Temporal resolved gas temperature measurement of 20 khz using 2.0 µm Semiconductor Diode Laser

Transcription

1 Temporal resolved gas temperature measurement of 2 khz using 2. µm Semiconductor Diode Laser Yuji IKEDA, Katsuhiko FUKUZATO, Keiichiro OKA and Tsuyoshi NAKAJIMA Department of Mechanical Engineering, Kobe University Rokkodai, Nada, Kobe , JAPAN Abstract Tunable diode laser absorption spectroscopy (TDLAS) measurement of H 2 O gas temperatures was carried out in a heated cell and a turbulent premixed burner at a repetition rate of up to 2 khz. A near 2. µm distributed feedback (DFB) laser operating at room temperature was developed and all optical components were assembled in a box shown in Fig. 1. The absorption lines for CO 2 and H 2 O gas were carefully selected from the HITRAN / HITEMP database, considering its sensitivity. To measure high temperatures over 1 K, the ratio of the H 2 O absorption line strength at cm -1 and cm -1 was used. A heated static cell filled with gas was used to evaluate the apparatus, the measured temperature agreed with the preset gas temperature, and the accuracy was within 5 %. A premixed turbulent burner was selected for actual measurements (Fig. 2). The gas velocity was measured using a particle image velocimetry (PIV) system, which revealed the characteristics of turbulent flows throughout the flame region. The measurement location was chosen carefully, based on this data. The temperature of the gas on the inner cone of a turbulent premixed flame was measured at 2 khz. The time-averaged measured gas temperature was 1351 K and 37 K root mean square (RMS), for which the thermo-couple showed 1336 K and 99 K RMS shown in Fig. 3. The profile of the measured temperature probability density function (PDF) was almost Gaussian. There was good agreement with the thermocouple results for both measurements. It proved the high temporal rate of temperature measurement could be used for real-time combustion monitoring and extended to feedback control. Figure 1 Developed system using 2 µm DFB lasers Temperature(K) Turbulent burner, (r, z)=(7, 56) ave.= K rms=36.7 K Time(ms) PDF(%) Figure 2 Measurement point Figure 3 Time series gas temperature measurement at 2 khz at the inner cone of turbulent premixed methane/air flame - 1 -

2 1. INTRODUCTION Combustion gas measurement has conventionally been done by gas sampling systems[1], which makes it difficult to measure high-speed events (>khz order). This makes gas sampling type systems unsuitable as a real-time sensor for combustor control. And also, laser-based diagnostics, like Rayleigh and CARS [2][3] has been restricted by its size, cost and time resolution. Using diode laser absorption spectroscopy, the measurement of temperature and concentration of multiple gaseous species, with a high temporal resolution has been proven [4-12], because a diode laser is small, robust, and has a high-speed wavelength modulation capability. In addition, a diode laser can easily be interfaced with optical fibers to enable optical access to many types of measurement systems. Hanson et al. [13-24] reported research on a diode laser absorption spectroscopy system using the wavelength of near infrared region. Their research initially focused on H 2 O absorption. Subsequently, other species were measured, including CO [25], CH 4 [26], and NO [27], followed by simultaneous measurement of multiple gaseous molecule species (CO, CO 2, and CH 4 ) [28]. Allen et al. measured H 2 O [29] and NO 2 [3] using a diode laser at 1.3~1.5 µm. In order to evaluate the accuracy of these measurements precisely, Tanahashi et al. [31] performed numerical simulations of diode laser H 2 O absorption spectroscopy using DNS data for a turbulent flame. Aizawa et al. [32] measured OH radicals, Iseki et al. [33] measured CH 4, Tanoura et al. [34] measured NO x, and Torniainen et al. [35] conducted tomographic research with 2-D measurements. In order to achieve a high degree of measurement accuracy, a method using frequency modulation is better, although high temporal resolution cannot be obtained. However, a scanned-area method exceeding khz order has not been reported. The resolution of the transmitted wavelength in an InGaAsP/InP DFB diode laser is.1 nm; it is restricted by the resolution of the temperature controller of a DFB laser, which is.1 K. In other words, it is difficult to fix the wavelength of a diode laser because precise control of temperature is too difficult. To understand turbulent flame characteristics, measurements over khz order of gas species concentration, velocity and temperature are strongly required. Furthermore, time series measurements of these physical parameters are essential to understand local flame front structure in turbulent flames. Gas velocity can be easily measured using laser techniques [36], but it has been difficult to measure gas temperatures at high data rates in a practical burner. Thus, we developed a TDLAS system using a 2. µm DFB laser to measure gas temperatures with high temporal resolution (2 khz) in a combustion system [37]. The measurement accuracy was examined with a static cell, a heated cell and a flat flame burner. The purpose of this study is to make time-series measurements of gas temperature in a practical turbulent burner at a very high repetition rate exceeding 2 khz using the TDLAS system. This paper describes the system developed and the results of measurement of a premixed turbulent flame. 2. PRINCIPLE OF MEASUREMENT Diode laser absorption spectroscopy utilizes the absorption characteristics of light, and follows the Lambert-Beer Law. Webber et al. [38] describes this principle in detail. Several parameters must be considered; we used a DFB type diode laser with a 2. µm absorption wavelength, and a sweep rate of 2 khz. Figure 4 shows a schematic of the layout used for the measurements, which was the same as reported in references [13-3, 38-42]. The optical wavelength was controlled so that the diode laser current could be swept (~15 ma) at a set frequency (2 khz) to cover gas absorption lines. When an absorption line is present, a dip appears in the transmitted intensity signal. The wavelength of an absorption line and the principle of gas temperature measurements are shown in Fig. 4. A second diode laser sweeps other absorption lines, and measures the dips. The ratio of the areas of two dips gives the gas temperature. By sweeping with two lasers, LD1 and LD2, at an optional frequency of 2 khz, it becomes possible to measure transient temperatures, as shown in the figure

