Modeling of nitrogen oxide formation in turbulent flames: Development of reduced mechanisms and mixing models

Development and application of reduced mechanisms and mixing models are conducted for predicting nitrogen oxides (NO{dollar}sb{lcub}rm x{rcub}{dollar}) formation in turbulent flames. Several reduced mechanisms are derived from the detailed hydrogen-air and methane-air combustion chemistry with the aid of an interactive computer code developed for automatically generating reduced mechanisms. The performance of reduced mechanisms is evaluated by comparisons with detailed chemistry under various simple laminar and turbulent flames. The use of reduced mechanisms in place of detailed mechanisms shows a significant reduction in computing demands. Accurate predictions can be achieved when reasonable reduced mechanisms are used.; Three mixing models are incorporated into a Partially Stirred Reactor (PaSR) code: the modified Curl's model, the Interaction-by-Exchange-with-the-Mean (IEM) model, and the linear-eddy model. Numerical simulations show that the unmixed nature of fluids has profound influence on the NO formation in PaSR with hydrogen and methane combustion. Results of the scalar mixing cases indicate that the preferential diffusion effect decreases as the Reynolds number increases, and has a stronger influence on the instantaneous thermo-chemical properties than on the mean statistics. For hydrogen reacting flows, statistical details of the mixing models are examined to explain differences in their predictions of NO concentrations at the same unmixedness in a PaSR.; Hydrogen reduced mechanisms are applied to the predictions of NO{dollar}sb{lcub}rm x{rcub}{dollar} emissions from turbulent diffusion jet flames with various amounts of helium dilution. Analysis of simulation results obtained with a detailed chemistry shows that the partial equilibrium assumption can break down in a turbulent flame even at temperatures as high as 2,000 K under intense turbulent mixing. Increasing the inert gas content in the fuel jet increases the departures from the partial equilibrium state. The steady state assumption is found reasonably good for intermediate species at temperatures above 1,200 K Measured NO{dollar}sb{lcub}rm x{rcub}{dollar} emissions are found to scale with the square root of the Damkohler number as EINOX = {dollar}rm 0.09{lcub}Lsb{lcub}v{rcub}sp3/(Usb{lcub}rm jet{rcub} dsp*sb{lcub}jet{rcub}sp2)/tausb{lcub}NO{rcub}{rcub}sp{lcub}1/2{rcub}{dollar} over four decades. This scaling relation is reproduced by the joint scalar Probability Density Function (PDF) model when better reduced chemistry is used.