CHAPTER III EXPERIMENTAL SYSTEM

Transcription

1 CHAPTER III EXPERIMENTAL SYSTEM 3.1 Introduction The basic design and implementation of laser induced Raman scattering systems is, in general, relatively simple. Laser light from a single laser source is focused to a point (small cross-sectional area) in a flame/sample to induce Raman scattering activity. The light collection optics are positioned 90 to the laser beam and on the same plane as the direction of propagation to improve the spatial resolution of the system. The added benefit of this arrangement is the minimization of Mie scattering interferences, i.e. scattering from large particles other than molecular species. The collection optics are as large as possible since Raman scattering is relatively weak. A spectrograph with high efficiency and throughput (low f#) is used to disperse the collected light into its component wavelengths, or Raman signals, while providing excellent spatial imaging resolution. Filters are used at the spectrograph entrance plane to block out Rayleigh scattered and stray light from interfering with the measurement of the Raman signals. An intensified charged coupled device (CCD) camera simultaneously measures the intensity and spatial distribution of all the Raman signals. The first part of this work deals with Raman spectroscopy performed in steady atmospheric flames to develop the Raman bandshape fitting technique for use as a temperature probe in practical combustion devices. These laboratory experiments are used to identify the response of the UV Raman system and validate the accuracy of the temperature measurements. The validated Raman bandshape technique is then used to measure temperature during testing of a sub-scale rocket-engine-like test article. 3.2 Laser Source The H 2 Stokes Raman scattered signals are induced by a Lambda-Physik COMPex 150T narrowband KrF excimer laser that is tunable from cm -1 ( nm). Typically, when the laser is operated in the narrow bandwidth mode, a small fraction of the emitted light is broadband (resulting from unpolarized amplifier spontaneous emission) (Grünefeld et al. 1996). The ratio of the intensity of the narrowband component of the laser 37

2 output to the total output of the laser is defined as the locking efficiency, η. To increase η, also the degree of linear polarization which is directly proportional to η, and locking repeatability from shot-to-shot, the laser is modified to operate in a single-pass configuration (Grünefeld et al. 1996). A fraction of the laser beam is sent through an etalon to monitor the laser locking efficiency in real-time. The locking efficiency for the single-pass configuration used in the present work is ~85%. The broadband output spectrum, obtained by collecting the Rayleigh scattered light as shown in Fig. 3-1, is the asymmetrical emission spectrum of the KrF excimer which has a full spectral bandwidth of ~250cm -1 and a peak emission, ν B, at ~ cm -1. Since the broadband laser linewidth is much broader than the spectrometer/detector linewidth (~6.75cm -1 ) and the spectrometer entrance slit is extremely small, the instrumental linewidth for the broadband laser emission, ν B = 50cm -1, is determined by comparing the Rayleigh spectrum with a calculated profile using a 80%Gaussian/20%Lorentzian lineshape distribution function. The asymmetry in the broadband KrF emission spectrum is clearly visible. A sample spectrum of the Rayleigh scattered light from the narrowband laser is shown in Fig The narrowband Gaussian/Lorentzian lineshape fitted to the spectrum reveals asymmetry and broadening induced by the incomplete locking efficiency of the laser. Therefore instead of the single narrowband 80%Gaussian/20%Lorentzian profile, a superposition of two 80%Gaussian/20%Lorentzian functions using the narrowband and broadband laser bandwidths using the broadband laser center frequency of peak intensity scaled by the locking efficiency is used to fit the experimental Rayleigh spectrum with sufficient accuracy. The importance of this signal mixing process, the lineshape distribution functions, and their implications on accurately fitting the experimental spectra are discussed in Chapter Four. Additionally, fitting the narrowband Rayleigh spectrum using the method described in Chapter Four reveals the bandwidth of the narrowband laser, ν L = 0.8cm -1 (0.005nm), and verifies the locking efficiency with the value obtained from the etalon, ~85%. The output of the KrF laser has enough photon energy to excite several rotational transitions of the A 2 Σ(v =3) X 2 Π(v =0) band of the OH molecule. The resulting fluorescence via the (3 1) band of strong emission lines between cm -1 interfere with the H 2 O and H 2 Raman signals. Fluorescence interferences can be significantly reduced by properly tuning the narrowband laser emission away from electronically accessible molecular transitions 38