3 I o L I ν Lambert Beer Theory T= I ν =exp(-k ν L) I o Wavelength sweep 4 LD1 LD2 Absorption 6 Absorption Absorption A 1 A 2 2 khz Time Time Absorption spectrum (HITRAN) 5 A 1 /A 2 : Temperature 7 Temperature 2 khz resolution Time Fig. 4 Measurement principle 3. SELECTION OF THE ABSORPTION LINE In considering conditions as follows, two lines (58.11 cm -1 and cm -1 ) for H 2 O gas temperature measurement were selected. 1. A strong absorption line was required in the target temperature measurement range. 2. The strength ratio of the two absorption lines had to remain highly sensitive over a wide temperature range. 3. The selected wavelength could not overlap other gas absorption lines, although there was little interference in the baseline. As an advantage of these lines, it is less sensitive at measuring temperatures below 6 K. This means that no calibration for low temperature is required. For instance, if the laser passes through a region at room temperature, the effect of the path length at room temperature is negligible. Since most semiconductor diode lasers used for H 2 O gas measurements have been developed for telecommunications purposes, they use the 1.3 µm band. The ratios of cm -1 /58.11 cm -1 in the 2. µm band and cm -1 / cm -1 in the 1.3 µm band were compared, and the results indicated that there was a high S/N ratio for the 2. µm band. Obviously, the 2. µm band is very sensitive at high temperatures, while the 1.3 µm band is very sensitive at low temperatures, and would require calibration at room temperature. Therefore, the ratio of cm -1 /58.11 cm -1 in the 2. µm band is useful for measuring H 2 O gas temperatures in actual combustion

4 4. EXPERIMENTAL EQUIPMENT We considered the following in order to improve the S/N ratio of the measured signal: In DFB diode laser development, these subjects were considered and developed: longer wavelength relative to the strong absorption line, stabilized temperature control, high beam power, high repetition rate, and triangular signal for fast response. For the detector, these parameters were taken into account: high quantum efficiency, low noise and effective response time. Here, we used a low-noise AC amplifier instead of a DC amplifier in order to prevent drift in time-series measurements. Figure 5 shows a schematic diagram of the experimental package for the TDLAS that we developed. It contains two 2. µm DFB diode lasers (NTT Electronics: K, 18.5 ma, K, 18.5 ma), and all the necessary optics and electronics. This TDLAS can sweep at frequencies of 1 Hz, 1 khz, 1 khz, and 2 khz. In order to increase the temporal resolution and stability of the DFB diode laser, a triangular wave sweep was selected, rather than a ramp wave sweep, at high frequencies. Each frequency can be set for various temporal applications. A heated static gas cell was filled with water vapor at 1 K and an R-type thermocouple was used to evaluate the measured temperature. A turbulent pre-mixed CH 4 /air burner was selected as a real combustion application. Gas velocity and turbulence characteristics were measured using a PIV system [43, 44]. The average velocity of the unburned gas in the center of the burner was ū = 3.18 m/s. The Reynolds number was Re = The equivalence ratio was φ = 1.1. The point of measurement was X = 56 mm from the end of the burner; the radial distance was r = 7 mm from the center. The beam path was 27 mm, using a reflecting mirror. All temperature measurements were made at a high temporal resolution of 2 khz. 1. mm PC 3 mm 2 mm Optics Turbulent burner TDLAS system Fig 5. Developed TDLAS system and experimental setup of turbulent premixed burner, Re=

