Computational Studies of Premixed Flame Characteristics with Surface Effects.

Abstract

As a fundamental study to understand characteristics in catalyst-assisted combustion, numerical simulations of two canonical models are performed.

For a stagnation-point flow combustor with a catalytic surface, parametric studies are conducted to investigate the effects of strain rate, equivalence ratio and heat loss on the combustion and extinction modes. The steady results showed that catalysis largely extends the strain-induced extinction limit, while suppressing the gas phase reaction at lower strain rates. The temperature versus strain rate response curves exhibit multiple branches of stable solutions, implying a possibility of hysteresis behavior in a coupled homogeneous-heterogeneous reactor. In the lean extinction limit investigation, results show that the level of flammability extension by surface reaction depends strongly on the mixture dilution, such that the benefit of catalyst-assisted lean combustion can be fully realized only with a diluted system. These observations are explained by consideration of characteristic time scales calculated from the fuel consumption rate. The extinction response to an oscillatory strain rate also shows consistent behavior. Experimental validation was carried out to demonstrate simulation conclusions. It was found the substrate surface heat loss condition is a key factor to match simulation result with experimental results. Radiation and conductive heat loss through the surface in the experiment could be modeled as one effective heat loss coefficient, and that coefficient is relevant to both the flow rate (related with strain rate) and the surface temperature at that time. Thus an iteration process could be used to find the extinction condition for a fixed flow rate.

Parametric studies on heat loss, equivalence ratio and device dimension for micro-channel flow configuration are conducted to research the combustion stability at near quenching conditions. A parallel DNS code supplemented by flame position capturing ability is applied. The results show the flame quenching condition is mainly affected by channel width and equivalence ratio at current parameter range. Channel wall heat loss shows influence on flame propagation speed but not on quenching conditions. Flame shape is affected by both equivalence ratio and heat loss.

This research is expected to provide insight into improving the combustion stability and efficiency of catalyst-assisted combustors.