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Time resolved 2D concentration and temperature measurement using CT tunable laser absorption spectroscopy
Flow Measurement and Instrumentation 46 (2015) 312–318
Contents lists available at ScienceDirect
Flow Measurement and Instrumentation journal homepage: www.elsevier.com/locate/flowmeasinst
Time resolved 2D concentration and temperature measurement using CT tunable laser absorption spectroscopy Y. Deguchi n, T. Kamimoto, Y. Kiyota Graduate school of Advanced Technology and Science, The University of Tokushima, Tokushima, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 1 December 2014 Received in revised form 21 May 2015 Accepted 22 June 2015 Available online 24 June 2015
In this study, the noncontact and simultaneous 2D temperature and concentration measurement method has been developed to elucidate the reaction characteristics and improve the relevant simulation code. The technique is based on a CT method using absorption spectra of molecules such as H2O, NH3 and CH4. The CT Tunable diode laser absorption spectroscopy (TDLAS) method using 16-path laser beams was applied to measure 2D temperature, NH3 and CH4 distributions in engine exhausts and oscillating flames. Simultaneous and time resolved 2D temperature, NH3 and CH4 distributions were successfully reconstructed using a set of 16-path absorption spectra. Since CT TDLAS has a potential of kHz response time, this method enables real-time 2D temperature and species concentration measurements in various industrial processes including engine applications. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Measurement and instrumentation 2D temperature and concentration measurement Combustion Engine CT Tunable diode laser absorption spectroscopy
1. Introduction It has been a major challenge to reduce anthropogenic carbon dioxide emissions from engines. Behind the trend is the fact that increased carbon dioxide in the air causes global warming and adversely affects natural ecosystems. Further, the demands for lowering the burdens on the environment will continue to grow steadily. It is thus becoming important to understand the emission characteristics to minimize environmental disruption and improve the efficiency of combustors. Considering the above situation, it is important to monitor controlling factors of combustors to improve the efficiency and exhaust gas treatment processes. In particular, measurement techniques for the parameters such as temperature and species concentration are necessary to elucidate the overall nature of combustion systems. Especially, 2D temperature and concentration distributions play an important role for the combustion structure and the combustor efficiency in engines, burners, gas turbines and so on. In engines, exhaust gas concentration and temperature distribution is an important factor in NOx, THC and PM emissions. It is also a catalytically important parameter in both gasoline and diesel engines. Especially real-time 2D concentration and temperature distribution plays an important role for the catalytic efficiency. In gas turbine combustors, the combustion oscillation is one of the major problems. The combustion n
Corresponding author. Fax: þ81 88 656 9082. E-mail address: [email protected] (Y. Deguchi).
http://dx.doi.org/10.1016/j.flowmeasinst.2015.06.025 0955-5986/& 2015 Elsevier Ltd. All rights reserved.
oscillation of gas turbines has many complex causes such as pressure fluctuations, combustion instabilities, and mechanical designs of the combustion chamber. Elimination of combustion oscillations is often time-consuming and costly because there is no single approach to solve this problem. The complexity and nonlinearity of this phenomenon occur partly due to the complicated interaction between flame chemistries, turbulent flow structures and acoustic modes of combustors. Recently, as a measurement technique with high sensitivity and fast response, laser diagnosis has been developed and applied to the actual industrial fields [1]. Tunable diode laser absorption spectroscopy (TDLAS) has been developed for applications [1–7]. With these engineering developments, transient phenomena such as start-ups and load changes in engines have been gradually elucidated in various conditions. In this study, the theoretical and experimental research has been conducted to develop the noncontact and fast response 2D temperature and concentration distribution measurement method. The technique is based on a CT method [8–19] using absorption spectra of molecules such as H2O, NH3 and CH4. The CT-TDLAS method using 16-path laser beams was applied to measure 2D temperature NH3 and CH4 distributions in engine exhausts and oscillating flames. NH3 is an important molecule for a urea SCR (Selective Catalytic Reduction) system and the 2D detection of NH3 in this system becomes a key technique for the improvement of its performance. The 2D distribution of CH4, which is the major fuel of gas turbine combustors, is a key factor to elucidate the combustion oscillation phenomena
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of gas turbine combustors. 2D temperature, NH3 and CH4 distributions were successfully reconstructed using a set of 16-path absorption spectra. Since CT-TDLAS has a potential of kHz response time, this method enables real-time 2D temperature and species concentration measurements in various industrial processes including engine applications.
