Effect of Filter Choice on OH* Chemiluminescence Kinetics at Low and Elevated Pressures

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Jayson Albert Summers
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1 7 th US National Technical Meeting of the Combustion Institute Hosted by the Georgia Institute of Technology, Atlanta, GA March 20-23, 2011 Effect of Filter Choice on OH* Chemiluminescence Kinetics at Low and Elevated Pressures M. M. Kopp 1, M. L. Brower 1, F. Guethe 2, and E. L. Petersen 1 1 Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77845, USA 2 Combustion Technology Group, Alstom Power, Baden, Switzerland Shock-tube experiments have been performed to assess the performance of various optical filters in capturing time-resolved chemiluminescence emission. The purpose was to see if choice of filter bandwidth changed the observed results. Filters included narrow-band ones at ± 10 nm and ± 40 nm and a broadband UG-11 schott filter. The main species of interest was OH*; therefore most of the filters used in this study were centered near 307 nm. Experiments were performed in highly diluted, stoichiometric mixtures of H 2 /O 2 /Ar and CH 4 /O 2 /Ar at a wide range of temperatures and pressures behind reflected shock waves in a shock tube. A dual optical setup on both sides of the shock tube was optimized to allow for simultaneous measurements using two different filters. In addition, experimental results were compared with two chemical kinetics models containing OH* chemistry. Good agreement was seen at atmospheric pressures, while the agreement at higher pressures shows that improvements to the model can still be made. For the cases near 1 atm, the results are not filter specific, but some differences are seen at the highest temperatures for the higher-pressure experiments near 16 atm. 1. Introduction Chemiluminescence measurements have proved to be a common, cost-effective method to determine emission intensities and ignition delay times in shock-tube studies [1]. These measurements can lead to valuable information, such as temperature distributions, pressure dependencies, and reactant mixing, among others, for many aerospace propulsion applications [2]. This optical diagnostic technique requires precise alignment of reflecting mirrors to direct the light emitted from the reaction inside the shock tube, through narrow slits on sidewall optical ports and typically a narrow-bandpass UV filter, into a photomultiplier tube (PMT) to collect the signal and send it to a data acquisition system. The quick response of the system produces time histories of excited species which can provide kinetics model validation through ignition delay times as well as the shapes and magnitudes of the profiles themselves. Of particular interest in this study was the effect of using different types of optical filters in these chemiluminescence measurements. Typically, a narrowband filter is utilized so that interference from possible emitters (other species and grey body emission sources) at overlapping wavelengths is minimized. However, changes in the excited species spectroscopy at elevated temperatures and pressures could alter the emission intensity within the filter bandwidth. To

2 narrow the scope of the present project, OH* chemiluminescence was the primary focus, which experiences peak intensities around 307 nm. Therefore, most studies from the present group dealing with OH* chemiluminescence utilize narrow-bandpass UV filters centered at around 307 nm to capture this feature. However, as can be seen in Fig. 1a, the width of the entire spectroscopic feature occurs over a 25-nm range at 1 bar, and a lower but not insignificant intensity emission is seen between 275 and 300 nm. To explore the potential of capturing the entire OH* feature while minimizing background emission from species such as CO 2 *, experiments were conducted in highly dilute H 2 /O 2 and CH 4 /O 2 (a) mixtures at a range of temperatures and pressures using three 1.0 different filters: a narrow-bandpass UV filter centered at 307 nm with a 20-nm bandwidth, a [ ] c e second narrow-bandpass UV filter centered at 307 nm with a ita n0.6 s m wider bandwidth of 80 nm, and a n broadband UG-11 schott filter T ra 0.2 (Fig. 1). In addition to studying the effect of various optical filters used in OH* chemiluminescence measurements, the pressure dependence of the main OH* formation reaction, H + O + M OH* + M, was investigated. The following paper includes a brief experimental setup description along with the results from this study in various forms, including comparisons between signals from the different filters and comparisons between the experimental data and two commonly used chemical kinetics mechanisms. Following these sections is an interpretation of the findings along with future work to better understand the subject matter. 2. Experimental Setup and Models Wavelength [nm] All experiments were performed in the shock-tube facility described in detail by Aul [2]. Made entirely of 304 stainless steel, the shock-tube has a 4.72-m driven section with an internal diameter of cm and a 4.93-m driver section with a 7.62-cm internal diameter. To determine the test conditions behind the reflected shock wave, the standard one-dimensional shock relations (b) Figure 1. Chemiluminescence spectrum showing the wavelengths and widths of (a) the two narrow-bandpass filters [3], and (b) the transmittance spectrum for the broadband UG-11 schott filter [4].

