LASER-INDUCED FLUORESCENCE OF SEEDED NITRIC-OXIDE AS A FLAME THERMOMETER

Gas temperatures deduced from one-line and two-line laser-induced fluo rescence (LIF) measurements of NO seeded into low-pressure methane fla mes are compared with those determined from LIF measurements of OH rot ational distribution. The purpose is to make a quantitative assessment of the accuracy of one-and two-line LIF temperature strategies. Tempe rature determination from NO LIF intensity from a single transition mu st 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 the rmal equilibrium of radical species and incomplete combustion; this di lution depends on gas temperature and fuel/air stoichiometry. Chemical reactions of the NO produce an even larger variation in the NO mole f raction. Reburning of NO in the flame front (via reaction with CH) red uces the added NO by as much as 16% in the burnt gases of a slightly f uel-rich (Phi = 1.07) methane/air flame. The size of the reduction dep ends on both the amount of NO addition and the fuel/air stoichiometry. This effect can alter the apparent temperature determined from one-li ne NO LIF by as much as 25%. Two-line LIF strategies for gas temperatu re avoid these complications. Temperatures determined from all three L IF 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 b e applied, one-line LIF strategies for gas temperature from seeded NO are not reliable.