Combustion in meso-scale vortex chambers

Vortex flows were utilized as a means to stabilize gaseous flames in meso-/micro-scale non-premixed combustors for use in small-scale power and propulsion systems. Scaling studies were performed with a series of combustors ranging in size from ∼10.6 to 124 mm³ (i.e., combustor diameters of 2.4-6.4 mm). Three-dimensional modeling of reacting flows was also conducted to explore flame stabilization mechanism and flow evolution in the chamber. Combustor performance was evaluated by analyzing the stability limit and chemical efficiency. Both hydrogen and hydrocarbons (methane and propane) have been studied with the chemical energy varying from 25 to 174 W. For the largest combustion volume, hydrogen and hydrocarbons burned efficiently in air. However, for smaller volumes, oxygen enrichment in air was required to stabilize hydrocarbon flames. Flame stabilization was achieved by a large, relatively quiescent, hot core flow that was formed by the flow recirculation at the head- and tail-ends of the combustor. The stability limits for hydrogen/air mixtures, in terms of the overall equivalence ratio, were found to be ~0.25 on the lean-side and in the range of 3-6 on the rich-side. The corresponding chemical efficiency exceeded 97%. For propane/air combustion, the stability limits ranged from ∼0.25 to 2 for the 124 mm³ combustor. Methane was the most difficult fuel to stabilize the flame and required oxygen-enriched air containing 40% O₂ and 60% N₂ by volume to operate in the 10.6 mm³ combustor with a chemical efficiency of more than 85%.