optoelectronic interrogator, assembled in-house and consisting mainly of: interrogator. • A passive optoelectronic interrogator, assembled in-house and consisting mainly of: # a custom-made 1  3 mm step-index fibre coupler/splitter with 600 m core diameter, NA = 0.22 and FC connectors on all ports; o a custom-made 1 × 3 mm step-index fibre coupler/splitter with 600 µm core diameter, NA # = 0. thr 22 ee an photodetector d FC connectors o assemblies, n all port using s; o -the-shelf components, for measuring optical o three photodetector asse thermal radiation at 3 dimblies, using erent wavelengths: off-the-sh  =elf components, for me 850 nm,  = 1050 nm and asur  ing optical = 1300 nm; 1 2 3 thermal radiation at 3 different wavelengths: λ1 = 850 nm, λ2 = 1050 nm and λ3 = 1300 nm; # a power supply unit to power the photodetectors. o a power supply unit to power the photodetectors. A National Instrument (NI) data acquisition (DAQ) system, with maximum sampling rate • fA Nat = ion 1 MHz, al Instrument connected (NI) to dthe ata ac optoelectr quisition onic (DAQ interr ) sy ogator stem, w via ithBNC maxim cables um sand ampto ling a r Personal ate fMAX MAX Computer = 1 MHz, connected to the optoel (PC) via a USB cable. ectronic interrogator via BNC cables and to a Personal Computer (PC) via a USB cable. Figure 1. Schematic of the system. Figure 1. Schematic of the system. Fibr Fibres w es with ith large large core core d diameter iameter an and d lar large ge NA NA w wer eree c chosen hosen t to o m maximise aximise collection of optic collection of optical al thermal thermal radia radiation; tion; t the he gold gold (Au) co (Au) coating ating all allows ows t the he fi fibr bre to e to withstand withstand high high t temperatur emperatures, up es, up to ~1000 to ~1000 K K, , although although t the he cor core di e diameter ameter of of Au-coated Au-coated fibre fibress is is limited limited t to o 4 400 00 µm. m. The The wave wavelengths lengths of the of the photodetector assemblies photodetector assemblies we wer re chosen b e chosen a based sed on on previo previous us experien experience ce to to avoid avoid spspectral ectral feat featur ureses (e(emission mission an and d ab absorption sorption lilines) nes) from fromtthe he combust combustion ion by-product by-products s and and the the components components of the pyrot of the pyrotechnic echnic ch chargesarges (see (see figures fi below gure,s below taken from , ta earlier ken fr spectr om e oscopic arlier spect experiments), roscopic as experi well ments), a as to test s wel thel as to assumption test the assumpti that the measur on that the measur ed combustion ed combustion p process behaves rocess behave like a blackbody s like a (emissivity blackbody (e " miss = 1.0)—good ivity ε = 1 agr .0)eement —good amongst agreement a the temperatur mongst the te es estimated mperature at s e di s timated erent wavelengths at different can wavelen be used gths to can be confirm use that d tothe confirm th blackbody at th condition e blackbois dy cond met. ition is met. Figur Figure e 2 2a sho a shows ws the emission spectrum the emission spectrum captur captured ed wit with h a a Si Si spectro spectrometer meter, where , where the fo the following llowing featur features were es were identified: identified: A. 589 nm—Sodium (Na) emission lines; A. 589 nm—Sodium (Na) emission lines; B. 619 nm—CaOH emission lines; B. 619 nm—CaOH emission lines; C. 693 nm—Potassium (K) emission lines; C. 693 nm—Potassium (K) emission lines; D. 767 nm—K emission and absorption lines; D. 767 nm—K emission and absorption lines; E. 960 nm—Uncertain of assignment. Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 3 of 15 Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 3 of 15 Int. J. Turbomach. Propuls. Power 2020, 5, 31 3 of 16 E. 960 nm—Uncertain of assignment. E. 960 nm—Uncertain of assignment. Figure 2. Pyrotechnic emission spectrum from (a) Si spectrometer, (b) InGaAs spectrometer, ~36 ms after ignition. The coloured vertical lines identify the chosen wavelength: 850 (blue line in (a)), 1050 Figure 2. Pyrotechnic emission spectrum from (a) Si spectrometer, (b) InGaAs spectrometer, ~36 ms Figure 2. Pyrotechnic emission spectrum from (a) Si spectrometer, (b) InGaAs spectrometer, ~36 ms after (green line in (b)) and 1300 nm (red line in (b)). after ignition. The coloured vertical lines identify the chosen wavelength: 850 (blue line in (a)), 1050 ignition. The coloured vertical lines identify the chosen wavelength: 850 (blue line in (a)), 1050 (green (green line in (b)) and 1300 nm (red line in (b)). line in (b)) and 1300 nm (red line in (b)). Figure 2a also shows the blackbody spectrum from a tungsten calibration lamp (with a temperature of 3165 K) overlapped to the measured spectrum. The agreement between the shape of Figure 2a also shows the blackbody spectrum from a tungsten calibration lamp (with a Figure 2a also shows the blackbody spectrum from a tungsten calibration lamp (with a temperature the two spectra suggests that the blackbody assumption for a fireball is a valid hypothesis. temperature of 3165 K) overlapped to the measured spectrum. The agreement between the shape of of 3165 K) overlapped to the measured spectrum. The agreement between the shape of the two spectra Figure 2b shows the emission spectrum captured with an InGaAs spectrometer, where the the two spectra suggests that the blackbody assumption for a fireball is a valid hypothesis. suggests that the blackbody assumption for a fireball is a valid hypothesis. following features were identified: Figure 2b shows the emission spectrum captured with an InGaAs spectrometer, where the Figure 2b shows the emission spectrum captured with an InGaAs spectrometer, where the following features were identified: following features were identified: A. 1104 nm—K emission lines; A. B. A. 11 11 01104 6 49 nm—K nm—K nm—K emiss emiss emission ion line ion line lines; ss ; ; B. 1169 nm—K emission lines; B. C. 11 Broad 69 nm—K OH ab emiss sorption in ion