3 were used. The incident-shock velocity was calculated by extrapolating the velocities calculated at four locations along the shock tube using five PCB 113 pressure transducers which send signals to four Fluke PM 6666 timer counter boxes that signal the instant of shock passage. OH* chemiluminescence measurements were collected through two CaF 2 windows at sidewall locations on both sides of the shock tube located 1.6 cm from the endwall. This dual setup allowed for simultaneous emission measurements using two different optical filters. Each filter was housed in a custom-made enclosure outside of two Hamamatsu 1P21 photomultiplier tubes (PMT) which captured the emission from each test. Experimental conditions were chosen such that peak voltages produced were within the range of PMT linearity [6]. Identical optical settings were maintained throughout the entire series of experiments to allow for direct comparison of the OH* profiles. Three filters with different bandwidths were employed, with the specific bandwidths mentioned above (20 nm, 80 nm, and UG-11). The 20-nm and UG-11 filters were used for experiments in the stoichiometric hydrogen/oxygen mixture (diluted in 98.5% argon), and the two filters used in the methane/oxygen mixture (stoichiometric, diluted in 99.1% argon) were the 20-nm and 80-nm ones. The 20-nm narrow-bandpass filter was employed in both mixtures as a baseline measurement. The UG-11 filter was chosen for the hydrogen/oxygen mixture because it was the widest of the three filters, and chances of it capturing stray emission from other species would be less in a system without hydrocarbons. Two chemical kinetics models were employed for the comparison with the experimental results. is based on the GRI 3.0 mechanism for the ground state chemistry, and is based on the C4_49 mechanism by the National University of Ireland Galway [7, 8]. Added to each of these core models is the OH* mechanism of Hall and Petersen [2]. The C4_49 Galway mechanism can be found at and the GRI 3.0 mechanism can be accessed at 3. Results and Discussion Figure 2 shows normalized experimental OH* traces from the 20-nm bandwidth filter and the UG-11 schott filter compared with predicted OH* concentration from the two mechanisms for the H 2 -O 2 mixture. Because the experimental traces represent a qualitative measurement of OH* chemiluminescence in Volts and the mechanism predicts OH* concentration, the results herein are normalized to peak values and shifted in time to allow for a direct comparison between experimental data and model predictions. As seen in Fig. 2a, there is excellent agreement between data and model (especially ) and between the two different optical filters for this 1-atm example. However, agreement seems to lessen as experimental pressure increases, as seen in Fig. 2b. At this higher pressure, the models tend to predict thinner time histories than the experiment. The slight mismatch between the results for each filter in Fig. 2b on the rise portion of the time history is unexplained at this time but is thought to be due to differences in alignment between the two setups. Sensitivity of the present results to optical configuration is the subject of an ongoing study in the authors laboratory, the results of which will be presented in the near future.

4 Normalized OH* Profile Experiment (UG11) T=1505 K P=1.4 atm Normalized OH* Profile Experiment (UG11) T=1619 K P=17.1 atm (a) (b) Figure 2. Representative OH* profiles from stoichiometric hydrogen/oxygen mixtures in 98.5% Ar compared to model predictions for 1.4 atm (a) and 17.1 atm (b). While Fig. 2 shows a comparison of the time history and basic shape of the OH* profiles, it is also of interest to assess the impact of filter width and pressure on the absolute intensity of the signals, shown in Fig. 3, over a range of conditions in the present study. To develop Fig. 3, it was necessary to perform a set of calibration experiments to correlate the output voltage of the PMT to the OH* concentration, as the two are proportional. This calibration can be done because kinetics modeling of OH* chemiluminescence at atmospheric pressure is fairly well known, as shown previously in Fig. 2a. Peak OH* (mol/cm 3 ) 5x x x x x Using a second set of 1-atm experiments with a different argon dilution level (in this case, 97% Ar), a calibration curve can be developed and extrapolated out to conditions at higher pressures, as in the work of Donato [9]. In Figs. 2 and 3, the UG-11 schott filter is denoted by UG11 in the legend, and the 20-nm wide narrow-bandpass filter centered at 307 nm is denoted by ±10nm. As shown in Fig. 3, the results from the two different filters show relatively the same trends over the entire range of temperature and pressure, except at the highest temperatures and pressures, where the schott filter captures more emission than the 20-nm wide bandpass filter. Reasons for this discrepancy are not known at this time and are the subject of ongoing study. Possible explanations might be that at elevated pressures the OH* feature is more broad or that at elevated temperatures and pressures the measured signal is susceptible to broadband emission other than from OH*, which the broader filter is more likely to capture atm Data (UG11) 15 atm Data (±10nm) 11 atm Data (UG11) 11 atm Data (±10nm) 5.5 atm Data (UG11) 5.5 atm Data (±10nm) 1.4 atm Data (UG11) 1.4 atm Data (±10nm) Temperature (K) Figure 3. Peak OH* concentration as a function of temperature for both filters for the H 2 -O 2 mixture. Trendlines are shown through each data set.