2. Theory Gas temperature and species concentration can be determined by measuring molecular absorbance at multiple wavelengths. TDLAS was used in this research. It is possible to continuously scan laser wavelengths and measure absorption spectra. The principle of TDLAS is based on Lambert Beer's law. When light permeates an absorption medium, the strength of the permeated light is related to absorber concentration according to Lambert Beer's law. TDLAS uses this basic law to measure temperature and species concentration. The number density of the measured species n is related to the amount of light absorbed as in the following formula [1,10,13]:
Iλ/Iλ0 = exp { − A λ }
⎧ ⎪ = exp ⎨ − ⎪ ⎩
⎞⎫ ⎪
⎛
∑ ⎜⎜ni L ∑ Si, j (T ) GVi, j ⎟⎟ ⎬⎪ i
⎝
j
⎠⎭
(1)
Here, Iλ0 is the incident light intensity, Iλ the transmitted light intensity, Aλ is the absorbance, ni is the number density of species i, L is the path length, Si,j is the temperature dependent absorption line strength of the absorption line j, and Gvi,j is the line broadening function. In this study NH3 and H2O absorption spectra were used to measure NH3 concentration and temperature. NH3 and H2O absorption spectra were shown in Fig. 1. Fig. 1(a)–(c) are a theoretical H2O absorption spectrum calculated using a HITRAN database [20], measured NH3 and CH4 absorption spectra in the condition of 0.1 MPa and room temperature (293 K), respectively. In this study the 1512.22 nm and 1635.34 nm absorption lines of NH3 and CH4 were used for NH3 and CH4 concentration measurements. Three absorption lines located at 1388.135 nm (#1), 1388.326 nm (#2), and 1388.454 nm (#3) were used to measure temperature. It is important to use several absorption lines with different temperature dependence to reduce the temperature error induced by a CT algorism. Absorption of transmitted light through absorption medium occurs on the optical path. The absorption signal strength becomes an integrated value of the optical path. In this study, several optical paths are intersected to each other to form the analysis grids, reconstructing the 2D temperature distribution by a CT method [8,9,13,15]. Concept of analysis grids and laser beam paths is shown in Fig. 2. The integrated absorbance in the path p is given by
A λ, p =
∑ n q L p, q α λ, q
(2)
q
Because the integrated absorbance is dependent on both temperature and concentration, the temperature distribution has to be calculated by more than two different absorbance values. Temperature and H2O concentration at each analysis grid were determined using a multi-function minimization method to minimize the spectral fitting error at 1388.0–1388.6 nm.
Error =
Fig. 1. H2O, NH3 and CH4 absorption spectra. (a)H2O absorption spectrum, (b) NH3 absorption spectrum, and (c) CH4 absorption spectrum.
2
∑ { (A λ, q )theory − (A λ, q )experiment }
(3)
A set of measured H2O absorption spectra was compared to the theoretical spectra [20] to measure temperature. NH3 and CH4 concentrations were also determined using the same algorism.
Fig. 2. CT grid and laser path.
Sets of temperature, NH3 and CH4 distributions at analysis grids were determined separately by each minimization procedure shown in Fig. 3. The reconstruction of temperature and concentrations had separate minimization loops and the set of temperature and concentrations were determined to minimize the total error shown in Eq. (3). This error was evaluated by the spectra fitting method and a polynomial noise reduction technique [5] was also used to reduce noises such as the effect of laser beam steering. The spectra fitting and polynomial noise reduction
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Fig. 3. CT algorithm.
technique have played an important role to achieve a stable and accurate CT reconstruction procedure.
3. Experimental setup Fig. 4 shows the outline of a CT-TDLAS measurement system used in this study. DFB lasers at 1388 nm (NTT Electronics Co., NLK1E5GAAA), 1512 nm (NTT Electronics Co., NLK1S5GAAA), and 1635 nm (NTT Electronics Co., NLK1U5EAAA) were used to measure water vapor (temperature), NH3 and CH4 concentration, respectively. The laser wavelength was scanned at 1–4 kHz and absorption spectra were measured simultaneously to calculate the instant 2D concentration and temperature using 16 path measurement cell as shown in Fig. 4(a). Laser beams were modulated with opposite input current modulations as shown in Fig. 4(a). These beams were combined by a beam combiner to make a less intensity-modulated laser beam and then separated by an optical fibre splitter (OPNETI CO., SMF-28e SWBC 1 16) to 16 laser beams. The separated laser beams were irradiated into the target area by 16 collimators (THORLABS Co., 50-1310-APC). The transmitted light intensities were detected by photodiodes (Hamamatsu Photonics and G8370-01), and taken into a recorder (HIOKI E.E. Co., 8861 Memory Highcoda HD Analog16). The experiment was performed using three types of experimental setups. One was an experiment system for the accuracy evaluation of 2D concentration measured by CT-TDLAS. The experimental setup is shown in Fig. 4(b). A jet flow of 1% CH4 with a buffer gas of N2 was introduced into the 16 path CT-TDLAS measurement call at the flow rate of 1.7 10–5 m3/s. The inner diameter of the jet pipe was 8 mm and the N2 guard flow from an outer pipe with the inner diameter of 65 mm was formed at the flow rate of 3.3 10–4 m3/s. The CH4 concentration distribution at 3 mm above the CH4 jet pipe was measured by CT-TDLAS. The CH4 concentration distribution at Y ¼0 mm was also measured by sampling the gas and measuring the CH4 concentration by TDLAS. The experiment was also performed using gasoline engine (FUJI HEAVY INDUSTRIES, Inc., EX13, 126 cc) as shown in Fig. 4(c). The laser paths were set at the outlet of the engine exhaust pipes. The diameter of 16 path measurement cell was 70 mm. The exhaust pipe length was 160 mm and it had an inner pipe with diameter of 8 mm to input NH3 into the exhausts. The measurement cell was set at 10 mm above the NH3 exhaust nozzle and the 2% NH3 standard gas was introduced at the flow rate of 1.0 10–3 m3/s. The temperature of engine exhaust gas was 400–450 K and the input NH3 gas was heated at this temperature. The input of NH3 was controlled on and off manually using a valve shown in Fig. 4 (c).