3 Broadband KrF Laser Linewidth 80%Gaussian+20%Lorentzian Profile Measured Intensity 1 Normalized Intensity (arb. units) Frequency (cm -1 ) Fig. 3-1 Rayleigh spectrum of the broadband KrF excimer laser compared to 80%Gaussian/20%Lorentzian lineshape distribution with Γ = 50 cm

4 Narrowband KrF Laser Linewidth Narrowband (80%Gaussian/20%Lorentzian) Mixed Narrow and Broadband (80%Gaussian/20%Lorentzian) Profiles Measured Intensity 1 Normalized Intensity (arb. units) Frequency (cm -1 ) Fig. 3-2 Rayleigh spectrum of the narrowband KrF excimer laser compared to narrowband 80%Gaussian/20%Lorentzian and a superposition of the narrowband 80%Gaussian/20%Lorentzian and broadband 80%Gaussian/20%Lorentzian profiles scaled by the locking efficiency (η = 85%). The narrowband laser bandwidth, ν L = 0.8cm -1, is determined using the convolution method described in Chapter 4 which includes the spectrometer linewidth and slit function. 40

5 (Andresen et al and Wehrmeyer et al. 1992). Therefore, an OH excitation spectrum is obtained to determine the optimum narrowband laser emission frequency, ν L. For the excitation spectrum the spectrograph is centered at cm -1 with a spectral coverage of 1175cm -1 to record the OH fluorescence emission from the (3 1) band, which is also used as the calibration source to characterize the frequency response of the spectrograph and intensified CCD (Charge Coupled Device) detector. A near stoichiometric H 2 -air flame is probed with the laser by tuning the laser, in cm -1 increments, through its locking range. The resulting excitation spectrum is shown in Fig The 3 0 OH transition frequencies pumped by the laser are listed in Table 3-1 along with the associated 3 1 fluorescence frequencies (Diecke and Crosswhite 1962). Tuning the laser to cm -1 ( nm), a frequency close to the peak emission frequency of the KrF excimer, maximizes the laser emission intensity and the Raman scattered signal strengths while minimizing OH fluorescence (shown by the arrow in Fig. 3-3). 3.3 UV Raman Light Collection System A schematic of the experimental UV Raman system is shown in Fig The line integrated Raman scattered light is collected at 90 to the laser beam using a 50 mm diameter f/4 lens and relayed by another lens through a Laser Optik 0 dielectric thin-film notch filter into a 0.5m spectrograph (Acton Research model 500M). The thin-film notch filter is used at the spectrograph entrance to prevent Rayleigh and Mie scattered light from interfering with the Raman signals. An absorption/transmission scan was performed to characterize the frequency response of the thin-film notch filter which is non-uniform over the H 2 Stokes frequency range. A high-efficiency 3600 groove/mm holographic grating (56cm -1 /mm dispersion at cm -1 ) is used to disperse the Raman signals onto an intensified CCD array with a 576 x 384 pixel chip consisting of22µm x 22µm pixels. For the H 2 /N 2 gas mixtures and the H 2 -air flames the photomultiplier gain, or the detected number of counts on the CCD, is ~30counts/e- and ~100counts/e-, respectively. Maximum spectral resolution is achieved without significantly attenuating the Raman signals by setting the spectrograph entrance slit to 37.5µm, which is equivalent to the spatial resolution of one pixel considering the pixel size and intensifier minification ratio (~1.65). The resolution of the spectrograph, ν spect, is 6.75cm -1 (0.05nm) determined from measurements of emission lines from a UV Hg-vapor lamp. Combined with the dispersion of the grating, the spectrograph entrance slit produces and additional rectangular 41

6 1 0.9 Q 1 (11) Q 2 (11) 0.8 Q 2 (10) Intensity (arb units) Q 1 (10) R 1 (14) P2 (7) P 1 (8) P 2 (8) R 2 (15) P 1 (9) Frequency (cm -1 ) Fig. 3-3 OH excitation spectrum showing the (3 0) band of the A 2 Σ - Χ 2 Π transition The spectrograph is set to cm -1. The arrow indicates the optimum operating frequency of the laser for the Raman flame spectra, ν L = cm -1 ( nm). 42

