Laser Diagnostics for Flowfields, Combustion, and MMD Applications.

Abstract

HE appearance of the laser and its introduction into the field of spectroscopy was a turning point in the development of light-scatter ing diagnostic techniques. In a relatively short period of time, laser-based diagnostic techniques emerged as major investigative tools in a number of branches of the physical sciences. In this paper the application of laser-based diagnostic techniques to flowfields in combustion systems and magnetohydrodynamics are of interest. Hence, only some of the techniques that have been successfully applied and that present some potential promise for these investigations will be discussed. A number of light-scatter ing processes have been considered for the analysis of flowfields and combustion systems. Among those extensively investigated have been elastic scattering process such as Rayleigh1'3 and Mie4 and inelastic scattering processes such as Raman,5'17 near resonant Raman,6'7 and fluorescence.18-20 Other techniques that can be utilized in combustion and flowfield diagnostics are the absorption and nonlinear optical processes. The latter are represented by coherent anti-Stokes Raman21-27 and stimulated Raman scattering.29'30 From this list of potential diagnostic techniques applicable to flowfields and combustion, the Mie and spontaneous Raman scattering techniques are the most versatile. The Mie scattering phenomenon has been utilized in laser Doppler velocimetry (LDV)31'32 and is capable of nonintrusively providing measurement of velocity, turbulent intensity, and particle size distribution in flowfields. The spontaneous Raman effect can simultaneously, remotely, and instantaneously provide the species concentrations and temperatures of a flowfield consisting of any number of species. In addition, when properly used, it can provide local turbulence properties, correlation and cross-correlation parameters, and the socalled mixedness or unmixedness parameters in reactive flows.33 However, due to the very low equivalent scattering cross-section occurring under certain conditions in hydrocarbon turbulent combustors, difficulties may develop in securing reliable measurements. These difficulties are related to the very high noise level generally attributed to carbon emissions. The signal-to-noise ratio under those conditions may become unacceptably low, thus making the utilization of the spontaneous Raman technique very difficult. Here the coherent anti-Stokes Raman spectroscopy (CARS) appears to fill the gap. The equivalent scattering cross section, in conjunction with the coherence of the radiation, combines to provide signals five to six orders of magnitude higher than the spontaneous Raman effect. In addition to the collection of the total generated signal, its coherent character permits the simultaneous suppression of the collected interference signals, resulting in high signal-tonoise ratios in very hostile environments. One of the major drawbacks of CARS is its nonlinear character, which may cause difficulties in a number of situations. A process that holds great promise for flowfield and combustion diagnostics has been recently demonstrated.28 This process, called stimulated Raman spectroscopy (SRS), has been known for over a decade34'35 and was applied in the first practical demonstration of the collinear CARS system. It has been used with high-power pulsed lasers and continuous wave (CW) low-power lasers.29 Being of a coherent nature, the SRS signals may under certain conditions exceed the strength of the CARS signals, with the added advantage of being linearly dependent on the power input and self-phase matched. In terms of high signal response, another technique known for several decades is the fluorescent diagnostic method. Here the major interfering phenomenon of the spontaneous Raman technique is being utilized as a diagnostic technique. This technique, in spite of its very high signal levels, has not been very successfully applied until recently. The major obstacle has been the strong collisional quenching process associated Samuel Lederman received his undergraduate education in Munich, Germany, and his graduate education at the Polytechnic Institute of Brooklyn. Since 1952 he has been associated with the Department of Mechanical and Aerospace Engineering and is currently a Professor of Mechanical and Aerospace Engineering. His research and experience cover a wide range of interests. Some of the major contributions were in the field of microwave plasma diagnostics, electrostatic probe theory and technique mass spectrometry, laser Raman spectroscopy, and in general, laser-based methods and techniques applicable to remote measurements of flowfields, combustion, magnetohydrodynamics, air pollution etc. He is an author or co-author of over 80 publications.