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Abstract
The effects of residence and micro mixing time scales on NOx formation in hydrogen combustion are modeled, using idealized Partially Stirred Reactors (PaSR), with stochastic Monte-Carlo simulations. The explicit dependence on residence and mixing time scales, of the mean and variance of mixture fraction, is derived and verified via the simulations. Transient responses of temperature and species mass fractions are studied as functions of the mean reactor mixture fraction, with varying residence and mixing times. Results of the transient studies are contrasted with steady-state values occurring in continuous combustion in a PaSR under identical conditions. Steady-state temperatures are only marginally higher than their peak values in the unsteady case. The effect of Exhaust Gas Recirculation (EGR) on emissions, particularly NOx, is studied by premixing the oxidizer inlet with exhaust gas. Adding EGR is seen to have an effect similar to that of increasing the mixing time scale. It is reasoned that this is due to faster chemistry occurring at higher levels of EGR, in effect weakening mixing relative to chemistry. A decrease from 3000 ppm to 900 ppm of NOx is predicted as the EGR level is increased from 0 to 40% by volume. This reduction is independent of thermal effects, commonly quoted as the reason for reduction in NOx. The effects of pressure are also studied by varying the pressure from 1 atm to 20 atm. It is found that at pressures higher than atmospheric, an equivalent amount of EGR brings about double the reduction in NOx achieved at atmospheric pressure, being caused by enhanced consumption rates.
This work was in part supported by a grant from the UC Energy Institute's EST Program, for which the authors are grateful. The authors are also grateful to Prof. J.-Y. Chen for helpful discussions.