7 Table 3-1. OH absorption and resulting fluorescence lines excited by the KrF laser from Diecke and Crosswhite (1962). (3 0) Absorption Lines (3 1) Fluorescence Lines Designation Frequency (cm -1 ) Trans. Prob. Designation Frequency (cm -1 ) Q 1 (10) 40, R 1 (9) 37, Q 1 (10) 36, P 1 (11) 36, Q 21 (10) 40, R 2 (9) 37, Q 2 (10) 36,826.7 P 2 (11) 36, R 1 (14) 40, R 1 (14) 36, Q 1 (15) 36, P 1 (16) 35, R 21 (14) 40, R 2 (14) 36, Q 2 (15) 36,377.3 P 2 (16) 35, P 2 (7) 40, R 2 (5) 37, Q 2 (6) 37, P 2 (7) 36, Q 12 (10) 40, R 1 (9) 37, Q 1 (10) 36, P 1 (11) 36, Q 2 (10) 40, R 2 (9) 37, Q 2 (10) 36, P 2 (11) 36, R 2 (14) 40, R 2 (14) 36, Q 2 (15) 36,377.3 P 2 (16) 35, P 1 (8) 40, R 1 (6) 37, Q 1 (7) 37, P 1 (8) 36, Q 1 (11) 40, R 1 (10) 37, Q 1 (11) 36, P 1 (12) 36, Q 21 (11) 40, R 2 (10) 37, Q 2 (11) 36,

8 P 2 (12) 36, P 12 (8) 40, R 1 (6) 37, Q 1 (7) 37, P 1 (8) 36, P 2 (8) 40, R 2 (6) 37, Q 2 (7) 37,006.4 P 2 (8) 36, R 1 (15) 40, R 1 (15) 36, Q 1 (16) 36,293 P 1 (17) 35, R 21 (15) 40, R 2 (15) 36, Q 2 (16) 36, P 2 (17) 35, Q 12 (11) 40, R 1 (10) 37, Q 1 (11) 36, P 1 (12) 36, Q 2 (11) 40, R 2 (10) 37, Q 2 (11) 36, P 2 (12) 36, R 2 (15) 40, R 2 (15) 36, Q 2 (16) 36, P 2 (17) 35, P 1 (9) 40, R 1 (7) 37,287.3 Q 1 (8) 37, P 1 (9) 36,

9 TO COMPUTER DIGITAL CAMERA WITH GATED INTENSIFIER BEAM DUMP 0.5 m IMAGING SPECTROGRAPH 50 mm DIA., f/4 SINGLET LENSES 37.5 µm 14 mm TRIGGER FOR LASER AND CAMERA INTENSIFIER 0.17 mm DIA. BEAM WAIST H 2 -AIR FLAME FOCUSING LENS SINGLE-PASS TUNABLE KRF EXCIMER LASER Fig Schematic of experimental UV Raman system. 45

10 lineshape function, ν slit, of ~2.1cm -1. The combined resolution of the spectrograph/ccd system, ν spect-slit, with a slit width of 37.5µm is ~7.1cm -1 (~0.055nm) as determined from measurements of emission lines from a UV Hg-vapor lamp. To increase the Raman scattered signal, the 5mm x 22mm laser beam is focused to a ~0.17mm x 0.75mm profile in the sample volume with the smallest dimension aligned with the spectrograph entrance slit width. The probe length or spatial resolution of the Raman system, as determined by the magnification ratio of the collection system (1:1) and the CCD intensifier minification ratio, is 8.64mm in the direction of beam propagation. The extremely narrow entrance slit width required to provide the necessary spectral resolution does limit the amount of light striking the CCD per laser pulse; however, integration of the Raman scattered light from the 8.64mm probe length significantly increases the total measured signal strengths and allows for statistically reliable single-pulse and time-averaged measurements. 3.4 Detector Response The intensified CCD camera is an excellent spectroscopy tool. However, a significant problem arises when trying to assign each camera pixel a specific frequency because the resolution of the spectrograph is not adequate to fully disperse the signal and the 22µm pixel width is large compared to the sharp atomic or fluorescence lines used for calibration. Typically, the peak centers of intensity of the 32 OH (3 1) fluorescence lines used to calibrate the spectrograph/ccd are between 1-2 pixels in width, as illustrated in Fig Thus for calibration of the spectrograph/ccd, the peak centers of the OH fluorescence lines between cm -1 are fitted using Lorentzian profiles with weighted line centers about the mean pixel location and a linear regression analysis to generate a 6 th order polynomial calibration curve. The error in the calibration fit for any one frequency measurement is equivalent to the variance of the fit, σ 2 = ±0.3cm -1, which is ~4µm on the image or about one fifth of a pixel (Tellinghuisen 1993). Spatial smearing of the experimental spectra occurs because of the finite size of the pixels, geometric response of the CCD array, and from the microchannel plate intensifier. These effects along with the focal length of the spectrograph and dispersive capability of the grating limit the resolving power of the collection system. Thus to provide the maximum spectral resolution without significantly attenuating the Raman signals, the spectrograph entrance slit is 46