Fig. 4. Experimental apparatus for 2D temperature and concentration measurements. (a) 16 path CT-TDLAS measurement cell, (b) accuracy evaluation of 2D CH4 concentration, (c) time resolved 2D NH3 concentration measurement apparatus of NH3 in engine exhausts, and (d) time resolved 2D CH4 concentration measurement in an oscillating CH4-Air flame.
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The other experiment system was aimed to measure time resolved 2D CH4 concentrations in an oscillating CH4-air Bunsentype flame using CT-TDLAS. The experimental setup is shown in Fig. 4(d). The flow rate of CH4 was oscillated at 50 Hz using a direct drive servo valve (Moog Inc., D633). The air and average CH4 flow rates were 6.0 10–5 m3/s and 9.0 10–6 m3/s, respectively. The inner diameter of the burner was 8 mm and the air guard flow from an outer pipe with the inner diameter of 65 mm was formed at the flow rate of 3.3 10–4 m3/s. A stainless steel mesh was placed at 10 mm above the burner to simulate a lifted flame and CH4 concentration distribution at 3 mm above the burner was measured by CT-TDLAS. The oscillating flame was also measured by a CCD camera (Panasonic, HX-WA30K) at 480 frames per second (fps) to confirm the flame oscillation characteristics.
4. Results and discussion Linearity and temperature dependence of NH3 and CH4 absorbance and absorption intensity are shown in Fig. 5. The NH3 and CH4 absorbances with the 220 mm path length at room temperature show a good linearity as shown in Fig. 5(a) and (b). Fig. 5 (c) shows the temperature dependence of NH3 absorption intensity. The NH3 absorption intensity gradually decreases as temperature increases because of the decrease of the NH3 number density at the lower energy level of its absorption line. This information is used for correction of temperature to measure NH3 concentration. Fig. 6 shows CH4 concentration measurement results by CT TDLAS and the gas sampling method shown in Fig. 4(a). Measured CH4 concentration distributions show almost the same structure between the two methods. The high concentration of CH4 was measured at X ¼Y¼ 0 mm and CH4 was distributed from X¼ 6 mm to 6 mm. Because of the limited path number of CT TDLAS, the sharp edge of CH4 concentration change was not measured by CT TDLAS at X ¼ 4 mm and 4 mm, because of the achievable spatial resolution using the 16 path CT-TDLAS measurement cell (3–4 mm at the center of the CT-TDLAS measurement cell). The spatial resolution of CT-TDLAS was determined by the number of laser path, the number of laser path angles, the laser path layout, and the CT algorism[8–10,13,15,16]. The spatial resolution can be modified mainly by increasing the number and angles of laser paths. 2D NH3 and temperature were measured by CT TDLAS using the 16 path measurement cell shown in Fig. 3. In this experiment the engine was operated at 1200–1300 rpm and 2% NH3 was injected into the engine exhaust. Fig. 7 shows the time history of NH3 absorption intensity of the laser path 3 (see Fig. 3). The NH3 injection was started at 0.5 s and it was shortly stopped at 6 s. Because of the fluctuation of the engine rotating speed, the absorption intensity fluctuated 10–20% during 2–5 s and 7–10 s. The on and off speed of NH3 input was about 0.5 s because of the valve on and off speed (manual) and the length of the NH3 pipe. Fig. 8 shows the absorption spectra of the laser path 3 and 4 measured at 4 s. Because the two spectra with different wavelengths (1388 nm and 1527 nm) were mixed into a single laser scan, Fig. 8 has a abscissa axis of the laser scanning time. The NH3 and H2O absorption spectra were simultaneously detected by combining the laser outputs of 1388 nm and 1527 nm. The noise level was quite low compared to the absorption signals because of large absorption line strengths for both NH3 and H2O. The laser path 3 was located above the inner pipe of NH3 injection and both NH3 and H2O absorption lines were recognized at the different time from the trigger of laser scanning. In the case of the laser path 4 the NH3 absorption intensity became lower because this path was located
Fig. 5. Linearity and temperature dependence of NH3 and CH4 absorption intensity. (a) Linearity measurement result of NH3, (b) linearity measurement result of CH4, and (c) temperature dependence of NH3.
apart from the NH3 injection pipe. The NH3 absorption line was used for the NH3 concentration measurement and the H2O absorption lines for temperature. Fig. 9 shows 2D temperature and NH3 distribution measured by CT-TDLAS. 2D temperature and NH3 concentration were successfully reconstructed by this CT method. The accuracy of the reconstructed results depends on the number of laser path (special resolution) and the accuracy of the absorption database. Using the 16 path measurement cell used in this study, the special resolution becomes 3-4 mm depending on the measurement position. Fig. 10 shows the time history of 2D NH3 concentration. Since 2% NH3 standard gas was mixed into the exhaust gas during t¼ 0.5–10 s and it was shortly stopped at t¼6 s, NH3 was not detected at t ¼0 s and 6 s. This method successfully detected the 2D NH3 concentration change around t¼6 s. This method can reach kHz response time depending on applications [3]. It was demonstrated from these results that the NH3 distribution has been successfully reconstructed and the fast 2D concentration measurement becomes possible using the newly developed CT-TDLAS method. Fig. 11 shows the time-history of 2D CH4 concentration
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Fig. 6. CH4 measurement results. (a) Sampling result (Y ¼0 mm), and (b) CT-TDLAS result (Y ¼0 mm).
