CHAPTER I INTRODUCTION
1.1 Motivation and Goals
Numerical simulations of complex flows and combustion processes aid in the development and improvement of flame models and practical combustion devices like piston engines, gas turbines, and rocket engines. However, it is still necessary to conduct experiments in these devices since numerical simulations have limitations because of computational cost and a lack of knowledge regarding fundamental physical processes. Experimental acquisition of essential qualitative and quantitative information allows insight into the combustion process while providing input conditions and validation of combustion models. Therefore, it is pertinent that accurate and reliable measurement techniques be developed and improved for combustion research.
In reactive and non-reactive fluid flow systems, precise temporal and spatial knowledge of the local temperature and temperature distributions is essential in understanding the thermo- chemical properties (reaction rates and equilibrium states) and physics (fluid interaction) of the flow-field. Traditional methods to measure temperature in combustion environments include the use of gas sampling systems and thermocouples. These measurement techniques have several major disadvantages in practical combustion devices where the environment can be chemically harsh (extreme temperatures), spatially inhomogeneous (unsteady), and rapidly changing (turbulent): 1) the physical probes provide only time-averaged information; 2) the probe disturbs the flow-field thus influencing local mixing; 3) the corrections due to radiation losses introduce large uncertainties in the measurements; and 4) the survivability of the probe is a concern at high temperatures (>2000K). Consequently there exists a need for non-intrusive methods that provide more accurate measurements while not perturbing the interrogated flows.
Laser-based diagnostics provide the capability of non-intrusively probing a combustion environment to measure thermodynamic properties. In addition to not perturbing the flow field, optical measurement techniques offer high temporal and excellent spatial resolution. The sensitivity and selectivity of optical methods also allows for exceptional measurement accuracy.
The most widely accepted laser-based techniques for temperature measurements (Kohse-
Höinghaus and Jeffries 2002) are coherent anti-Stokes Raman spectroscopy (CARS) (Alessandretti and Violono 1983, Eckbreth et al. 1984, Hussong et al. 2000, and Smith et al.
2002), laser-induced fluorescence (LIF) (Andresen et al. 1988, Crosley and Jeffries 1992, Barlow and Carter 1994, Kohse-Höinghaus 1994, Cohen et al. 2001, and Bessler and Schulz 2004), Rayleigh scattering (Seasholtz et al. 1997 and Miles et al. 2001), and spontaneous Raman scattering (Wehrmeyer et al. 1992a, Gillespie 1993, Barlow and Carter 1994, Farhangi et al.
1994, Jones et al. 1996, de Groot 1998, Wehrmeyer et al. 2001, Yeralan et al. 2001, and Kojima and Nguyen 2004). These techniques have been successfully applied to laboratory flames (Andresen et al. 1988, Crosley and Jeffries 1992, Wehrmeyer et al. 1992a, Kohse-Hoinghaus 1994, Hussong et al. 2000, and Bessler and Schulz 2004), supersonic wind tunnels (Seasholtz et al. 1997), and a variety of practical combustion devices such as internal combustion engines (Alessandretti and Violono 1983), jet engines (Eckbreth et al. 1984), shock tubes (Gillespie 1993), and rocket engines (Farhangi et al. 1994, Jones et al. 1996, de Groot 1998, Cohen et al.
2001, Wehrmeyer et al. 2001, Yeralan et al. 2001, and Smith et al. 2002).
Optical thermometry principally relies either on the temperature dependence of the population of different rotational, vibrational, or electronic states of atomic or molecular probes (e.g. spontaneous Raman scattering, Rayleigh scattering, LIF, and CARS) or the temperature dependence of the total number density, nD, (e.g. traditional spontaneous Raman and Rayleigh scattering) (Bessler and Schulz 2004). Each of these optical measurement techniques has inherent advantages and disadvantages depending on the particular application. The techniques are briefly discussed with respect to their ability to provide accurate temperature measurements.
