Influence of Swirl Number on NO_X Formation in a Turbulent Non-Premixed Flame

A numerical simulation of swirling methane/air non-premixed flame (TECFLAME swirl burner) in a two-dimensional model combustion chamber is carried out to assess the influence of entrance flow swirl number on temperature distribution, flow behavior and NO pollutant formation. A Finite Volume staggered grid approach is employed to solve the governing equations. The second-order upwind scheme is applied for the space derivatives of the advection terms in all transport equations. The eddy dissipation-finite rate model is employed to predict the heat release and the Reynolds stress turbulence model is applied to simulate the flow behavior. NO formation is modeled as a post-processing solution. NO formation prediction has reasonable agreement with experiments for smaller swirl numbers but with the increase of swirl number, deviations between numerical results and the experimental data increase. It may be due to incorrect prediction of temperature distribution in higher swirl numbers. With the increase of swirl number, maximum temperature of chamber decreases from 1900 (K) to 1650 (K). With temperature decline, NO concentration in the exhaust decreases from 27 (ppm) at swirl number of 0.7, to 4 (ppm) at swirl number of 2. On the other hand, with increase of swirl number, ratio of prompt NO formation to thermal NO increase rapidly. In another word, with decrease of flame temperature, prompt NO roles increase noticeably.