Electronic-Resonance-Enhanced CARS for Quantitative in-situ Nitric-Oxide Measurements in High-Pressure Combustors

The primary objective of this Phase-I effort is to develop an advanced coherent anti-Stokes Raman scattering (CARS) technique for quantitative concentration measurements of nitric oxide (NO) in high-pressure flames. The objectives of the proposed work are (1) to develop and demonstrate electronic-resonance enhanced (ERE) CARS for measurements of NO in high-pressure flames, (2) to compare ERE-CARS and laser-induced fluorescence (LIF) measurements, and (3) to develop a computer code for interpreting measured signals and quantifying NO concentrations. The proposed method has three very significant advantages over the conventional LIF-based measurements of NO: (1) in ERE-CARS the signal comes out as a laser-like beam, (2) for a given temperature and NO mole fraction, the LIF signal will remain the same or decrease slightly as the pressure increases whereas in ERE-CARS that the signal will increase as the square of the pressure as the pressure increases, and (3) CO2 and H2O will not interfere with the ERE-CARS signal, because the signal is generated with significant intensity only when the conditions of Raman resonance and electronic resonance are simultaneously met. Consequently, the ERE CARS technique offers the potential for much higher selectivity compared to LIF detection of NO in high-pressure flames.