11 1 OH Calibration Normalized Intensity (arb. units) Pixel # Fig A 40 pixel region of a typical (3 1) OH fluorescence spectrum used for calibration of the spectrograph/ccd. The OH fluorescence line illustrated is Q 2 (15) at cm -1 resulting from laser excitation of R 2 (14) at cm -1. The frequency scale is ~2cm -1 per pixel. 47

12 set to 37.5µm, which is equivalent to the spatial resolution of one pixel based on the pixel size and intensifier minification ratio. The spectrograph instrumental linewidth, including the slit function and the smearing on the detector, is measured by recording the lineshape of an isolated and intense 3 1 OH fluorescence line in the spectral region of interest as shown in Fig Unlike the typical fluorescence spectrum shown in Fig. 3-5, the selected OH spectra for the linewidth calibration have well defined peak centers that are sharp (<0.025cm -1 or less than the 6.75cm -1 resolving power of the spectrograph) and contained within a few pixels. Fluorescence spectral shapes are typically dominated by radiation damping or predissociative processes and for instruments with moderate to weak spectral resolution are adequately described by Lorentzian profiles. The OH spectrum in Fig. 3-6, upon de- convolution of the slit function and laser bandwidth as described by methods in Chapter Four, confirms the measured spectrograph instrumental linewidth, ν spect, of 6.75cm Burners Temperature measurements are taken in dilute H 2 -N 2 mixtures issuing from a gas heater jet (Sylvania Series III model , 12.5mm diameter) and in atmospheric pressure H 2 -air flames of various rich equivalence ratios produced in an adiabatic Hencken burner (12.5mm diameter). The contour fit Raman derived temperatures are compared to measurements made with Pt6%Rhodium-Pt13%Rhodium bare uncoated thermocouples. Radiation corrections are made using methods suggested by Kaskan (1957). Fuel- and air-flow rates for the conditions probed are listed in Table 3-2 and are measured with Teledyne-Hastings hot-wire mass flow meters. The flow meters have accuracies of ±1% of their full scale. Based on calibrations using laminar flow elements, the flow rates used in the experiments are accurate to within ±1%, and the predicted adiabatic flame temperatures, based on the combined relative error of the mass flow meters, are accurate to within ±1.5%. 48

13 Normalized Intensity (arb. units) OH Fluorescence Linewidth Gaussian Lorentzian Measured Intensity Frequency (cm -1 ) Fig Measurement of the instrumental linewidth of the spectrograph/ccd using the Q 2 (16) 3 1 OH fluorescence line resulting from excitation of the R 2 (15) 3 0 OH absorption line. The spectrograph instrumental linewidth, ν spect, is 6.75cm -1 after de-convolution of the slit function and laser bandwidth. 49

14 Table 3-2. Flow-rates for the laboratory Raman measurements in the H 2 /N 2 mixtures and H 2 -air flames. Flow Rates (L/min) T adiabatic T thermocouple Mixture H 2 N 2 Air (K) (K) H 2 /N H 2 /N H 2 -air H 2 -air