line the fibre; s; C. Broad OH absorption in the fibre; C. D. Broad 1243 a OH nd 12 ab 52 sorption in nm—K emissi the fibre; on lines; D. 1243 and 1252 nm—K emission lines; D. E. 1Broad 243 an OH d 12ab 52sorption in nm—K emissi the fibre; on lines; E. Broad OH absorption in the fibre; E. F. Broad 1517 nm—K OH ab emiss sorption in ion line the fibre; s. F. 1517 nm—K emission lines. F. 1517 nm—K emission lines. As photodetectors with variable gain G were used, a simple theoretical model was developed to As photodetectors with variable gain G were used, a simple theoretical model was developed to estimate the optical power measured by each photodetector and how their voltage signals change As photodetectors with variable gain G were used, a simple theoretical model was developed to estimate the optical power measured by each photodetector and how their voltage signals change with with G. Their bandwidth B also decreases with increasing G, adjustable in 10 dB steps from 0 to 70 estimate the optical power measured by each photodetector and how their voltage signals change G. Their bandwidth B also decreases with increasing G, adjustable in 10 dB steps from 0 to 70 dB. dB. with G. Their bandwidth B also decreases with increasing G, adjustable in 10 dB steps from 0 to 70 First of all, the blackbody radiation power coupled into the core of the optical fibre (see geometry First of all, the blackbody radiation power coupled into the core of the optical fibre (see geometry dB. sketched in Figure 3) was calculated, assuming an emissivity " = 1 and optical transmission of the fibre sketched in Figure 3) was calculated, assuming an emissivity ε = 1 and optical transmission of the First of all, the blackbody radiation power coupled into the core of the optical fibre (see geometry over the range  = (0.3–2.4) m as specified in the Au-coated fibre datasheet. fibre over the range λ = (0.3–2.4) µm as specified in the Au-coated fibre datasheet. sketched in Figure 3) was calculated, assuming an emissivity ε = 1 and optical transmission of the fibre over the range λ = (0.3–2.4) µm as specified in the Au-coated fibre datasheet. Figure 3. Geometry of the end of the sensor (Au-coated fibre): d = fibre core diameter; θ = maximum Figure 3. Geometry of the end of the sensor (Au-coated fibre): d = fibre core diameter;  = maximum acceptance half-angle. acceptance half-angle. Figure 3. Geometry of the end of the sensor (Au-coated fibre): d = fibre core diameter; θ = maximum acceptance half-angle. Neglecting the Fresnel reflection losses from the end-facet of the fibre and from the sapphire Neglecting the Fresnel reflection losses from the end-facet of the fibre and from the sapphire window, the total blackbody radiation power coupled into the fibre core over the full blackbody window, the total blackbody radiation power coupled into the fibre core over the full blackbody Neglecting the Fresnel reflection losses from the end-facet of the fibre and from the sapphire radiation spectrum (i.e., all wavelengths) is: radiation spectrum (i.e., all wavelengths) is: window, the total blackbody radiation power coupled into the fibre core over the full blackbody radiation spectrum (i.e., all wavelengths) is: 4 Pin, TOT(T) = ΩAεσT /π Pin, TOT(T) = ΩAεσT /π where: where: Int. J. Turbomach. Propuls. Power 2020, 5, 31 4 of 16 P (T) = WA"T / in, TOT where: T is the blackbody temperature in K; 8 2 4 = 5.67 10 Wm K is the Stefan–Boltzmann constant; 2 7 2 A = d /4 = 1.25664 10 m is the fibre core area; W = tan () is the maximum solid acceptance angle of the Au-coated fibre, with  the maximum acceptance half-angle of the Au-coated fibre, which is related to the NA of the fibre as: NA = nsin() = 0.22 As the refractive index of air is n 1, the maximum solid acceptance angle can be re-written as: W = tan (arcsin(NA)) 0.16 sr Hence, the total blackbody radiation power coupled into the optical fibre is: 16 4 P (T) 3.624 10 T W in, TOT The fractional power coupled into the optical fibre over the wavelength range  = (0.3–2.4) m, P (T), can be calculated numerically or through tabulated values, considering the in wavelength-temperature products. With such a model, approximately 10 mW of optical thermal radiation is coupled into the optical fibre for T  2500 K; however, for T = 300 K: P (T) < 10 pW. in To calculate the optical power measured by each photodetector and the associated voltage signals, the losses in the optical transmission line from the sensor head to the detectors need to be considered. To estimate the signals accurately, the following contributions need to be taken into account: 1. The transmission factor of the sapphire window placed in front of the fibre end-facet, due to Fresnel reflection losses (7% at each interface/surface): t = 0.93 0.93 = 0.8649. 2. The transmission factor at the end-facet of the Au-coated fibre, due to Fresnel reflection losses: t = 0.96. 3. Transmission losses of 12 m of fibre (2 m sensor + 10 m of extension lead fibre)—considering that typical losses for large-core multi-mode fibre are of the order of 10 dB/km or less at  = (0.6–1.6)m: t =0.12 dB 0.973. fibre 4. Losses due to optical connectors (3), typically of the order of 0.3 dB each—i.e., a transmission factor t = 0.933 connector 5. The splitting ratio of the 1 3 optical coupler/splitter: t  0.333. splitter 6. The optical transmission (t ) of the bandpass filters in the photodetector assemblies—it filter is worth noting that the filters used have di erent values of optical transmission peak and Full-Width-at-Half Maximum (FWHM): # t = 70%; 850 nm # t = 45%; 1050 nm # t = 40%; 1300 nm # FWHM = 40 nm 8 nm; 850 nm # FWHM = 10 nm 2 nm; 1050 nm # FWHM = 30 nm 6 nm. 1300 nm Hence, the optical power incident on the photodetectors can be calculated as: Int. J. Turbomach. Propuls. Power 2020, 5, 31 5 of 16 P (, T) = t t t t (t ) t ()P (, T)  0.22t ()P (, T) (1) connector i 0 1 fibre splitter filter in filter in Finally, to calculate the voltage signal, we also need to consider the photodetector transimpedance gain