A Study of Combustion in Supersonic Streams

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Supersonic combustion of reactive aluminum alkyl fuels has been experimentally demonstrated in two-dimensional ducted combustors and adjacent to a flat plate. Fuel was injected from the combustor walls through multiple orifices and ignited spontaneously. Stable supersonic heat release was maintained as evidenced by schlieren and direct motion pictures of the flow field and deduced from static and pitot pressure measurements in the combustion zone. The results of the ducted combustor tests were correlated with elementary one-dimensional and pseudo-one-dimensional theoretical models of the flow field. This agreement permitted a reason.able determination of combustion efficiency to be made. In the ducted combustor tests a favorable effect of preheating the fuel to approximately 250°F was noted and a simple empirical factor was found which satisfactorily correlated all of the data for the range of conditions tested. A theoretical model of constant pressure heat release on a flat plate in supersonic flow is postulated. Normal force coefficients and specific impulse values are tabulated for a variety of flight Mach numbers and altitudes. Additional refinements in this theoretical model were required to adequately describe the experimental results. In a test simulating Mach 5 flight at 66,000 feet altitude a side force specific impulse of 1350 seconds was measured at equivalence ratio of one. Combustion was only partially completed 12 inches downstream of fuel injection. Based on the theoretical mode l an additional 12 inches of combustor length and 36 inches of expansion length would be required to obtain the estimated theoretical impulse of 5760 sec. The interaction of a vaporizing liquid droplet with a supersonic stream is considered. Additional refinements were made in the existing theories on droplet trajectory to include the influences of a separated zone and the normal component of velocity of the external stream. Calculations of the trajectory and evaporation of the estimated mean droplet size based on the modified technique were in general agreement with the observed flame zone and deduced combustion efficiency.