15 3.6 Marshall Space Flight Center Tests The configuration of the Raman system for the hot-fire tests of the sub-scale rocketengine like test article at NASA Marshall Space Flight Center Test Stand 115 is very similar to the laboratory system previously described. An identical Lambda-Physik COMPex 150T narrowband KrF excimer laser is used to induce the H 2 Stokes Raman scattered signals. However, the laser is operated in the standard configuration and not in single-pass mode to maintain minimal divergence of the laser beam (~0.2mrad for the conventional mode vs. ~1mrad for the single-pass mode). The better coherence is required to transmit the beam over the long distance from the instrumentation building to the test article without considerable beam expansion. This allows for tighter focusing of the beam in the sample volume, thus maintaining a high energy density for Raman scattering activity. The narrowband/broadband linewidth parameters and locking efficiency are comparable to those in the single-pass mode. For the hotfire test, the laser is operated at 10Hz and a pulse energy of ~425mJ at the exit plane of the laser cavity with only ~225mJ reaching the test article due to losses from optical components along the path length. Also to prevent damaging the test article windows, the laser beam is focused with a 2m lens. The Raman scattered light is collected with a 50mm diameter f/5 air-spaced doublet, used for its excellent spatial imaging properties, and relayed by an identical lens through the same Laser Optik 0 dielectric thin-film notch filter and Acton Research model 500M 0.5m spectrograph equipped with the 3600 groove/mm holographic grating and the same ICCD as used in the laboratory experiments. The test article for the hot-fire test consists of an injector plate assembly, GH 2 /GO 2 hotgas torch igniter, choked flow convergent-divergent section, rapid expansion nozzle plate, and several spacers, thermocouple sections, calorimeter rack, or annular windowed sections. Schematics of this modular combustion test article (MCTA) showing the thermocouple, calorimeter, and shadowgraphy/raman test configurations are illustrated in Fig The combustion chamber of the MCTA is 10cm in diameter and 48cm in length. The 7 element coaxial injector is shown in Fig Each injector element is of co-annular design consisting of an inner flow of LO X shrouded by LH 2. Table 3-3 lists the test conditions for the hot-fire tests of the MCTA. Figure 3-9 shows a hot-fire test of the MCTA with a windowed section aft of the injector and torch igniter for shadowgraph imaging. Shadowgraphy measurements are performed at the injector face and at several downstream axial positions to analyze injector spray 51

16 performance with thermocouple and calorimeter measurements to obtain combustion temperatures and thermal efficiency for comparison with computational fluid dynamic (CFD) calculations. Raman temperature measurements, for validation of the contour-fit temperature measurement technique for practical combustion devices and comparison with thermocouple measurements and CFD predictions, are performed in the combustion chamber ~40cm downstream of the injector face to prevent liquid-phase interferences and to ensure complete mixing of the individual injector element spray cores and chemical/thermal equilibrium. 52

17 Co-Axial 7 element Injector Thermocouple Racks Nozzle Plate GH 2 /GO 2 Torch Igniter Calorimeter Choked Flow Section Windowed Section Fig. 3-7 Cross-section schematic of the modular combustion test article (MCTA) in various test configurations. The top schematic shows the MCTA with the thermocouple rack installed for spatial temperature measurements, the middle schematic shows the calorimeter installed for combustion efficiency measurements, and the bottom schematic shows the windowed section installed for shadowgraphy and Raman measurements. 53

18 Fig. 3-8 View of the 7 element co-axial injector looking through the exhaust nozzle down the combustion chamber. The injector is composed of two annular flows, the inner injector post supplies LO X, while the outer shroud supplies LH 2. 54

19 Table 3-3. MCTA hot-fire test conditions. a Combustion Chamber Conditions Nozzle a Ignition c Mainstage 1 d Mainstage 2 d Exhaust Plume MR b Total Flow Rate (lbm/s) P c (psig) >>14.7 T c (K) ρ (g/cc) V (m/s) - ~15 ~15 ~300 N H2 (molecules/cm 3 ) - ~1 x ~1 x ~2 x [H 2 ] (%) - ~0.9 ~0.9 - Laser Fluence (GW/cm 2 ) - ~1 ~1 ~5 Raman Signal Gain e Choked flow at the throat exit. Listed values are approximations 10cm downstream of the throat in the rapidly expanding exhaust plume. b MR is the oxidizer to fuel mass ratio. Stoichiometric is 8:1. c The chamber conditions for ignitions. d During the test sequence the MCTA is operated at two different mainstage combustion conditions. e Raman signal gain based on equilibrium calculations of in-chamber combustion conditions and compared with laboratory experiments of N H2 = , T = 293K, and a laser fluence of ~10 (GW/cm2). For a 2%/98% H 2 /N 2 mixture at P = 1800psi and a N H2 = with ~0.1 (Gw/cm2) laser fluence the expected gain in Raman signal is ~12. LH 2 inlet T = ~40K LO x inlet T = ~110K Test Duration = ~20-42s 55

20 Fig. 3-9 Hot-fire test of the MCTA. Windowed section is installed just aft of the injector to provide shadowgraphy measurements during injection. 56