G [V/A] and responsivity R( ) [A/W], which is a function of wavelength; hence: i i Z Z Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 5 of 15 V = G R ()P (, T)d = 0.22G t ()R ()P (, T)d (2) i i i i i filter i in 𝑃 (𝜆, 𝑇 ) = 𝐴 𝛺𝐿 (𝜆, 𝑇) where: P (, T) = AWL (, T) in b with: with: 2𝑐 1 2𝑐 ( ) 2c 1 2c 𝐿 𝜆, 𝑇 = ≈ 𝑒 1 1 c /T ⁄ 2 L (, T) =  e 𝜆 𝑒 −1 𝜆 5 5 c /T e 1 −16 −2 −1 where c1 and c2 are the first and second radiation constants equal to 0.59552197 × 10 W·m ·sr and 16 2 1 where c and c are the first and second radiation constants equal to 0.59552197 10 Wm sr and 1 2 −2 1.438769 × 10 m·K, respectively, and the expression after the ≈ symbol is the Wien approximation 1.438769 10 mK, respectively, and the expression after the symbol is the Wien approximation valid for c2 >> λT. valid for c >> T. Equations (1) and (2) were evaluated at different temperatures, with the results compared with Equations (1) and (2) were evaluated at di erent temperatures, with the results compared with preliminary experimental data. These showed a lower signal than expected (by a factor of ~2), likely preliminary experimental data. These showed a lower signal than expected (by a factor of ~2), likely due due to extra connection losses. This was fed back into the model, which gave the results shown in to extra connection losses. This was fed back into the model, which gave the results shown in Figure 4 Figure 4 in terms of optical power incident onto the photodetectors—noise floors are shown only for: in terms of optical power incident onto the photodetectors—noise floors are shown only for: G = 0 dB, G = 0 dB, B = 12 MHz (highest noise floor); G = 20 dB, B = 1 MHz and G = 70 dB, B = 3 kHz (lowest B = 12 MHz (highest noise floor); G = 20 dB, B = 1 MHz and G = 70 dB, B = 3 kHz (lowest noise floor). noise floor). Figure 4. Blackbody radiation power incident onto photodetectors as a function of blackbody Figure 4. Blackbody radiation power incident onto photodetectors as a function of blackbody temperature. temperature. Noise floor was estimated for each photodetector as: Noise floor was estimated for each photodetector as: 1/2 P = NEP B  R /R( ) n, i / MAX i 𝑃 =𝑁𝑃𝐸 𝐵 𝑅 /𝑅(𝜆 ) where NEP is the Noise Equivalent Power and R is the peak responsivity—both provided in the where NEP is the Noise Equivalent Power and RMAX is the peak responsivity—both provided in the MAX photodetector datasheets. photodetector datasheets. Figure 4 shows that, in any case, the instrument should be capable of measuring temperatures Figure 4 shows that, in any case, the instrument should be capable of measuring temperatures T T > 1600 K at all wavelengths, with photodetectors set at G = 0 dB and B = 12 MHz. However, > 1600 K at all wavelengths, with photodetectors set at G = 0 dB and B = 12 MHz. However, considering that the maximum sampling rate of the DAQ system is f = 1 MHz, a gain setting of considering that the maximum sampling rate of the DAQ system is fMAX = 1 MHz, a gain setting of G MAX = 20 dB (B = 1 MHz) would allow measurement of temperatures as low as ~1000 K with the 1300 nm photodetector (but not at λ1 = 850 nm and λ2 = 1050 nm), with no penalty in terms of speed. The minimum and maximum temperatures measurable by the instrument are dictated, respectively, by the noise level (experimentally measured as ~1 mV for most values of G) and the saturation level (~10 V) of the photodetectors. To find the photodetector settings that optimise the measurable temperature range, Equation (2) was evaluated at different temperatures for different G and B settings of the photodetectors. Voltage signals generated by the three photodetectors were plotted versus temperature for all values of G and B. Figure 5 shows two of these plots for representative values of G and B. Int. J. Turbomach. Propuls. Power 2020, 5, 31 6 of 16 Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 6 of 15 G = 20 dB (B = 1 MHz) would allow measurement of temperatures as low as ~1000 K with the 1300 nm • With a gain of G = 20 dB (B = 1 MHz—Figure 5a), the instrument can measure a minimum photodetector (but not at  = 850 nm and  = 1050 nm), with no penalty in terms of speed. temperature of ~1150 K at 1 a single wavelengt 2 h (λ3 = 1300 nm) or ~1400 K at all three wavelengths. The minimum and maximum temperatures measurable by the instrument are dictated, respectively, • With a gain of G = 30 dB (B = 260 kHz), the minimum measurable temperature can be brought by the noise level (experimentally measured as ~1 mV for most values of G) and the saturation level down to ~1025 K for single-wavelength measurement (λ3 = 1300 nm) and ~1275 K at all three (~10 V) of the photodetectors. To find the photodetector settings that optimise the measurable wavelengths, but at cost of reduced sampling speed (f ≤ B = 260 kHz), while still avoiding temperature range, Equation (2) was evaluated at di erent temperatures for di erent G and B settings saturation at 3300 K, our maximum temperature of interest. of the photodetectors. Voltage signals generated by the three photodetectors were plotted versus • With a gain of G ≥ 40 dB (B = 90 kHz—Figure 5b), the photodetectors would start saturating at temperature for all values of G and B. Figure 5 shows two of these plots for representative values of G TMAX < 3300 K and their bandwidth would decrease significantly, down to B = 3 kHz at G = 70 and B. dB. Figure 5. Modelled signals from photodetectors versus blackbody temperature at di erent G and B Figure 5. Modelled signals from photodetectors versus blackbody temperature at different G and B settings: (a) G = 20 dB and B = 1 MHz, (b) G = 40 dB and B = 90 kHz. settings: (a) G = 20 dB and B = 1 MHz, (b) G = 40 dB and B = 90 kHz. With a gain of G = 20 dB (B = 1 MHz—Figure 5a), the instrument can measure a minimum Hence, the optimum photodetector gain is G = 30 dB, which allows temperatures above 1025 K temperature of ~1150 K at a single wavelength ( = 1300 nm) or ~1400 K at all three wavelengths. to be measured for λ3 = 1300 nm, or temperatures above 1275 K to be measured for all wavelengths, With a gain of G = 30 dB (B = 260 kHz), the minimum measurable temperature can be brought with a maximum sampling rate f ≤ B = 260 kHz. down to ~1025 K for single-wavelength measurement ( = 1300 nm) and ~1275 K at all three Int. J. Turbomach. Propuls. Power 2020, 5, 31 7 of 16 wavelengths, but at cost of reduced sampling speed (f  B = 260 kHz), while still avoiding saturation at 3300 K, our maximum temperature of interest. With a gain of G 40 dB (B = 90 kHz—Figure 5b), the photodetectors would start saturating at T < 3300 K and their bandwidth would decrease significantly, down to B = 3 kHz at G = 70 dB. MAX Hence, the optimum photodetector gain is G = 30 dB, which allows temperatures above 1025 K to be measured for  = 1300 nm, or temperatures above 1275 K to be measured for all wavelengths, with a maximum sampling rate f  B = 260 kHz. Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 7 of 15 3. Instrument Calibration 3. Instrument Calibration 3.1. Test Rig 3.1. Test Rig The instrument was calibrated using a Thermo Gauge blackbody radiation furnace and a The instrument was calibrated using a Thermo Gauge blackbody radiation furnace and a KE- KE-Technologie GmbH LP3 linear pyrometer calibrated traceably to the ITS-90 [3], with the Technologie GmbH LP3 linear pyrometer calibrated traceably to the ITS-90 [3], with the stainless- stainless-steel tube of the packaged sensor filled with sand to avoid overheating the Au-coated steel tube of the packaged sensor filled with sand to avoid overheating the Au-coated fibre that could fibre that could be irreversibly damaged. A photograph of part of the test rig is shown in Figure 6: be irreversibly damaged. A photograph of part of the test rig is shown in Figure 6: the hot Thermo the hot Thermo Gauge blackbody furnace and temperature sensor are visible in the background and Gauge blackbody furnace and temperature sensor are visible in the background and foreground, foreground, respectively. The latter is placed on a stainless-steel V-groove mounted on an optical respectively. The latter is placed on a stainless-steel V-groove mounted on an optical breadboard and breadboard and safely held in place by removable brackets bolted to the breadboard. This breadboard is safely held in place by removable brackets bolted to the breadboard. This breadboard is installed on installed on a motorised stage, controlled by a computer, for horizontal and vertical alignment. The LP3 a motorised stage, controlled by a computer, for horizontal and vertical alignment. The LP3 (not (not visible in Figure 6) is mounted on the same framework, so that it can be easily moved in front of visible in Figure 6) is mounted on the same framework, so that it can be easily moved in front of the the blackbody, in place of the sensor, to measure the temperature at each setpoint of the calibration. blackbody, in place of the sensor, to measure the temperature at each setpoint of the calibration. Figure 6. Photograph of the calibration furnace: the instrument sensor, housed in a steel tube, is sitting Figure 6. Photograph of the calibration furnace: the instrument sensor, housed in a steel tube, is sitting on the V-groove placed in front of blackbody furnace, ready to be manually moved in and out of it on the V-groove placed in front of blackbody furnace, ready to be manually moved in and out of it for for dynamic calibration at a set temperature. dynamic calibration at a set temperature. Data from the instrument were acquired using a NI LabVIEW program written in-house and Data from the instrument were acquired using a NI LabVIEW program written in-house and executed on the PC that is part of the system, whereas the blackbody furnace was controlled with a executed on the PC that is part of the system, whereas the blackbody furnace was controlled with a separate desktop computer that also controls the motorised framework. separate desktop computer that also controls the motorised framework. 3.2. Test Method The instrument was configured with the photodetectors set with optimum gain G = 30 dB (B = 260 kHz) and the sampling rate set at f = 250 kHz. At the beginning of the calibration, the voltage offset from the three photodetectors was measured once, to zero the photodetectors. The instrument was calibrated in the temperature range T = (1073–2873) K, in steps of ∆T = 200 K, according to the following procedure: 1. The blackbody furnace was set at the required temperature set-point. 2. The temperature of the blackbody cavity was monitored using the LP3. 3. Once the blackbody temperature reached stability, a measurement was taken from the LP3, by measuring the average and standard deviation over ~30 s (the LP3 is sampled at 1 Hz). 4. The LP3 was moved out of the way and the sensor moved into place, so that it was in line with and parallel to the long axis of the blackbody, as shown in Figure 6. Int. J. Turbomach. Propuls. Power 2020, 5, 31 8 of 16 3.2. Test Method The instrument was configured with the photodetectors set with optimum gain G = 30 dB (B = 260 kHz) and the sampling rate set at f = 250 kHz. At the beginning of the calibration, the voltage o set from the three photodetectors was measured once, to zero the photodetectors. The instrument was calibrated in the temperature range T = (1073–2873) K, in steps of DT = 200 K, according to the following procedure: 1. The blackbody furnace was set at the required temperature set-point. 2. The temperature of the blackbody cavity was monitored using the LP3. 3. Once the blackbody temperature reached stability, a measurement was taken from the LP3, by measuring the average and standard deviation over ~30 s (the LP3 is sampled at 1 Hz). 4. The LP3 was moved out of the way and the sensor moved into place, so that it was in line with and parallel to the long axis of the blackbody, as shown in Figure 6. 5. Data acquisition and logging were started on the instrument. 6. Manually, the sensor was quickly moved into and out of the blackbody (within a few seconds). 