Laser-induced fluorescence of seeded nitric oxide as a flame thermometer

Gas temperatures deduced from one-line and two-line laser-induced fluorescence (LIF) measurements of NO seeded into low-pressure methane flames are compared with those determined from LIF measurements of OH rotational distribution. The purpose is to make a quantitative assessment of the accuracy of one-and two-line LIF temperature strategies. Temperature determination from NO LIF intensity from a single transition must accurately account for any variation in the mole fraction of seeded NO. In the hot post-flame region. this added NO is diluted by the thermal equilibrium of radical species and incomplete combustion; this dilution depends on gas temperature and fuel/air stoichiometry. Chemical reactions of the NO produce an even larger variation in the NO mole fraction. Reburning of NO in the flame front (via reaction with CH) reduces the added NO by as much as 16% in the burnt gases of a slightly fuel-rich (Phi = 1.07) methane/air flame. The size of the reduction depends on both the amount of NO addition and the fuel/air stoichiometry. This effect can alter the apparent temperature determined from one-line NO LIF by as much as 25%. Two-line LIF strategies for gas temperature avoid these complications. Temperatures determined from all three LIF methods show good agreement in low-pressure flames, when a detailed model of the chemistry is used to correct the one-line NO temperature. However, in reactive systems where a modal of the chemistry cannot be applied, one-line LIF strategies for gas temperature from seeded NO are not reliable.