Flow Characteristics in an Optically Accessible Solid Fuel Scramjet

A direct connect, optically accessible combustor was used to reveal flow characteristics inside a reacting solid fuel supersonic combustor with air inlet total pressure of 12.9 atm and total temperature of 1225 K using polymethyl-methacrylate fuel grains. Flow visualization techniques such as shadowgraph, chemiluminescence, and high-speed imaging/planar pyrometry were used to characterize the flowfield, which has not been previously reported on for a solid fuel scramjet. Shadowgraphs revealed the role of the cavity flameholder geometry in defining the flowfield. Large length to depth ratios force the recirculation zone to expand into the core flow, causing flow biasing. Small length to depth ratios allow the recirculation zone to be fully contained within the flameholding cavity. [Formula: see text] chemiluminescence shows that most of the heat release takes place in the cavity flameholder favoring the upstream region near the air inlet. Both the flow visualization and measured fuel regression rates indicate that the majority of the fuel that contributes to combustion pyrolyzes from the converging angle of the flameholding cavity. Fuel regression rates decrease as fuel is consumed and the cross sectional area of the flowpath increases, consistent with historical observation of solid fuel regression rate dependency on air mass flux.