Fig. 8. Absorption spectra measured at 4 s. (a) Laser path 3, and (b) Laser path 4.
Fig. 7. Time history of NH3 absorption intensity of laser path 3.
distribution in oscillating flames measured by CT-TDLAS. The high concentration of CH4 was measured at X ¼Y¼0 mm and CH4 was distributed from X ¼ 6 mm to 6 mm. The 2D CH4 concentration was successfully reconstructed by the set of 16 path absorbance. The precise structure of 50 Hz oscillation of 2D CH4 concentration distribution was also detected showing the agreement with the 50 Hz frequency of CH4 concentration oscillated by the direct drive servo-valve. The spectra fitting and polynomial noise reduction technique were important to achieve stable and accurate 2D concentration and temperature distributions. The newly developed laser beam combination method also made it possible to achieve both the simultaneous temperature and concentration measurement and improvement of measurement accuracy. Fig. 9. 2D temperature and NH3 distribution measured by CT-TDLAS.
5. Conclusions The 2D temperature measurement method using CT-TDLAS was developed and successfully demonstrated to measure 2D NH3 and CH4 concentration and temperature distributions in engine exhausts and oscillating flames using 16 path measurement cells. The measurement technology was developed to detect NH3 and
H2O absorption lines simultaneously for NH3 concentration and temperature measurements. Time-resolved 2D NH3 and CH4 concentration measurements using CT-TDLAS were achieved and the results show that it has a good time resolution to measure continuous 2D concentration and temperature information in the practical fields. CT-TDLAS has a potential of kHz response time and
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Fig. 10. Time history of NH3 concentration between 0–7 s with the time resolution 1 s. (a) 0 s, (b) 1 s; (c) 2 s, (d) 3 s; (e) 4 s, (f) 5 s; (g) 6 s, and (h) 7 s.
the method enables the real-time 2D species concentration and temperature measurements in various fields.
References [1] Y. Deguchi, Industrial Applications of Laser Diagnostics, CRS Press: Taylor & Francis, Boca Raton, FL, 2011. [2] Y. Zaatar, J. Bechara, A. Khoury, D. Zaouk, J.-P. Charles, Diode laser sensor for process control and environmental monitoring, Appl. Energy 65 (2000) 107–113. [3] Y. Deguchi, M. Noda, Y. Fukuda, Y. Ichinose, Y. Endo, M. Inada, Y. Abe, S. Iwasaki, Industrial applications of temperature and species concentration monitoring using laser diagnostics, Meas. Sci. Technol. 13 (10) (2002) R103–R115. [4] X. Liu, J.B. Jeffries, R.K. Hanson, K.M. Hinckley, M.A. Woodmansee,
Fig. 11. Time-history of 2D CH4 concentrations distribution in 50 Hz oscillating flame. (a) 0 s, (b) 2.5 s; (c) 5 s, (d) 7.5 s; (e) 10 s, (f) 12.5 s; (g) 15 s, (h) 17.5 s; and (i) 20 s.
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Development of a tunable diode laser sensor for measurements of gas turbine exhaust temperature, Appl. Phys. B 82 (3) (2006) 469–478. M. Yamakage, K. Muta, Y. Deguchi, S. Fukada, T. Iwase, T. Yoshida, Development of direct and fast response exhaust gas measurement, SAE Paper, 2008, 20081298. H. Sumizawa, H. Yamada, K. Tonokura, Real-time monitoring of nitric oxide in diesel exhaust gas by mid-infrared cavity ring-down spectroscopy, Appl. Phys. B 100 (4) (2010) 925–931. V.L. Kasyutich, R.J. Holdsworth, P.A. Martin, In situ vehicle engine exhaust measurements of nitric oxide with a thermoelectrically cooled, cw DFB quantum cascade laser, J. Phys. Conf. Ser. 157 (2009) 012006. P. Wright, N. Terzijaa, J.L. Davidsona, S. Garcia-Castillo, C. Garcia-Stewart, S. Pegrumb, S. Colbourneb, P. Turnerb, S.D. Crossleyc, T. Litt, S. Murrayc, K. B. Ozanyana, H. McCann, High-speed chemical species tomography in a multicylinder automotive engine, Chem. Eng. J. 158 (1) (2010) 2–10. L. Ma, W. Cai, Numerical investigation of hyperspectral tomography for simultaneous temperature and concentration imaging, Appl. Opt. 47 (21) (2008) 3751–3759. F. Wang, K.F. Cen, N. Li, J.B. Jeffries, Q.X. Huang, J.H. Yan, Y. Chi, Two-dimensional tomography for gas concentration and temperature distributions based on tunable diode laser absorption spectroscopy, Meas. Sci. Technol. (2010) 21. Y. Deguchi, D. Yasui, A. Adachi, Development of 2D temperature and concentration measurement method using tunable diode laser absorption spectroscopy, J. Mech. Eng. Autom. 2 (9) (2012) 543–549. Y. Deguchi, T. Kamimoto, Y. Kiyota, D. Choi, J. Shim, Real-time 2D concentration and temperature measurement method using CT tunable diode laser absorption spectroscopy, in: Proceedings of WCIPT7 2013, 2013, pp. O4-4. Y. Deguchi, T. Kamimoto, Z.Z. Wang, J.J. Yan, J.P. Liu, H. Watanabe, R. Kurose, Applications of laser diagnostics to thermal power plants and engines, Appl. Therm. Eng. 73 (2) (2014) 1453–1464. G. Morthier, P. Vankwikelberge, Handbook of Distributed Feedback Laser