As a result of the deficiencies of CARS, LIF, and Rayleigh scattering, a method using spontaneous Raman scattering is developed to measure temperature in practical combustion applications. It should also be noted that laser absorption methods are capable of providing temperature measurements but are generally not favored for combustion applications because they are line-of-sight techniques that do not in general allow for spatial resolution along the beam path and are limited to combustion systems with homogeneous density and species concentrations (Kohse-Höinghaus 1994).
CARS is a multi-laser pump-probe technique producing a strong resonant signal capable of in situ temperature and species concentration measurements. Concentration measurements using CARS are generally inaccurate because of the nature of the measurement (e.g. the presence
of a non-resonant background contributed by all the chemical species). Also one probe laser is required to measure each gas species. Therefore, CARS is most widely used to obtain single- laser pulse temperature measurements. The strong coherent signal affords temperature measurements with ~1-2% accuracy (Hancock et al. 1997). CARS is also capable of measurements in “dirty” or harsh combustion environments because the strong signal can be easily discriminated from background luminosity and other interferences. Eventhough CARS provides very accurate temperature measurements, it is only capable of point-wise measurements with spatial resolutions of ~2mm in even the most advanced systems. Additionally, CARS systems have inherent complications such as system complexity (multiple laser beam and abundant optical components), limitations on the number of measured species, sensitivity to beam steering due to high laser fluences, geometrical requirements for optical access to the combustion device, and potential non-resonant background interference. The biggest disadvantage of CARS involves the complex theoretical analysis that must be performed to derive information from the measurements. Accurate measurements of temperature depend on knowledge of the nonlinear processes that result in the generation of the CARS spectrum and the physics of spectral line formation (Rosasco and Hurst 1992). These processes include the non- linear variation in the third-order susceptibility with respect to laser intensity and the species number density, nD, and kinetic effects on molecular energy distribution (i.e. exact information regarding the number of species and nD’s in the sampling environment and corresponding effect, through molecular collisions, on the observed energy of the Raman transitions and energy distributions or spectral lineshapes).
LIF is an electronic spectroscopy process that has widespread applications because of its sensitivity. The LIF technique produces strong non-resonant signals capable of providing either concentration or temperature measurements. If a pulsed laser is used, LIF has the ability to provide good temporal resolution and the strong signal strength also makes LIF attractive for line-wise measurements. However, similar to CARS, LIF is limited to single-species detection because the electronic transitions for most molecules do not overlap, thus requiring a specific laser source for the detection of each species. LIF has mainly been used for qualitative combustion analysis. For example, LIF can provide insight into spray formations or chemical mechanisms and pathways by monitoring the presence/absence of a specific species during the progress of an elementary reaction (Kohse-Höinghaus 1994). Accurate quantitative temperature
measurements with LIF are difficult to perform because the fluorescence signals depend upon collisional quenching and intra-molecular energy transfer processes, which are dependent on the pressure, temperature, and chemical composition of the sample volume. These collisional and non-radiative processes result in a redistribution of the thermal population of states and can vary locally within the sampling environment. Thus, quantitative temperature measurements are only reliably obtained, with significant effort, in steady, laminar, low-pressure environments.
Rayleigh scattering is an incoherent method based on the elastic scattering of light from molecules. Since there is no energy exchange (elastic), the scattered signal is not specific to any particular species in a spectral sense because the scattered light remains at the same energy/frequency of the incident light. Thus, Rayleigh scattering methods are incapable of species concentration measurements, but the strong scattered signal does allow for total density and temperature measurements (Eckbreth 1996). Traditional Rayleigh scattering methods are used for temperature and density measurements only when the local gas composition and the effective Rayleigh scattering cross-section are accurately known. Temperature measurements can also be obtained with Rayleigh scattering by resolving the Doppler-broadened linewidth of the scattered light, since the resulting spectral shape reflects the density, temperature, and energy state of the probed environment. The Doppler Rayleigh method also allows for velocity measurements due to the physics of the process, but is applicable only to situations where pressure broadening does not dominate the overall line broadening process and the Doppler shift is large, i.e. at low to moderate pressures and high velocities. Rayleigh methods suffer from background interferences, spuriously scattered laser light, and Mie interferences (scattering from small particles), thus limiting its application to very clean, particle-free environments and preventing practical combustion device probing (Eckbreth 1996). These interferences can be removed by filtered Rayleigh techniques; methods that separate the scattering interferences from the Doppler-broadened gas-phase signal through the use of molecular or atomic absorption lines as spectral/spatial filters. However, filtered Rayleigh (Doppler-shifted) methods are still only applicable in low to moderate pressure, high velocity (typically supersonic) flows where there is a sufficient Doppler shift to ensure separation of the Raman signal from the interference signals.
