Modelling of ammonia/air non-premixed turbulent swirling flames in a gas turbine-like combustor at various pressures

This study simulated bluff body stabilised non-premixed turbulent ammonia (NH3)/air flames using swirling flows. Although the space and time-averaged emission (STAE) of NO was found to decrease with increase in the global equivalence ratio (global f) under stoichiometric and rich conditions, similar to the behaviour of premixed flames, the local NO concentration distribution within the combustor was heterogeneous, unlike the almost uniform NO distribution in premixed flames. Zones of high NO concentration were identified near the combustor wall boundaries, whereas there was almost nil NO in the combustor centre, irrespective of the global f. The localised NO concentrations in non-premixed flames were shown to depend on the local f within the combustor and the model reproduced the NO emission characteristics of premixed flames in terms of f. This study also found that the effect of pressure on NO emissions is significant irrespective of the combustion type, because reducing the OH radical concentration in NH3/air flame at high pressure limits the NO generation. However, the STAE of NO of non-premixed flames were slightly higher than those of premixed flames, especially under rich conditions, possibly owing to zones of elevated NO concentrations near the wall boundaries. The introduction of an additional NH3 stream external to the air stream (by splitting the original NH3 flow) may mitigate this effect by creating an almost uniform f distribution within the combustor and thus a homogeneous NO distribution. The STAE of NO was minimised (and equalled that of a premixed flame) by applying a volumetric flow ratio between the innermost and outermost NH3 flows of 0.6: 0.4.