Diodes, Artech House Applied Photonics, 2013. [15] W. Cai, C.F. Kaminski, Tomographic technique for the simultaneous imaging of temperature, chemical species, and pressure in reactive flows using absorption spectroscopy with frequency agile lasers, Appl. Phys. Lett. 104 (2014) 034101/ 1–034101/5. [16] X. An, T. Kraetschmer, K. Takami, S.T. Sanders, L. Ma, W. Cai, X. Li, S. Roy, J. R. Gord, Validation of temperature imaging by H2O absorption spectroscopy using hyperspectral tomography in controlled experiments, Appl. Opt. 50 (4) (2011) A29–A37. [17] L. Ma, X. Li, S.T. Sanders, A.W. Caswell, S. Roy, D.H. Plemmons, J.R. Gord, 50kHz-rate 2D imaging of temperature and H2O concentration at the exhaust plane of a J85 engine using hyperspectral tomography, Opt. Express 21 (1) (2013) 1152–1162. [18] S. Pal, H. McCann, Auto-digital gain balancing: a new detection scheme for high-speed chemical species tomography of minor constituents, Meas. Sci. Technol. 22 (11) (2011) 115304/1–115304/13. [19] S.A. Tsekenis, N. Tait, H. McCann, Spatially resolved and observer-free experimental quantification of spatial resolution in tomographic images, Rev. Sci. Instrum. 86 (3) (2015) 035104/1–035104/17. [20] L.S. Rothman, I.E. Gordon, A. Barbe, D. ChrisBenner, P.F. Bernath, M. Birk, V. Boudon, L.R. Brown, A. Campargue, J.P. Champion, K. Chance, L.H. Coudert, V. Dana, V.M. Devi, S. Fally, J.M. Flaud, R.R. Gamache, A. Goldman, D. Jacquemart, I. Kleiner, N. Lacome, W.J. Lafferty, J.-Y. Mandin, S.T. Massie, S. N. Mikhailenko, C.E. Miller, N. Moazzen-Ahmadi, O.-V. Naumenko, A.V. Nikitin, J. Orphal, V.I. Perevalov, A. Perrin, A. Predoi-Cross, C.P. Rinsland, M. Rotger, M. Simeckova, M.A.H. Smith, K. Sung, S.A. Tashkun, J. Tennyson, R.A. Toth, A. C. Vandaele, J. VanderAuwera, The HITRAN2008 molecular spectroscopic database, J. Quant. Spectrosc. Radiat. Transf. 110 (2009) 533–572.
Contents lists available at ScienceDirect
Flow Measurement and Instrumentation journal homepage: www.elsevier.com/locate/flowmeasinst
Time resolved 2D concentration and temperature measurement using CT tunable laser absorption spectroscopy Y. Deguchi n, T. Kamimoto, Y. Kiyota Graduate school of Advanced Technology and Science, The University of Tokushima, Tokushima, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 1 December 2014 Received in revised form 21 May 2015 Accepted 22 June 2015 Available online 24 June 2015
In this study, the noncontact and simultaneous 2D temperature and concentration measurement method has been developed to elucidate the reaction characteristics and improve the relevant simulation code. The technique is based on a CT method using absorption spectra of molecules such as H2O, NH3 and CH4. The CT Tunable diode laser absorption spectroscopy (TDLAS) method using 16-path laser beams was applied to measure 2D temperature, NH3 and CH4 distributions in engine exhausts and oscillating flames. Simultaneous and time resolved 2D temperature, NH3 and CH4 distributions were successfully reconstructed using a set of 16-path absorption spectra. Since CT TDLAS has a potential of kHz response time, this method enables real-time 2D temperature and species concentration measurements in various industrial processes including engine applications. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Measurement and instrumentation 2D temperature and concentration measurement Combustion Engine CT Tunable diode laser absorption spectroscopy
1. Introduction It has been a major challenge to reduce anthropogenic carbon dioxide emissions from engines. Behind the trend is the fact that increased carbon dioxide in the air causes global warming and adversely affects natural ecosystems. Further, the demands for lowering the burdens on the environment will continue to grow steadily. It is thus becoming important to understand the emission characteristics to minimize environmental disruption and improve the efficiency of combustors. Considering the above situation, it is important to monitor controlling factors of combustors to improve the efficiency and exhaust gas treatment processes. In particular, measurement techniques for the parameters such as temperature and species concentration are necessary to elucidate the overall nature of combustion systems. Especially, 2D temperature and concentration distributions play an important role for the combustion structure and the combustor efficiency in engines, burners, gas turbines and so on. In engines, exhaust gas concentration and temperature distribution is an important factor in NOx, THC and PM emissions. It is also a catalytically important parameter in both gasoline and diesel engines. Especially real-time 2D concentration and temperature distribution plays an important role for the catalytic efficiency. In gas turbine combustors, the combustion oscillation is one of the major problems. The combustion n
Corresponding author. Fax: þ81 88 656 9082. E-mail address: [email protected] (Y. Deguchi).
