Advanced Laser Diagnostics for Quantitative Species Measurements in Turbulent Combustion

Jet engines and other turbine combustors operate at high pressures, with turbulence length and time scales on the order of 100 um and 100 us. Accurate experimental characterization of turbulent reacting flows is critical for validating computational fluid dynamics and chemistry codes used in the design of advanced turbine combustors. Laser-based diagnostics are typically used to characterize velocity, temperature, and concentrations of key chemical species in flames. However, most diagnostics in current use are not capable of resolving all of the time and length scales of interest. We propose to develop new diagnostic tools and techniques that take advantage of the characteristics of high-power, ultrashort pulse, mode-locked Ti:sapphire lasers. Phase I emphasis will be placed on the development of a spatially resolved pump-probe absorption technique to determine absolute number densities of the free radical CH, a key trace species in flames. Further Phase I work will study the use of picosecond degenerate-four-wave-mixing (DFWM) for CH number density measurements. This background-free technique promises to have greater sensitivity than absorption based techniques. Measurement of OH and NO using these techniques will require the third harmonic of Ti:sapphire. Determining the design for laser systems to measure these trace species will lay the foundation for the Phase II effort.