Coherent Laser Diagnostics for Temperature/Species Measurements in Advanced Engines

Abstract

Nonintrusive laser diagnostics are very attractive for providing spatially-and temporally-resolved measurements of temperature and species concentrations in combustion systems. Practical applications in actual or simulated engines require strong techniques capable of discriminating against the high interference environments typically found in such devices. Coherent laser techniques are particularly well-suited to engines since they produce strong signals in laser-like beams. The coherent or beam-like nature permits full capture of the signal from small optical apertures with extremely good background suppression. This paper reviews coherent anti-Stokes Raman spectroscopy (CARS) and resonant degenerate four wave mixing (DFWM). The two techniques are complementary in regard to species sensitivity; CARS is most suited to thermometry and major species, while DFWM is capable of minor, radical species measurements. CARS is quite well developed and is illustrated with applications to solid propellant combustion, turboramjet combustion and scramjets. DFWM is developing and appears extremely promising for combustion applications.