7. Two measurements were made at each set-point temperature. 3.3. Test Data Analysis Method The raw voltage signals from the three photodetectors of the optoelectronic interrogator were analysed to find the optimum calibration point in each signal. This is explained in Figure 7, showing typical measurement traces—the signal from the 1050 nm photodetector was the lowest, because of the combined e ect of the responsivity of the photodetector and the transmission and the bandwidth of the optical bandpass filter. Considering Figure 7b: t < 1.3 s: the blackbody cavity had a temperature gradient along the cavity wall and across its rear surface, it was hotter to the outside, and this was seen as the sensor approached: the radiance signal increased as the field of view of the sensor was initially filled. t (1.3–1.6) s: the signal fell as the sensor progressively saw more of the cooler central section of the back wall. t  (1.6–1.77) s: there was a period when the blackbody temperature fell due to heat lost to the cold sensor. t (1.77–1.92) s: as the sensor was withdrawn, the hotter regions of the blackbody cavity were seen again, so that the signal increased. t > 1.92 s: the signal decreased, as the sensor was withdrawn from the blackbody cavity. The maximum in the signal during sensor removal was lower than during insertion. This is consistent with the cooling of the blackbody cavity. The voltages recorded for calibration were chosen at the inflection point of each signal, highlighted by the blue circle, as it corresponds to the point when the field of view of the fibre is filled with thermal radiation from the back wall, before any further cooling caused by the sensor. Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 8 of 15 5. Data acquisition and logging were started on the instrument. 6. Manually, the sensor was quickly moved into and out of the blackbody (within a few seconds). 7. Two measurements were made at each set-point temperature. 3.3. Test Data Analysis Method The raw voltage signals from the three photodetectors of the optoelectronic interrogator were analysed to find the optimum calibration point in each signal. This is explained in Figure 7, showing typical measurement traces—the signal from the 1050 nm photodetector was the lowest, because of Int. J. Turbomach. Propuls. Power 2020, 5, 31 9 of 16 the combined effect of the responsivity of the photodetector and the transmission and the bandwidth of the optical bandpass filter. Figure 7. Typical calibration measurement traces: the black line in the middle of chart (a) identifies Figure 7. Typical calibration measurement traces: the black line in the middle of chart (a) identifies the the points used for calibration; (b) is a close-up on the 1300 nm signal and its calibration point. points used for calibration; (b) is a close-up on the 1300 nm signal and its calibration point. Considering Figure 7b: Calibration was performed by fitting experimental data to the Planckian version of the Sakuma–Hattori equation with three adjustable parameters A , B and c for each wavelength  [4]: • t < 1.3 s: the blackbody cavity had a temperature gradient i along i the cav i ity wall and across itis rear surface, it was hotter to the outside, and this was seen as the sensor approached: the radiance signal increased as the field of view of the sensor was initially filled. V = (3) c /( T+B ) i i i e 1 At each calibration point, the two voltage measurements for each wavelength/photodetector were averaged; each average was then converted into temperature using the inverse function of Equation (3): c 1 T =   (4) ln + 1 Optimum values of the adjustable parameters were found using the Generalized Reduced Gradient (GRG) solving method for smooth non-linear problems, to minimise the sum of the squares of temperature di erences with the LP3. Int. J. Turbomach. Propuls. Power 2020, 5, 31 10 of 16 3.4. Calibration Test Results The average signal at each setpoint was measured for each wavelength/photodetector and plotted versus the set-point temperature measured from the LP3 linear pyrometer. Figure 8 shows good agreement between experimental data and the theoretical model at G = 30 dB, as used for the calibration, with the signal from the 1300 nm photodetector higher than predicted, most likely due to overestimated losses, as a single figure was used for all three wavelengths. Figure 8 also shows that the instrument can measure a temperature as low as 1073 K at  = 1300 nm or 1273 K at all three wavelengths—these minimum temperatures match with those expected from the theoretical model. Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 10 of 15 Figure 8. Experimental data compared with theoretical model. Figure 8. Experimental data compared with theoretical model. Using the inverse Planck function—i.e. Equation (4)—average voltage measurements for each Using the inverse Planck function—i.e. Equation (4)—average voltage measurements for each wavelength at each set-point were converted into temperatures and the adjustable parameters were wavelength at each set-point were converted into temperatures and the adjustable parameters were optimised to minimise the sum of the squares of calibration residuals. The optimum calibration optimised to minimise the sum of the squares of calibration residuals. The optimum calibration coefficients are shown in Table 1 (Bi coefficients are not included, because they were found to be equal coecients are shown in Table 1 (B coecients are not included, because they were found to be equal to zero) and the residuals are shown in Figure 9, showing relative temperature differences within to zero) and the residuals are shown in Figure 9, showing relative temperature di erences within1% ±1% (absolute differences are within ±15 K). (absolute di erences are within15 K). Table 1. Optimum values of the adjustable calibration coefficients, minimising sum of squares of Table 1. Optimum values of the adjustable calibration coecients, minimising sum of squares of relative errors. relative errors. λi [nm] Ai [V] ci [µm K] [nm] A [V] c [m K] i i i 850 527.6 14,082.2 850 527.6 14,082.2 1050 53.0 14,431.8 1050 53.0 14,431.8 1300 91.7 14,307.3 1300 91.7 14,307.3 Figure 9. Calibration residuals. Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 10 of 15 Figure 8. Experimental data compared with theoretical model. Using the inverse Planck function—i.e. Equation (4)—average voltage measurements for each wavelength at each set-point were converted into temperatures and the adjustable parameters were optimised to minimise the sum of the squares of calibration residuals. The optimum calibration coefficients are shown in Table 1 (Bi coefficients are not included, because they were found to be equal to zero) and the residuals are shown in Figure 9, showing relative temperature differences within ±1% (absolute differences are within ±15 K). Table 1. Optimum values of the adjustable calibration coefficients, minimising sum of squares of relative errors. λi [nm] Ai [V] ci [µm K] 850 527.6 14,082.2 Int. J. Turbomach. Propuls. Power 2020, 5, 31 11 of 16 1050 53.0 14,431.8 1300 91.7 14,307.3 Figure 9. Calibration residuals. Figure 9. Calibration residuals. Having calibrated the sensor, the maximum measurable temperatures can be estimated by extrapolation of the Planck function in Equation (3), until the photodetector saturation level (V = 10 V) is reached or, more accurately, by replacing this value in the inverse Planck MAX function—i.e., Equation (4). In a similar way, minimum measurable temperatures were estimated by replacing the noise level (V  1 mV) in the inverse Planck function. Table 2 shows minimum and noise maximum measurable temperatures. Table 2. Minimum and maximum measurable temperatures. [nm] T [K] T [K] i MIN MAX 850 1260 4160 1050 1260 7470 1300 960 4740 4. Dynamic Tests 4.1. Test Rig To demonstrate the speed of the instrument, dynamic tests were performed using theatrical flash charges [5] in the pyrotechnic facility at NPL (National Physical Laboratory). This consists of a vented enclosure where pyrotechnic charges, placed on a stage, are remotely triggered with a controller that is connected and synchronised with the instrument. The sensor is mounted such that its front end protrudes into the enclosure with its tip ~15 cm above and ~5 cm away from the centre of the charge. The optimum position of the sensor is based on experience from previous tests, when we also conducted absorption/transmission experiments, from which no optical transmission was observed during the explosion, thus suggesting that the fireball is opaque and supporting our blackbody assumption, and an initial absorption coecient = 0.25 cm was estimated at  850 nm. 4.2. Test Method Two sets of explosion tests were performed: a preliminary set of 3 tests with medium pyrotechnic charges and another set of 3 tests with large pyrotechnic charges. In all cases, the photodetector gain was set at G = 30 dB, as the instrument was calibrated only with this setting. Sampling rate and number Int. J. Turbomach. Propuls. Power 2020, 5, 31 12 of 16 of samples were set, respectively, at f = 50 kHz and N = 50,000 (giving an acquisition time t = N/f = 1 s) for the first 4 tests and then at f = 250 kHz and N = 25,000 (giving an acquisition time t = N/f = 0.1 s) for the last 2 tests. Sampling rate f and number of samples N were initially chosen based on experience from previous explosion tests, to collect enough data at a high speed but without having to record excessive data. N and f were changed in the last two tests, based on observations from the previous test, again to avoid recording data where no signal was present, but also to capture finer details and test the maximum sampling speed. 4.3. Test Results The preliminary set of tests with medium pyrotechnic charges (shown in Figure 10) demonstrated that f = 50 kHz was sucient to measure the rapid temperature rise and decay and identify signal structure in between. Variability in temperature evolution was observed from test to test—this was to be expected as no two charges are the same. Nevertheless, there was a good correlation among all traces for a given test, although the temperature agreement was poor—in particular, the temperature estimated from the signal at  = 1300 nm was significantly lower than the other two, by up to ~600 K. This suggested that the e ective emissivity at the longest wavelength was significantly less than unity—i.e., the blackbody condition necessary for successful thermometry is not met. On the contrary, the set of tests with large pyrotechnic charges shown in Figure 11 produced more consistent results and better agreement among temperatures measured at di erent wavelengths, meaning that the blackbody Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 12 of 15 condition (" = 1) is more closely met than with medium charges. In explosion tests with large charges, the maximum temperatures estimated at di erent wavelengths agree with each other within up to charges. In explosion tests with large charges, the maximum temperatures estimated at different ~137 K or ~4.5%. wavelengths agree with each other within up to ~137 K or ~4.5%. Figure 10. Time trend of temperatures for pyrotechnic tests with medium charges—temporal offset Figure 10. Time trend of temperatures for pyrotechnic tests with medium charges—temporal o set introduced for clarity. introduced for clarity. Figure 11 shows again that a sampling rate of f = 50 kHz was still fast enough to capture events from large charges, despite shorter pulse duration (<20 ms versus ~200 ms), sharper rise time (<1 ms versus ~10 ms) and faster decay times (~10 ms versus ~100 ms) than medium charges. The sampling rate was increased to f = 250 kHz in the last two trials to test the maximum sampling frequency allowed by the gain set in the photodetectors (G = 30 dB). A temperature rise of up to ~3.25 K/µs was estimated for explosions of large charges. Figure 11. Time trend of temperatures for pyrotechnic tests with large charges—temporal offset