http://dx.doi.org/10.1016/j.flowmeasinst.2015.06.025 0955-5986/& 2015 Elsevier Ltd. All rights reserved.
oscillation of gas turbines has many complex causes such as pressure fluctuations, combustion instabilities, and mechanical designs of the combustion chamber. Elimination of combustion oscillations is often time-consuming and costly because there is no single approach to solve this problem. The complexity and nonlinearity of this phenomenon occur partly due to the complicated interaction between flame chemistries, turbulent flow structures and acoustic modes of combustors. Recently, as a measurement technique with high sensitivity and fast response, laser diagnosis has been developed and applied to the actual industrial fields [1]. Tunable diode laser absorption spectroscopy (TDLAS) has been developed for applications [1–7]. With these engineering developments, transient phenomena such as start-ups and load changes in engines have been gradually elucidated in various conditions. In this study, the theoretical and experimental research has been conducted to develop the noncontact and fast response 2D temperature and concentration distribution measurement method. The technique is based on a CT method [8–19] using absorption spectra of molecules such as H2O, NH3 and CH4. The CT-TDLAS method using 16-path laser beams was applied to measure 2D temperature NH3 and CH4 distributions in engine exhausts and oscillating flames. NH3 is an important molecule for a urea SCR (Selective Catalytic Reduction) system and the 2D detection of NH3 in this system becomes a key technique for the improvement of its performance. The 2D distribution of CH4, which is the major fuel of gas turbine combustors, is a key factor to elucidate the combustion oscillation phenomena
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of gas turbine combustors. 2D temperature, NH3 and CH4 distributions were successfully reconstructed using a set of 16-path absorption spectra. Since CT-TDLAS has a potential of kHz response time, this method enables real-time 2D temperature and species concentration measurements in various industrial processes including engine applications.
2. Theory Gas temperature and species concentration can be determined by measuring molecular absorbance at multiple wavelengths. TDLAS was used in this research. It is possible to continuously scan laser wavelengths and measure absorption spectra. The principle of TDLAS is based on Lambert Beer's law. When light permeates an absorption medium, the strength of the permeated light is related to absorber concentration according to Lambert Beer's law. TDLAS uses this basic law to measure temperature and species concentration. The number density of the measured species n is related to the amount of light absorbed as in the following formula [1,10,13]:
Iλ/Iλ0 = exp { − A λ }
⎧ ⎪ = exp ⎨ − ⎪ ⎩
⎞⎫ ⎪
⎛
∑ ⎜⎜ni L ∑ Si, j (T ) GVi, j ⎟⎟ ⎬⎪ i
⎝
j
⎠⎭
(1)
Here, Iλ0 is the incident light intensity, Iλ the transmitted light intensity, Aλ is the absorbance, ni is the number density of species i, L is the path length, Si,j is the temperature dependent absorption line strength of the absorption line j, and Gvi,j is the line broadening function. In this study NH3 and H2O absorption spectra were used to measure NH3 concentration and temperature. NH3 and H2O absorption spectra were shown in Fig. 1. Fig. 1(a)–(c) are a theoretical H2O absorption spectrum calculated using a HITRAN database [20], measured NH3 and CH4 absorption spectra in the condition of 0.1 MPa and room temperature (293 K), respectively. In this study the 1512.22 nm and 1635.34 nm absorption lines of NH3 and CH4 were used for NH3 and CH4 concentration measurements. Three absorption lines located at 1388.135 nm (#1), 1388.326 nm (#2), and 1388.454 nm (#3) were used to measure temperature. It is important to use several absorption lines with different temperature dependence to reduce the temperature error induced by a CT algorism. Absorption of transmitted light through absorption medium occurs on the optical path. The absorption signal strength becomes an integrated value of the optical path. In this study, several optical paths are intersected to each other to form the analysis grids, reconstructing the 2D temperature distribution by a CT method [8,9,13,15]. Concept of analysis grids and laser beam paths is shown in Fig. 2. The integrated absorbance in the path p is given by
A λ, p =
∑ n q L p, q α λ, q
(2)
q
Because the integrated absorbance is dependent on both temperature and concentration, the temperature distribution has to be calculated by more than two different absorbance values. Temperature and H2O concentration at each analysis grid were determined using a multi-function minimization method to minimize the spectral fitting error at 1388.0–1388.6 nm.
Error =
Fig. 1. H2O, NH3 and CH4 absorption spectra. (a)H2O absorption spectrum, (b) NH3 absorption spectrum, and (c) CH4 absorption spectrum.
2
∑ { (A λ, q )theory − (A λ, q )experiment }
(3)
A set of measured H2O absorption spectra was compared to the theoretical spectra [20] to measure temperature. NH3 and CH4 concentrations were also determined using the same algorism.
Fig. 2. CT grid and laser path.