5 The next set of figures represents the results from the methane/oxygen experiments. Figure 4 shows normalized experimental OH* traces from the 20-nm bandwidth filter and the 80-nm bandwidth filter compared with the two models. Agreement between model and data is mostly good at low pressure, as seen in Fig. 4a, as well as agreement between the two models and between the two filters themselves. At higher pressures, both models predict thinner profiles than the data, as seen in Fig. 4b. The Galway mechanism in seems to better model the trailing edge of the OH* profile than the GRI mechanism in for higher pressures. Normalized OH* Profile T=2169 K P=1.3 atm Normalized OH* Profile T=1830 K P=16.2 atm (a) (b) Figure 4. Representative OH* profiles from stoichiometric methane/oxygen mixtures in 99.1% Ar compared to model predictions for 1.3 atm (a) and 16.2 atm (b). Figure 5 shows the temperature dependence of the peak OH* chemiluminescence at two average pressures, 1.3 and 16 atm. To compare the experimental results with the predictions from the models (since no OH* concentration calibration was available for the CH 4 -O 2 experiments), the data were normalized to one condition, which in this case was 2255 K and 1.3 atm. The condition at 1.3 atm was chosen (rather than a higher-pressure point) because the kinetics of OH* are better known at the lower pressure. Peak OH* Normalized to 2255 K, 1.3 atm 1.2 P = 1.3 atm Peak OH* Normalized to 2255 K, 1.3 atm P = 16 atm Temperature (K) (a) 1.3 atm Temperature (K) (b) 16 atm Figure 5. Experimental and model results for normalized peak OH* as a function of temperature for the methane mixture.

6 For the 1.3-atm experiments, the agreement between the two different optical filters is excellent, seen in Fig. 5a. However, at higher pressure, it is apparent in Fig. 5b that the wider bandpass filter captures more emission than the narrower filter. It also seems like this trend increases as temperature increases. Figure 5 also shows that falls short in predicting experimental OH* chemiluminescence at both pressures. In the lower-pressure case, seen in Fig. 5a, over-predicts the peak OH* chemiluminescence, while shows great agreement with the data. In Fig. 5b, for the high-pressure case, both models tend to under-predict peak OH* chemiluminescence. Again, for the high-pressure case, disagreement becomes more exaggerated as temperature increases. 4. Conclusions and Future Work It can be concluded from this study that improvements to the OH* mechanism can still be made with regards to modeling the temperature dependence of peak OH* levels in methane/oxygen mixtures. For example, the pressure dependence can also be determined for H + O + M OH* + M to better model the OH* chemiluminescence at higher temperatures and pressures, which are also more representative of real engine conditions in the gas turbine industry. The Galway and GRI mechanisms agree in most cases, but the Galway mechanism better models the OH* profile trailing shape at higher pressures in the methane/oxygen system. This mechanism comparison could provide insight into how to better improve the final mechanism to better capture the experimental profiles at higher pressures. The excellent agreement between model and data for OH* profiles in the H 2 /O 2 mixture at low pressure confirms that the calibration method in relating PMT output to OH* concentration is valid. To better understand the reaction kinetics involved in the OH* mechanism, sensitivity analysis should be done to identify the dominant reactions during OH* formation, depletion, and peak values. Conducting experiments using filters centered at different wavelengths within the OH* feature could provide insight into whether the background from the broad CO 2 * feature is significant or not when using OH* time histories to validate the OH* chemical kinetics. References [1] E. L. Petersen, D. M. Kalitan, and M. J. A. Rickard, AIAA paper (2003). [2] J. M. Hall and E. L. Petersen, AIAA paper (2004). [3] M. Lauer and T. Sattelmayer, J. Eng. Gas Turbines Power 132 (2010) /8. [4] [5] C. J. Aul, M.S. Thesis, Texas A&M University (2009). [6] M. J. A. Rickard and E. L. Petersen, Aerospace Corporation, Internal Report. [7] D. Healy, M. M. Kopp, N. L. Polley, E. L. Petersen, G. Bourque, and H. J. Curran, Energy & Fuels 24 (2010) [8] D. Healy, H. J. Curran, N. S. Donato, C. J. Aul, E. L. Petersen, C. M. Zinner, G. Bourque, and H. J. Curran, Combustion and Flame 157 (2010) [9] N. S. Donato, M.S. Thesis, Texas A&M University (2009).

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