Knowledge Bank

Theoretical and experimental investigations of the feasibility of using CARS for high pressure combustion diagnostics will be described. An expression has been derived for the third-order, nonlinear electric susceptibility governing vibrational Q-branch CARS generation that is valid for overlapping spectral lines. Experimentally, the CARS spectrum of $N2$ has been investigated as function of pressure in the range one to 95 atmospheres in a high pressure vessel. The experimental signatures show evidence of significant pressure-induced, or collisional narrowing. Theoretical calculations based on the generalized, third-order susceptibility expression are in good agreement with these results, which show that CARS has considerable promise for high pressure combustion diagnosis. Recent demonstrations of CARS thermometry in a sooting flame and a liquid-fuelled combustor will also be reviewed. Theoretical calculations of Q-branch homogeneous linewidths needed for high accuracy in $N2$ CARS diagnostics will also be described.