Combustion, NOx formation and mixing processes in Helmholtz pulse combustors
This thesis presents a laser diagnostic investigation into the combustion, NOx formation and mixing processes occurring within the optically assessed combustion chamber of a methane-fired (10kW), fully premixed, self-aspirating, Helmholtz pulse combustor. The inlet geometry of the combustion chamber consisted of a step expansion and a bluff body obstacle formed by a stagnation plate. The focus of the investigation was the effects of the stream-wise position of the stagnation plate on the pulse combustion processes. A comprehensive parametric study of the performance of the pulse combustor is presented with stagnation plate position, air/fuel ratio and tailpipe length as the variables. The operating frequency and peak pressure amplitude trends were found to vary in accordance with the Rayleigh criterion. The operation of the combustor was more stable with the effective heat-release point preceding the resonant acoustic peak. Operation outside of this regime produce increased levels of CO. Time-resolved, laser-sheet flow visualisation images are presented of the flow structures within the combustion chamber. The inlet mixing - between the reactants and residual gases - was dominated by the formation of two counter-rotating toroidal vortices. In general, the inlet mixing was found to decrease as the stagnation plate was moved further into the combustion chamber. However, other mechanisms that tended to counter this trend were observed. Under certain conditions, significant flow reversals were imaged with gases penetrating the combustion chamber from the tailpipe. The combustion event was investigated using cycle-resolved chemiluminescence and laser induced fluorescence imaging of OH* radicals. Ignition of the fresh reactants by residual combustion/radical activity was found to occur along the interface between reactants and residual gases. The increase in reaction zone area generated by the action of the toroidal vortices provided the necessary mechanism for the rapid combustion of the reactants. The reduced mixing associated with moving the stagnation plate further into the combustion chamber produced a more compact combustion zone with less interaction between combusting reactants and cooler residual gases. This modification to the combustion zone was consistent with the measured trends of rising NOx tailpipe emissions and decreasing N02/NOx ratio. Under certain conditions, a reversal in the NOx and N02/NO, ratio trends was observed. This was explained by an augmentation of heat transfer rate out of the combustion chamber, characterised by increased flow reversal strength, which lead to cooler residual gases. Additional mechanisms, which modified the inlet mixing process, were also identified as contributing to the reversal of the NOx trends.