introduced for clarity. Figure 12 shows that large pyrotechnic charges produced not only more consistent results, but also higher peak temperatures than medium charges, by ~700 K. It is also worth observing that the Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 12 of 15 charges. In explosion tests with large charges, the maximum temperatures estimated at different wavelengths agree with each other within up to ~137 K or ~4.5%. Figure 10. Time trend of temperatures for pyrotechnic tests with medium charges—temporal offset introduced for clarity. Figure 11 shows again that a sampling rate of f = 50 kHz was still fast enough to capture events from large charges, despite shorter pulse duration (<20 ms versus ~200 ms), sharper rise time (<1 ms versus ~10 ms) and faster decay times (~10 ms versus ~100 ms) than medium charges. The sampling rate was increased to f = 250 kHz in the last two trials to test the maximum sampling frequency Int. J. Turbomach. Propuls. Power 2020, 5, 31 13 of 16 allowed by the gain set in the photodetectors (G = 30 dB). A temperature rise of up to ~3.25 K/µs was estimated for explosions of large charges. Figure 11. Time trend of temperatures for pyrotechnic tests with large charges—temporal offset Figure 11. Time trend of temperatures for pyrotechnic tests with large charges—temporal o set introduced for clarity. introduced for clarity. Figure 12 shows that large pyrotechnic charges produced not only more consistent results, but Figure 11 shows again that a sampling rate of f = 50 kHz was still fast enough to capture events also higher peak temperatures than medium charges, by ~700 K. It is also worth observing that the from large charges, despite shorter pulse duration (<20 ms versus ~200 ms), sharper rise time (<1 ms versus ~10 ms) and faster decay times (~10 ms versus ~100 ms) than medium charges. The sampling rate was increased to f = 250 kHz in the last two trials to test the maximum sampling frequency allowed by the gain set in the photodetectors (G = 30 dB). A temperature rise of up to ~3.25 K/s was estimated for explosions of large charges. Figure 12 shows that large pyrotechnic charges produced not only more consistent results, but also Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 13 of 15 higher peak temperatures than medium charges, by ~700 K. It is also worth observing that the temperature measured at  = 850 nm was always the highest, whereas the temperature measured at temperature measured at λ1 = 850 nm was always the highest, whereas the temperature measured at = 1300 nm was always the lowest. λ3 = 1300 nm was always the lowest. Figure 12. Plot of maximum temperatures versus test number. The inset on the upper left shows some Figure 12. Plot of maximum temperatures versus test number. The inset on the upper left shows some statistics: Mean TMAX and σ (TMAX) are, respectively, the average and the standard deviation of peak statistics: Mean T and  (T ) are, respectively, the average and the standard deviation of peak MAX MAX temperatures measured at different λ. temperatures measured at di erent . In any case, having calibrated our instrument with a blackbody cavity, it is possible to state that the fireball will have reached at least the highest measured temperature, regardless of the emissivity and of the blackbody assumption. In fact, if the blackbody assumption is made (ε = 1), but the true emissivity is ε < 1 and constant with wavelength (i.e., the fireball is not a blackbody, but a grey body), then the difference between the true temperature T and the measured temperature, also called colour temperature, Tc, can be written as: 1 1 𝜆 = + ln (𝜀 ) 𝑇 𝑇 𝑐 From this expression, an error in emissivity of Δ𝜀 will lead to an error in the inferred temperature: 𝜆𝑇 Δ𝑇 = − Δ𝜀 where: Δ𝑇 = 𝑇 −𝑇 and Δ𝜀 = 1 − ε . From the expression above, it is clear that the temperature error is temperature- and wavelength- dependent and that a grey body would not provide identical temperature readings at different wavelengths (as in the blackbody case), as shown also in Figure 13, where temperature error is plotted versus wavelength at three given emissivities at T = 3000 K, and in Figure 14, where temperature error is plotted versus true temperature at the three wavelengths used for ε = 0.8. It is worth observing that, for a given emissivity, the error is smaller for shorter wavelength, which agrees with the experimental findings (see Figure 12). Int. J. Turbomach. Propuls. Power 2020, 5, 31 14 of 16 In any case, having calibrated our instrument with a blackbody cavity, it is possible to state that the fireball will have reached at least the highest measured temperature, regardless of the emissivity and of the blackbody assumption. In fact, if the blackbody assumption is made (" = 1), but the true emissivity is " < 1 and constant with wavelength (i.e., the fireball is not a blackbody, but a grey body), then the di erence between the true temperature T and the measured temperature, also called colour temperature, T , can be written as: 1 1 ( ) = + ln " T T c c 2 From this expression, an error in emissivity of D" will lead to an error in the inferred temperature: DT = D" where: DT = T T and D" = 1 ". From the expression above, it is clear that the temperature error is temperature- and wavelength-dependent and that a grey body would not provide identical temperature readings at di erent wavelengths (as in the blackbody case), as shown also in Figure 13, where temperature error is plotted versus wavelength at three given emissivities at T = 3000 K, and in Figure 14, where temperature error is plotted versus true temperature at the three wavelengths used for " = 0.8. It is worth observing that, for a given emissivity, the error is smaller for shorter wavelength, which agrees with the experimental findings (see Figure 12). Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 14 of 15 Figure 13. Plot of temperature error versus wavelength at T = 3000 K for different values of emissivity. Figure 13. Plot of temperature error versus wavelength at T = 3000 K for di erent values of emissivity. Figure 14. Plot of temperature error versus temperature for ε = 0.8 and for the three chosen wavelengths. 