Applications of Transient Grating Spectroscopy to Temperature and Transport Properties Measurements in High-Pressure Environments
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Date
2001-05-25
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Transient Grating Spectroscopy (TGS) is a four-wave mixing technique that involves the first-order Bragg scattering of a probe beam off of a dynamic grating generated by two crossed pump laser beams. The amplitude and temporal behavior of the spatially coherent signal beam reflects the physical and chemical dynamics of the investigated medium. The temporal behavior of the signal is a function of the local temperature and transport properties. The TGS signal is shown to grow with increasing pressure and is spatially coherent. TGS thermometry is frequency based rather than amplitude based. These characteristics make the TGS technique very promising for thermometry in high pressure environments.
TGS thermometry experiments were conducted in both laboratory-scale and practical scale high-pressure combustors. Algorithms for analyzing the TGS signal were developed and shown to work efficiently at calculating temperature real time. Temperature was successfully measured under various flame conditions (pressure, equivalence ratio, soot volume fraction) in the laboratory-scale combustor, and the results were consistent with thermocouple readings. Progress was made towards applying TGS thermometry to practical-scale combustors. Suggestions for future efforts were given.
By curve-fitting the TGS signal acquired with an unfocused beams geometry, the acoustic damping rate of various gas samples (Ar, N2, O2, CO, CO2, CH4, C2H4, etc.) was measured at various pressures (up to 25 atm). The acoustic wave was found to decay faster than that predicted by the classical theory, except for atomic gases. Molecular absorption of sound wave energy becomes important for molecules with complicated structures and becomes more important with increased pressure. Results from Ar/He mixtures indicate that collisions between atoms with widely different molecular weights may enhance the redistribution of translational energy, and thus accelerate the acoustic wave damping.
The TGS signal from a very rich CO/air mixture shows a fundementally different temporal behavior. A photo-chemical reaction appears to be initiated by the crossed pump beams at the spatial intensity peaks. The liberated heat generates very strong density gratings and thus a very strong TGS signal. The relatively slower heat release due to photo-chemical reactions needs a longer time to establish the density grating. The temporal behavior of the TGS signal is determined by the photo-chemical reaction rate, which is found to be influenced by pump beam energy, pressure (molecular collisional rate) and O2 concentration.
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PhD
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Mechanical Engineering
