Numerical Investigation of the Effects of Swirling Hot Co-Flow on MILD Combustion of a Hydrogen–Methane Blend

This paper examines the effects of swirl hot co-flow on the combustion behavior of a moderate or intense low oxygen dilution (MILD) burner fueled by a mixture of methane and hydrogen. Toward this goal, the realizable k-ɛ turbulence model, GRI. 2.11 reaction mechanism, and the discrete ordinates radiation model are incorporated into a computational modeling of the reactive flow. The numerical results are, first, favorably compared against the existing experimental data. Subsequently, a number of swirl co-flows are implemented, and structures of the resultant reactive flows are investigated systematically. The outcomes indicate that increasing the swirl velocity leads to the reduction of ignition delay and significantly enhances the reaction completion. The analysis of the spatial distribution of hydroxyl and formyl (OH and HCO) radicals reveals that swirling MILD combustion radially extends the reaction zone in comparison with the conventional MILD combustion. Yet, it reduces the length of the reactive region and allows for the occurrence of heat release in a shorter axial distance from the outlet fuel nozzle. Further, the addition of swirl reduces the production of carbon monoxide through its influences upon flow temperature and generation of formyl radical. However, it is found that swirling hot co-flow intensifies NOx emissions by strengthening of prompt and thermal mechanisms of NOx production. Reducing the temperature of the recycled flue gas is deemed to be an effective way of resolving this issue.