5. Conclusions In summary, a novel ultra-high-speed combustion pyrometer, based on collection of thermal radiation via an optical fibre, was successfully designed, developed and tested. The instrument was traceably calibrated to the ITS-90 over the temperature range T = (1073–2873) K with residuals <1%. Dynamic tests with pyrotechnic charges demonstrated that the instrument can measure rapid (sub- ms) events, due to its high sampling rate (up to 250 kHz): a temperature rise of up to ~3.25 K/µs was estimated for explosions of large pyrotechnic charges. The accuracy of the temperature measurements can be assessed by considering the extent of agreement between readings at the three wavelengths—a self-diagnostic feature that is a critical strength of the technique. However, even when agreement between temperatures is poor, we can say, with a high level of confidence, that the fireball temperature is at least that reported by the reading at 850 nm. In future, the instrument will be tested in a maritime test engine. Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 14 of 15 Int. J. Turbomach. Propuls. Power 2020, 5, 31 15 of 16 Figure 13. Plot of temperature error versus wavelength at T = 3000 K for different values of emissivity. Figure 14. Plot of temperature error versus temperature for ε = 0.8 and for the three chosen Figure 14. Plot of temperature error versus temperature for " = 0.8 and for the three chosen wavelengths. wavelengths. 5. Conclusions 5. Conclusions In summary, a novel ultra-high-speed combustion pyrometer, based on collection of thermal radiation In summar via any, optical a novel fibr ultr e, a-high-speed c was successfully ombustion designed, pyrometer, based on collec developed and tested. The tion of therm instrument al radiation via an optical fibre, was successfully designed, developed and tested. The instrument was was traceably calibrated to the ITS-90 over the temperature range T = (1073–2873) K with residuals < traceab 1%. Dynamic ly calibrated to the ITS-90 over t tests with pyrotechnic he temperature ran charges demonstrated ge T = (1 that 073the –2873) instr K ument with residua can measur ls <1%. e Dynamic tests with pyrotechnic charges demonstrated that the instrument can measure rapid (sub- rapid (sub-ms) events, due to its high sampling rate (up to 250 kHz): a temperature rise of up to ~3.25 ms) events, due to i K/s was estimated ts high for saexplosions mpling rate ( of u lar p to ge pyr 250 otechnic kHz): a t char emp ges. eratThe ure ris accuracy e of upof tothe ~3.2 temperatur 5 K/µs wae s estimated for explosions of large pyrotechnic charges. The accuracy of the temperature measurements can be assessed by considering the extent of agreement between readings at the three wavelengths—a measurements can be self-diagnostic assessed by con featuresthat idering the ex is a critical te str nt of ag engthreeme of then technique. t between readin However gs ,at the even when three wavelengths—a self-diagnostic feature that is a critical strength of the technique. However, even agreement between temperatures is poor, we can say, with a high level of confidence, that the fireball when agre temperatur em e is ent between temperature at least that reported by s the is poor, we c reading ata 850 n say nm. , wit Inhfutur a high leve e, the instr l of ument confidenc wille, t behtested at the fireball temperature is at least that reported by the reading at 850 nm. In future, the instrument will in a maritime test engine. be tested in a maritime test engine. Author Contributions: Conceptualization, G.S.; methodology, A.S. and G.S.; software, A.S., D.L. and G.S.; validation, A.S.; formal analysis, A.S.; investigation, A.S. and D.L.; resources, A.S., D.L. and G.S.; data curation, A.S.; writing—original draft preparation, A.S.; writing—review and editing, D.L. and G.S.; visualization, A.S.; supervision, G.S.; project administration, A.S. and G.S.; funding acquisition, G.S. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded through the European Metrology research Programme (EMRP) Project 17IND07 DynPT. The EMRP is jointly funded by EMRP participating countries within EURAMET and the European Union. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. References 1. Teichmann, R.; Wimmer, A.; Schwarz, C.; Winklhofer, E. Combustion Diagnostic. In Combustion Engines Development—Mixture Formation, Combustion, Emissions and Simulation; Merker, G.P., Schwarz, C., Teichmann, R., Eds.; Springer: Berlin/Heidelberg, Germany, 2012; p. 115. 2. Saxholm, S.; Högström, R.; Sarraf, C.; Sutton, G.; Wynands, R.; Arrhén, F.; Jönsson, G.; Durgut, Y.; Peruzzi, A.; Fateev, A. Development of measurement and calibration techniques for dynamic pressures and temperatures (DynPT): Background and objectives of the 17IND07 DynPT project in the European Metrology Programme for Innovation and Research (EMPIR). J. Phys. Conf. Ser. 2018, 1065, 162015. [CrossRef] Int. J. Turbomach. Propuls. Power 2020, 5, 31 16 of 16 3. Preston-Thomas, H. The International Temperature Scale of 1990 (ITS-90). Metrologia 1990, 27, 3–10. [CrossRef] 4. Saunders, P.; White, D.R. Physical basis of interpolation equations for radiation thermometry. Metrologia 2003, 40, 195–203. [CrossRef] 5. Safety Datasheet of LeMaitre Ltd. Theatrical Flashes. Available online: https://www.lemaitreltd.com/ products/pyrotechnics/pyroflash/theatrical-flashes-stars/theatrical-flashes/sds-flashes-robotics-spds/ (accessed on 22 November 2020). Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional aliations. © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY-NC-ND) license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Journal

International Journal of Turbomachinery, Propulsion and PowerMultidisciplinary Digital Publishing Institute

Published: Dec 15, 2020

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