Combustion Efficiency of a Premixed Continuous Flow Combustor

Experimental data in the form of radial profiles of mean temperature, gas composition and velocity at the combustor exit and combustion efficiency are reported and discussed for a swirling flow, continuous combustor. The combustor is composed of two confined, concentric independently swirling jets: an outer, annular air jet and a central premixed fuel-air jet, the fuel being propane or methane. Combustion is stabilized by a swirl-generated central recirculation zone. The primary objective of this research is to determine the effect of fuel substitution and of changes in outer flow swirl conditions on combustor performance. Results are very similar for both methane and propane. Changes in outer flow swirl cause significant changes in exit profiles, but, surprisingly, combustion efficiency is relatively unchanged. A combustion mechanism is proposed which qualitatively explains the results and identifies important flow characteristics and physical processes determining combustion efficiency. It is hypothesized that combustion occurs in a thin sheet, similar in structure to a premixed turbulent flame, anchored on the combustor centerline just upstream of the recirculation zone and swept downstream with the flow. Combustion efficiency depends on the extent of the radial propagation, across mean flow streamtubes, of this reaction sheet. It is concluded that, in general, this propagation and hence efficiency are extremely sensitive to flow conditions.