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AIAA-2004-6850-643 - AIAA 2004-6850 USAF Developmental Test...

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Transient Rarefaction Models for Hypersonic Blowdown Tunnels Kerrie A. Smith * and Mark J. Lewis University of Maryland, College Park, MD 20742-3015 In a hypersonic blowdown tunnel, upstream components may be affected by large pressure drops associated with the tunnel starting process. Two quasi-steady techniques for modeling the pressure drop within the heater vessel of a hypersonic wind tunnel have been investigated. In these techniques, the wind tunnel-heater system is modeled as an incident rarefaction moving through a series of area changes. A pattern of transmitted and reflected shocks and rarefactions is assumed at each area change. The Rankine-Hugoniot equations are used to construct a system of equations adequate to determine the relative pressures through the flow pattern. By combining these techniques, conditions throughout the entire system may be determined. Furthermore, effectiveness of the attenuation provided by a metering orifice is explored for various area constrictions and initial diaphragm pressure ratios. Nomenclature A = cross-sectional area a = speed of sound M = Mach number p = pressure T = temperature u = flow velocity γ = ratio of specific heats I. Introduction P ROVIDING sufficient enthalpy is critical for achieving hypersonic conditions in a high-speed wind tunnel. One approach is the use of a heater. This heater must transfer sufficient energy to the working fluid to prevent condensation during the expansion into the test section, while withstanding the high pressures necessary to provide hypersonic flow. Generally, in wind tunnel analysis, it is treated as a theoretical reservoir, i.e., a constant pressure boundary condition. However, during the start-up process of the tunnel, prior to the establishment of steady flow, the heater may encounter a transient pressure drop. This pressure drop can be detrimental for a number of reasons. Many wind tunnel heaters function by forcing the air through a bed of hot refractory pebbles. A pressure drop across the bed can cause the pebbles to lift and possibly be drawn out of the heater and into the test section. Pressure drop is considered to be the most critical parameter in design of a pebble-bed heater. 1 In facilities where batch heating is used, the pressure vessel must be insulated from the extreme temperature conditions of the test gas, required for hypersonic flow establishment. In such a case, protective liners may be inserted into the heater so that it may operate at temperatures higher than the yield of the pressure vessel material. In this case, pressure differences on either side of the liner can cause it to collapse during the startup process. One protective measure against this transient rarefaction is the use of a metering orifice. The metering orifice works in a number of ways. Primarily, it chokes the flow to Mach 1, limiting the upstream propagation of the expansion. A secondary effect is caused by separation losses as the working fluid moves through the abrupt area change. These losses also decrease the upstream pressure drop.
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