Sets of temperature, NH3 and CH4 distributions at analysis grids were determined separately by each minimization procedure shown in Fig. 3. The reconstruction of temperature and concentrations had separate minimization loops and the set of temperature and concentrations were determined to minimize the total error shown in Eq. (3). This error was evaluated by the spectra fitting method and a polynomial noise reduction technique [5] was also used to reduce noises such as the effect of laser beam steering. The spectra fitting and polynomial noise reduction
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Fig. 3. CT algorithm.
technique have played an important role to achieve a stable and accurate CT reconstruction procedure.
3. Experimental setup Fig. 4 shows the outline of a CT-TDLAS measurement system used in this study. DFB lasers at 1388 nm (NTT Electronics Co., NLK1E5GAAA), 1512 nm (NTT Electronics Co., NLK1S5GAAA), and 1635 nm (NTT Electronics Co., NLK1U5EAAA) were used to measure water vapor (temperature), NH3 and CH4 concentration, respectively. The laser wavelength was scanned at 1–4 kHz and absorption spectra were measured simultaneously to calculate the instant 2D concentration and temperature using 16 path measurement cell as shown in Fig. 4(a). Laser beams were modulated with opposite input current modulations as shown in Fig. 4(a). These beams were combined by a beam combiner to make a less intensity-modulated laser beam and then separated by an optical fibre splitter (OPNETI CO., SMF-28e SWBC 1 16) to 16 laser beams. The separated laser beams were irradiated into the target area by 16 collimators (THORLABS Co., 50-1310-APC). The transmitted light intensities were detected by photodiodes (Hamamatsu Photonics and G8370-01), and taken into a recorder (HIOKI E.E. Co., 8861 Memory Highcoda HD Analog16). The experiment was performed using three types of experimental setups. One was an experiment system for the accuracy evaluation of 2D concentration measured by CT-TDLAS. The experimental setup is shown in Fig. 4(b). A jet flow of 1% CH4 with a buffer gas of N2 was introduced into the 16 path CT-TDLAS measurement call at the flow rate of 1.7 10–5 m3/s. The inner diameter of the jet pipe was 8 mm and the N2 guard flow from an outer pipe with the inner diameter of 65 mm was formed at the flow rate of 3.3 10–4 m3/s. The CH4 concentration distribution at 3 mm above the CH4 jet pipe was measured by CT-TDLAS. The CH4 concentration distribution at Y ¼0 mm was also measured by sampling the gas and measuring the CH4 concentration by TDLAS. The experiment was also performed using gasoline engine (FUJI HEAVY INDUSTRIES, Inc., EX13, 126 cc) as shown in Fig. 4(c). The laser paths were set at the outlet of the engine exhaust pipes. The diameter of 16 path measurement cell was 70 mm. The exhaust pipe length was 160 mm and it had an inner pipe with diameter of 8 mm to input NH3 into the exhausts. The measurement cell was set at 10 mm above the NH3 exhaust nozzle and the 2% NH3 standard gas was introduced at the flow rate of 1.0 10–3 m3/s. The temperature of engine exhaust gas was 400–450 K and the input NH3 gas was heated at this temperature. The input of NH3 was controlled on and off manually using a valve shown in Fig. 4 (c).
Fig. 4. Experimental apparatus for 2D temperature and concentration measurements. (a) 16 path CT-TDLAS measurement cell, (b) accuracy evaluation of 2D CH4 concentration, (c) time resolved 2D NH3 concentration measurement apparatus of NH3 in engine exhausts, and (d) time resolved 2D CH4 concentration measurement in an oscillating CH4-Air flame.
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The other experiment system was aimed to measure time resolved 2D CH4 concentrations in an oscillating CH4-air Bunsentype flame using CT-TDLAS. The experimental setup is shown in Fig. 4(d). The flow rate of CH4 was oscillated at 50 Hz using a direct drive servo valve (Moog Inc., D633). The air and average CH4 flow rates were 6.0 10–5 m3/s and 9.0 10–6 m3/s, respectively. The inner diameter of the burner was 8 mm and the air guard flow from an outer pipe with the inner diameter of 65 mm was formed at the flow rate of 3.3 10–4 m3/s. A stainless steel mesh was placed at 10 mm above the burner to simulate a lifted flame and CH4 concentration distribution at 3 mm above the burner was measured by CT-TDLAS. The oscillating flame was also measured by a CCD camera (Panasonic, HX-WA30K) at 480 frames per second (fps) to confirm the flame oscillation characteristics.
