Inlet mixing and NO x formation in a helmholtz pulse combustor

Abstract

The inlet mixing field and combustion processes have been investigated in a fully premixed, methanefired (10 kW), self-aspirating, Helmholtz pulse combustor. The modification of the inlet mixing field by the variation of the streamwise position of a stagnation plate was investigated and related to the resultant NO x generation. Time-resolved, laser-sheet flow visualization images are presented of the flow structure within the combustion chamber. The combustion event was investigated using cycle-resolved chemiluminescence imaging. The inlet mixing was found to be dominated by the formation of two counterrotating toroidal vortices formed by the expansion geometry of the inlet and the transient nature of the inlet flow. The inlet geometry consisted of an expansion into the combustion chamber and a bluff body obstacle formed by the stagnation plate. The vortices formed by this inlet configuration were responsible for the rapid mixing between the incoming reactants and residual gases from the previous cycle. It was found that mixing at the inlet decreased as the stagnation plate was moved downstream into the combustion chamber. The observed modification of the reactant/residual mixing field was consistent with the measured trends in the combustor NO x emissions. It is demonstrated that the reduced mixing associated with moving the stagnation plate away from the inlet was consistent with the measured trends of rising NO x emissions and decreasing NO 2 /NO x ratio.