Spontaneous Raman scattering is an incoherent method resulting from inelastic collisions of light with molecular species. Unlike CARS and LIF, a Raman system only requires one laser source to monitor all the species of interest. The laser can operate at any frequency, although a
UV frequency is preferred due to the v4 dependence of the molecular scattering cross-section, because the process does not depend on resonance of the probed species. The Raman spectra are located at discrete frequency shifts from the laser, which are proportional to the quantized molecular energy states of the probed species. Thus the Raman scattered signals are species specific with generally no interference from Raman bands of other species. Most importantly, the Raman scattered signal is relatively independent of the surrounding gas composition (mainly due to limited spectral resolution inherent in most practical Raman systems) and is unaffected by collisional quenching (Eckbreth 1996). These aspects, in combination with the linear response of the scattered signal intensity with respect to the species nD’s, greatly simplify spectral synthesis and data interpretation procedures compared to CARS, LIF, and Rayleigh scattering.
The one drawback of Raman scattering is weaker signal strengths compared to CARS, LIF, and Rayleigh scaettering, which in general limit its application to relatively clean combustion environments.
The deficiency in strength of the Raman scattered signal can be overcome through the use of modern high-energy pulsed UV lasers. Although the scattering process is still weaker than CARS, LIF, and Rayleigh scattering, the enhanced signal does provide Raman scattering with the capability of obtaining highly-resolved temporal and spatial temperature measurements and, if desired, species concentrations. Typical spontaneous Raman scattering systems are capable of
~10-20ns temporal resolution and spatial resolution in the submillimeter region, which is an order of magnitude better than most single-laser pulse CARS systems (with ~2mm spatial resolution). Additionally, at high-pressures, which are typical in most practical combustion devices, the Raman scattered signal intensity is further enhanced through increased nD. In general, Raman systems are much less susceptible to beam steering problems and non-linear processes compared to CARS, LIF, and Rayleigh. Thus considering the aforementioned advantages and disadvantages of the other optical methods, Raman scattering is an extremely capable diagnostic tool for combustion analysis and in many cases is the preferred optical diagnostic in favorable combustion environments.
The successful application of optical diagnostics in combustion, whether in a laboratory or practical combustion device, depends on careful assessment of the ability of the various techniques to obtain the desired measurement given the environment conditions. Therefore, the fundamental objective of this work is to further develop a reliable non-intrusive optical
diagnostic method for thermometry applications in practical combustion devices, in particular as a temperature probe for high-pressure H2/O2 fueled combustion systems like rocket engines.
Combustion of this nature is generally clean, thus making a UV Raman scattering method the preferred diagnostic. To realize the goals of this work, a model for Raman thermometry is developed, validated through experimentation, and then applied during the hot-fire testing of a H2/O2 fueled rocket engine-like test article.
1.2 Past Raman Work
In traditional Raman thermometry methods, the measured species number densities (the frequency integrated intensity of the Raman bandshape for each species) are related to species concentrations and temperature via an ideal gas law analysis using an independent mechanical pressure measurement. Alternative methods using Raman bandshape methods can be used to measure temperature independently since the relative molecular energy level populations and distribution are determined directly from the spectral lineshape. These Raman bandshape techniques exploit the inherent temperature variation in the shape of the induced spectrum and allow direct temperature measurements without the need for an independent calibration of the Raman system or pressure measurement, which is advantageous in practical combustion devices with non-uniform flows and local pressure variations.
