Nitric oxide levels of turbulent jet diffusion flames: Effects of residence time and damkohler number

Abstract

The global residence time and the deviations from chemical equilibrium (i.e., the Damkohler number) were varied for a number of jet diffusion flames. The resulting effects on the nitric oxide emission index were measured and were compared with existing analysis. The global residence time is defined as Lf/UF, where Lf is the flame length and UF is the fuel jet velocity. Flame length is varied by increasing the jet diameter, by adding either premixed air or inerts to the fuel jet, or by adding a coaxial air stream. In particular, a unique jet flame was studied that is composed of helium---diluted hydrogen fuel; this flame is free of the complicating effects of flame radiation, buoyancy, and prompt NO and provides a useful baseline comparison to theory. It is found that NOx levels for three types of fuels were consistently less than levels predicted by thermal theory, which suggests that one or both of the two mechanisms that suppress NOx, namely strain and radiative cooling, are important. The use of a Damkohler number was found to successfully correlate the NOx data for the hydrogen/helium-air flames that have simple chemistry. As the helium concentration is increased in order to reduce the Damkohler number, the measured NOx emission index exceeds that of the equilibrium theory by as much as a factor of 24, which is further indication that it is important to add the correct nonequilibrium oxygen atom chemistry to current models.