4. Results and discussion Linearity and temperature dependence of NH3 and CH4 absorbance and absorption intensity are shown in Fig. 5. The NH3 and CH4 absorbances with the 220 mm path length at room temperature show a good linearity as shown in Fig. 5(a) and (b). Fig. 5 (c) shows the temperature dependence of NH3 absorption intensity. The NH3 absorption intensity gradually decreases as temperature increases because of the decrease of the NH3 number density at the lower energy level of its absorption line. This information is used for correction of temperature to measure NH3 concentration. Fig. 6 shows CH4 concentration measurement results by CT TDLAS and the gas sampling method shown in Fig. 4(a). Measured CH4 concentration distributions show almost the same structure between the two methods. The high concentration of CH4 was measured at X ¼Y¼ 0 mm and CH4 was distributed from X¼ 6 mm to 6 mm. Because of the limited path number of CT TDLAS, the sharp edge of CH4 concentration change was not measured by CT TDLAS at X ¼ 4 mm and 4 mm, because of the achievable spatial resolution using the 16 path CT-TDLAS measurement cell (3–4 mm at the center of the CT-TDLAS measurement cell). The spatial resolution of CT-TDLAS was determined by the number of laser path, the number of laser path angles, the laser path layout, and the CT algorism[8–10,13,15,16]. The spatial resolution can be modified mainly by increasing the number and angles of laser paths. 2D NH3 and temperature were measured by CT TDLAS using the 16 path measurement cell shown in Fig. 3. In this experiment the engine was operated at 1200–1300 rpm and 2% NH3 was injected into the engine exhaust. Fig. 7 shows the time history of NH3 absorption intensity of the laser path 3 (see Fig. 3). The NH3 injection was started at 0.5 s and it was shortly stopped at 6 s. Because of the fluctuation of the engine rotating speed, the absorption intensity fluctuated 10–20% during 2–5 s and 7–10 s. The on and off speed of NH3 input was about 0.5 s because of the valve on and off speed (manual) and the length of the NH3 pipe. Fig. 8 shows the absorption spectra of the laser path 3 and 4 measured at 4 s. Because the two spectra with different wavelengths (1388 nm and 1527 nm) were mixed into a single laser scan, Fig. 8 has a abscissa axis of the laser scanning time. The NH3 and H2O absorption spectra were simultaneously detected by combining the laser outputs of 1388 nm and 1527 nm. The noise level was quite low compared to the absorption signals because of large absorption line strengths for both NH3 and H2O. The laser path 3 was located above the inner pipe of NH3 injection and both NH3 and H2O absorption lines were recognized at the different time from the trigger of laser scanning. In the case of the laser path 4 the NH3 absorption intensity became lower because this path was located
Fig. 5. Linearity and temperature dependence of NH3 and CH4 absorption intensity. (a) Linearity measurement result of NH3, (b) linearity measurement result of CH4, and (c) temperature dependence of NH3.
apart from the NH3 injection pipe. The NH3 absorption line was used for the NH3 concentration measurement and the H2O absorption lines for temperature. Fig. 9 shows 2D temperature and NH3 distribution measured by CT-TDLAS. 2D temperature and NH3 concentration were successfully reconstructed by this CT method. The accuracy of the reconstructed results depends on the number of laser path (special resolution) and the accuracy of the absorption database. Using the 16 path measurement cell used in this study, the special resolution becomes 3-4 mm depending on the measurement position. Fig. 10 shows the time history of 2D NH3 concentration. Since 2% NH3 standard gas was mixed into the exhaust gas during t¼ 0.5–10 s and it was shortly stopped at t¼6 s, NH3 was not detected at t ¼0 s and 6 s. This method successfully detected the 2D NH3 concentration change around t¼6 s. This method can reach kHz response time depending on applications [3]. It was demonstrated from these results that the NH3 distribution has been successfully reconstructed and the fast 2D concentration measurement becomes possible using the newly developed CT-TDLAS method. Fig. 11 shows the time-history of 2D CH4 concentration
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Fig. 6. CH4 measurement results. (a) Sampling result (Y ¼0 mm), and (b) CT-TDLAS result (Y ¼0 mm).
Fig. 8. Absorption spectra measured at 4 s. (a) Laser path 3, and (b) Laser path 4.
Fig. 7. Time history of NH3 absorption intensity of laser path 3.
distribution in oscillating flames measured by CT-TDLAS. The high concentration of CH4 was measured at X ¼Y¼0 mm and CH4 was distributed from X ¼ 6 mm to 6 mm. The 2D CH4 concentration was successfully reconstructed by the set of 16 path absorbance. The precise structure of 50 Hz oscillation of 2D CH4 concentration distribution was also detected showing the agreement with the 50 Hz frequency of CH4 concentration oscillated by the direct drive servo-valve. The spectra fitting and polynomial noise reduction technique were important to achieve stable and accurate 2D concentration and temperature distributions. The newly developed laser beam combination method also made it possible to achieve both the simultaneous temperature and concentration measurement and improvement of measurement accuracy. Fig. 9. 2D temperature and NH3 distribution measured by CT-TDLAS.
5. Conclusions The 2D temperature measurement method using CT-TDLAS was developed and successfully demonstrated to measure 2D NH3 and CH4 concentration and temperature distributions in engine exhausts and oscillating flames using 16 path measurement cells. The measurement technology was developed to detect NH3 and
H2O absorption lines simultaneously for NH3 concentration and temperature measurements. Time-resolved 2D NH3 and CH4 concentration measurements using CT-TDLAS were achieved and the results show that it has a good time resolution to measure continuous 2D concentration and temperature information in the practical fields. CT-TDLAS has a potential of kHz response time and
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Fig. 10. Time history of NH3 concentration between 0–7 s with the time resolution 1 s. (a) 0 s, (b) 1 s; (c) 2 s, (d) 3 s; (e) 4 s, (f) 5 s; (g) 6 s, and (h) 7 s.
the method enables the real-time 2D species concentration and temperature measurements in various fields.
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