Four such methods are commonly used to accurately determine temperature from vibrational Raman scattering (Drake et al. 1982): 1) the band area method, as demonstrated with N2, H2O, and CO2 (Lapp et al. 1973 and Lapp 1974), derives temperature from the ratio of the integrated intensities of the excited “hot” bands with that of the ground state; 2) the band peak intensity method, as demonstrated for N2 and O2 (Lapp 1974, Setchell and Miller 1978, Stephenson and Aiman 1978, and Drake and Hastie 1981), extracts temperature from the sensitivity of the peak intensity ratio of the excited vibrational states to the ground state; 3) the Stokes/anti-Stokes intensity method, typically for N2 (Widhopf and Lederman 1971, Lapp 1974, and Wehrmeyer et al. 1992a and 1992b) but also achieved using H2 (Shirley and Hall 1977), determines temperature from the relative intensity ratio of the integrated Stokes vibrational Q- branch to that of the anti-Stokes vibrational Q-branch; and 4) the contour-fit of the Raman bandshape method, as shown for N2 (Lapp 1974, Boiarski et al. 1978, Stephenson and Blint 1979, Schoenung and Mitchell 1979, Wehrmeyer et al. 1992a, Gillespie 1993, and Cheng et al.
2002), O2 (Schoenung and Mitchell 1979, Stephenson and Blint 1979, Jones et al. 1996, and de Groot 1998), CO2 (Stephenson and Blint 1979, Blint et al. 1980, and Stephenson 1981), H2O (Bribes et al. 1976 and Stephenson 1981), and H2 (Lapp 1974, Shirley 1990, Wehrmeyer et al.
2002, and Kojima and Nguyen 2004), measures temperature by least squares fitting theoretical spectra at specified temperatures with experimental spectra. Temperature measurements using these Raman bandshape methods are accurate to better than ~5% and are often in good agreement with independently measured temperatures from thermocouples and adiabatic equilibrium calculations.
Of the Raman bandshape methods, the contour-fit method is the most accurate and simplest method because it is based on direct comparison with theory. The band area and band peak intensity methods are based on the relative ratios of scattering from the vibrational branches to derive temperature, which requires extrapolation of the contribution of the individual vibrational branches from the total signal to derive temperature. In vibrational Raman scattering, the rotational fine structure of lower lying vibrational states produces scattering that overlaps with higher vibrational states. This makes accounting for the scattering from each vibrational state difficult and is a significant source of error in these measurements. The Raman scattered signals in the Stokes/anti-Stokes intensity method must be detected over a wide spectral range and, in general, require the use of two separate collection systems. Thus, determination of the relative scattering intensity ratio, hence temperature, is dependent upon accurate characterization of the response of the two collection systems. Additionally, detectability is another concern because the anti-Stokes scattering process is considerably weaker than the normal Stokes Raman signal.
In the works previously mentioned, the contour-fit Raman bandshape methods have been applied solely to laboratory flames because of equipment limitations. During the era in which Raman bandshape methods received much attention for thermometry, the state-of-the-art in spectroscopy equipment were continuous wave and low-power long pulse-length visible lasers and scanning monochromators using photo-multiplier tubes for photon counting devices. As a result of the equipment, the measurements had to be time-integrated (in cases where extremely high-resolution is required, such as for N2 and O2, acquisition of a single spectrum could take over one hour).