5 5. RESULTS AND DISCUSSION H 2 O gas absorption lines were examined to evaluate the accuracy of measurements and operation frequency width, as shown in Fig. 6. Here, the measured absorption, reference beam, and etalon signal are shown. The rows in Fig. 6 show the signals for 1 Hz, 1 khz, 1 khz, and 2 khz, respectively. Although it is difficult to fill a heated static gas cell with water vapor at high concentration, a dip sufficient for analysis was recognized. 1Hz 1kHz 1kHz 2kHz Etalon Reference Transmitted λ=1996 nm, FSR=1.9 GHz Fig. 6 Measured absorption lines at different sweep frequencies Temperature(K) Heated static cell, H 2 O, T=1 K ave.=19.6 K rms=18.5 K Time(ms) PDF(%) Figure 7 Time series temperature measurement in a heated static cell at 2 khz 3-5 -

6 Figure 7 shows the temporal results for a temperature of 1 K in the heated cell. The data were plotted separately, and not combined and averaged. The average temperature was K. The RMS error was 18.5 K. This result demonstrates the utility of measurements at a 2 khz repetition rate. Figure 8 shows the instantaneous gas velocity vector measured by PIV and the turbulence characteristics as a velocity gradient. The temperature profile measured by the thermo-couple is also shown in the figure, in which the radiation effect on the thermo-couple is taken into account. The measurement location of this method is not a single point as shown in the figure due to use of a line of sight method. However, the temporal resolution of the gas temperatures gives a very sophisticated picture of the characteristics of a turbulent flame. This TDLAS system requires a long path, which makes the measurement volume mm. Here, the inner cone of the turbulent flame was selected for measurement. Figure 9 shows the temperatures measured in a premixed turbulent burner. Each dot in the figure represents raw data recorded at 2 khz, not an averaged ensemble. The average temperature was K and the RMS was 36.7 K, while the temperature as measured by the thermo-couple was 1336 K and the RMS was 99 K. This result indicates that gas temperatures can be measured at 2 khz in a turbulent flame. Although high temporal resolution of measured gas temperatures was obtained, the spatial resolution was not sufficient to understand the turbulent flame characteristics. Instantaneous direct image and PIV data m/s Cone: (r, z)=(7, 56) d=1 mm ū=3.18 m/s Φ=1.1 Re=9345 x/d (1/s) du dx r (mm) r (mm) 11 Z (mm) (K) Figure 8 Turbulent flow characteristic by PIV and temperature profile by thermocouple - 6 -

7 In this experiment, in order to improve the S/N ratio, the beam path had to be long, at 27 mm. A pair of mirrors was used to increase the length of the beam path. Consequently, the beam had to pass through a region at room-temperature, where it was not in the turbulent flame. As discussed above, no calibration or correction was required, so this method has the advantage of being able to monitor H 2 O gas temperatures in real time. However, ideally the beam path should be as short as possible, since the longer the beam path, the greater the effects on line of sight measurement, such as beam steering effects. If the beam path is too long, the spatial resolution decreases, which makes it difficult to specify the measuring point. Thus, there is a trade-off between beam path length and spatial resolution, which is a disadvantage of line of sight measurement. A longer relative wavelength was selected for these reasons. Other sources of error were beam steering effects and base line matching caused by wing effects. To avoid beam steering effects, which are expected to come from the flame edge, a large lens was selected for detecting light. The angle of beam steering was less than 3 degrees. Base line matching with wing effects is a more complicated process. 6. CONCLUSION The measurements of H 2 O gas temperatures were made at 2 khz using TDLAS techniques by 2. µm semiconductor laser. The ratio of the cm -1 and cm -1 absorption lines around 2. µm was chosen, and it was evident that it has advantages for the application of combustion measurement. The newly developed TDLAS system was examined in a heated static gas cell, gas temperature measurements at 2 khz were examined an actual turbulent premixed burner as well. There was good agreement with the thermocouple results for both measurements. It proved the high temporal rate of temperature measurement could be used for real-time combustion monitoring and extended to feedback control. Temperature(K) Turbulent burner, (r, z)=(7, 56) ave.= K rms=36.7 K Time(ms) 5 1 PDF(%) 15 Figure 9 Time series gas temperature measurement at 2 khz at the inner cone of turbulent premixed methane/air flame - 7 -

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