Thermometry in high pressure soot-laden reacting flows via non-resonant laser-induced gratings

Abstract

We are currently investigating the use of transient grating spectroscopy as a diagnostic for applications in high-pressure combustion. We have recorded nonresonant, laser-induced transient grating signals in pressurized, sooting flames. The signals are recorded in a time-resolved manner. Analysis of the corresponding power spectrum permits the extraction of the local temperature and pressure. I. INTRODUCTION The temperature field of combustion environments reflects the complex interaction of the local chemistry, fluid dynamics and heat transfer of the combustion gases. For the purposes of testing combustor design and the verification of predictive computer models of combustors, it is imperative that accurate temperature measurements be made. Major challenges to optical diagnostics of practical combustors include high pressure, the presence of participates (particularly soot), a high background luminosity, optical thickness and limited optical access. To address these challenges and provide quantitative diagnostic information on practical combustors, we are exploring non-resonant transient grating spectroscopy (TGS) for the point-wise measurement of temperature and pressure in highpressure, sooting flames. In short, the TGS technique involves the first-order Bragg scattering of a probe laser off of a grating induced by two crossed pump laser beams.1'2 Historically, transient grating techniques have been used to explore many timedependent phenomena in a wide variety of media.3'4 Recently, investigators have applied the technique to gas-phase studies relevant to the combustion community. Experimental work has been performed in static gases at high pressure,5'6 in the infrared7 and in atmospheric flames,8 and several groups have modeled9'10'n-12-13-14-15 the transient grating signal. The interference grating of the spatially and temporally overlapped pump beams induce a modulation of the local index of refraction which subsequently scatters the probe beam. Several mechanisms can produce the index modulation, two of which are important here. In the absence of a molecular or broadband optical absorber, electrostriction produces the desired index change. When soot is present, it absorbs light, heats up vaporizes, and the vapor particulates heat the surrounding gas producing an index modulation. The probed fluid responds hydrodynamically and attempts to recover local equilibrium via thermal diffusion and acoustic motion. Typically, the transient grating signal is recorded in a time-resolved manner. The temporal behavior of the signal is a function of the local temperature and transport properties. It appears as an acousticallymodulated slow decay. Determination of the acoustic oscillation frequency of the signal provides a measurement of the local temperature. This analysis is most often achieved by fitting the acquired signal with the theoretical expressions found in the literature. The model expressions are now quite