Practical application of the contour-fit Raman temperature method can be realized with the advent of modern photon counting devices like photo-diode arrays (PDA) and gated- intensified charge coupled device (CCD) cameras. Jones et al. (1996) and de Groot (1998) were able to obtain vibrational O2 temperature measurements in a lab-scale rocket thruster operating at extremely high oxidizer to mass mixture ratios (MR = 13-65, O2-rich); however, the measurements were still time-integrated from multiple laser pulses because low peak-power long pulse-length visible laser were used to induce the Raman scattering. Kojima and Nguyen (2004) were able to use the rotational H2 Raman spectrum as a thermometer for measurements in a high- pressure combustion cell. The measurements were time-averaged (~700 laser pulses) because of the low-power long pulse-length visible laser. The development of high-energy short pulse- length UV lasers has allowed single-laser pulse, contour-fit temperature measurements to be obtained. Gillespie (1993) used contour-fitting of the N2 Raman spectrum in conjunction with a KrF excimer laser and PDA to obtain single-pulse temperature measurements in the stagnation chamber of a shock tube. The contour-fit method using modern UV Raman systems has also been demonstrated in adiabatic laboratory burners by Cheng et al. (2002) for single-pulse N2
temperature measurements up to 30 bar and Shirley (1990) and Wehrmeyer et al. (2002) for H2
temperature measurements. The work of Shirley and Wehrmeyer et al. identified the potential of H2 thermometry but exact quantitative measurements were prevented because of limited spectral resolution.
1.3 Present Raman Work
In advanced rocket engines fueled with H2-O2 propellants, the system is operated in excess H2 conditions for reasons of safety and performance. Thus, the main species in the combusted gas that may serve as temperature-sensitive thermometers are H2 and H2O. H2 is the preferential temperature indicating species because it is present throughout the combustion process (as a reactant and as an unburnt product species), its molecular structure is well characterized, and detection is much simpler than for H2O or other polyatomic and diatomic species since its Raman scattering cross-section is large and the vibrational energy levels are widely spaced.
Limited applications of Raman scattering for temperature measurements in H2-O2 fueled high-pressure rocket engine-like test articles have been reported (Farhangi et al. 1994, Jones et al. 1996, de Groot 1998, Wehrmeyer et al. 2001, and Yeralan et al. 2001).
Farhangi et al. and Yeralan et al. were able to obtain single-pulse temperature measurements in a fuel-rich lab-scale thruster; however the temperatures were derived with traditional Raman scattering methods using visible lasers. As previously noted, Jones et al. and de Groot used the contour-fit Raman bandshape method for O2 temperature measurements in a lab-scale rocket thrust chamber, but the operating conditions (O2-rich) were not representative of practical rocket engines. The only work to take advantage of UV laser excitation, while studying the feasibility of UV Raman scattering, is Wehremeyer et al.; however, the rocket chamber was O2-rich and the measurements were strictly qualitative. The results from Wehrmeyer et al. suggest that the Raman scattered signal is strong enough to merit spatially-resolved single laser-pulse temperature measurements in high-pressure rocket engines.
Thus, the focus of the present work is to demonstrate the potential of H2 Raman contour- fitting for practical thermometry applications in high-pressure H2-O2 fueled rocket engines. To realize this goal, a detailed H2 Raman scattering model is developed using fundamental Raman scattering principles and then experimentally validated in laboratory conditions. A Raman system employing a UV laser source to enhance the scattering activity is constructed to afford single laser-pulse temperature measurements. The Raman system is used to probe H2/N2 heated mixtures and steady adiabatic laminar flames. Temperatures are derived from comparison of the experimentally obtained spectra with simulated spectra using non-linear least squares parameter estimation methods. A detailed uncertainty analysis using Monte-Carlo methods is performed to determine the accuracy of the measurement technique. Results of the first-reported successful application of H2 Raman contour-fit thermometry are reported. The H2 Raman thermometer is applied during the fuel-rich hot-fire testing of a rocket engine-like test article in an attempt to demonstrate the technique at high-pressure.
1.4 Organization
To facilitate the reader and provide a better understanding of the goals for the current work, the organization of the dissertation is presented. The fundamentals of Raman scattering are discussed in Chapter II. Details of the UV Raman scattering system and measurement
conditions are described in Chapter III. A description of the Raman scattering model, including fitting procedures and uncertainty analysis techniques, is presented in Chapter IV. Results from laboratory testing and an application in a rocket engine-like test article are shown in Chapter V.
Conclusions on UV-induced vibrational H2 Raman thermometry are summarized in Chapter VI.
