Optimization of CH Fluorescence Diagnostics in Flames: Range of Applicability and Improvements with Hydrogen Addition

This study quantifies the range of premixed flame conditions for which CH fluorescence diagnostics are applicable, and it shows that the CH fluorescence signal can be increased if some of the hydrocarbon fuel is replaced with hydrogen. The CH fluorescence signal is found to be adequate for fuel-air equivalence ratios ((phi)) as small as 0.85 for both methane-air and propane-air flames. The CH signal increases until a maximum at (phi) = 1.25 and (phi) = 1.35 for methane-air and propane-air flames, respectively, and then decreases for richer conditions. A strategy to increase the CH fluorescence signal and decrease interference from soot precursors is proposed by addition of the proper amount of hydrogen to the hydrocarbon fuel. Hydrogen addition reduces the background signal from soot precursors by as much as a factor of 10 and increases the CH fluorescence signal by as much as 80%. The normalized CH fluorescence measurements are compared with computations that utilize GRI-mech 3.0 chemistry. Sources of experimental uncertainties are discussed.