Nitrogen oxides emissions from buoyancy and momentum controlled turbulent methane jet diffusion flames

Abstract

This paper presents an investigation on unconfined, vertical, turbulent methane jet diffusion flames issuing from a straight tube into quiescent air at atmospheric pressure and temperature, and is focused on NO x emissions. In addition to post-flame NO x emissions, measurements were made for flame geometry (lift-off heights and flame heights) and flame radiant fractions as a means of explaining the observed NO x emission scaling for both buoyancy and momentum dominated turbulent methane jet diffusion flames. The test conditions analyzed covered a wide range of flow conditions varied by changing the nozzle exit internal diameter and the jet exit mean velocity. Three different sized nozzles with inner diameters of 5, 6 and 8 mm were employed and the ranges of jet exit velocities, Reynolds and Froude numbers considered were 9.4–89.9 m/s, 4181–44,547 and 1117–102,738, respectively. The main conclusions of the present study are: (1) the experimental results show that the transition from buoyancy to momentum controlled turbulent jet diffusion flames occurs at a Froude number of about 10 4; (2) in common with previous studies, the results show that for buoyancy dominated flames the main factor that controls the NO x scaling is the flame volume (proportional to Fr 3/5); and (3) in the momentum dominated regime, the EINO x data yield a Froude number dependence of Fr 0.35; in this regime, all flames present constant flame heights and radiant fractions so that departure from the zeroth-order scaling is attributed to chemical non-similarity.