The Hypersonic Boundary Layer Facility
The HyperBLaF is a Mach 8 blowdown wind tunnel used for fundamental studies of compressible turbulence, shock wave/boundary layer interactions, shock/shock interactions and configuration studies of hypersonic vehicles. The working section is circular, with a diameter of 229mm (9"), with an overall length of 2.0m (6 ft). A maximum stagnation temperature of 870K (1100F) at a maximum stagnation pressure of 10 MPa (1500 psia) is possible; run times will vary from approximately 2 to 10 minutes. The tunnel operating conditions were chosen to give a Reynolds number range so that at the lowest Reynolds number the flow will be laminar (even on the tunnel walls), and at the highest Reynolds number fully turbulent boundary layers will be generated on a flat plate mounted in the test section. Even at Mach 8 a Reynolds number based on momentum thickness of about 15,000 seems possible, with natural transition and a highly cooled wall. The key part of the installation is a storage heater which consists of a coil or array of heavy walled stainless steel pipe which is preheated electrically to the desired stagnation temperature, so that the pipe stores the heat and contains the high pressure gas. The vacuum system uses an air ejector driven by the existing high pressure air supply to provide low back pressures. The tunnel can be operated with air or other gases (the mass flow rates are relatively small).
The tunnel is approximately 30 ft (9 m) long, not including the assembly used to heat the incoming flow. The high pressure air supply is connected to the inlet of the heater assembly by a valve which expands the air from 3000 psi (20 MPa) to 1500 psi (maximum) through 250 psi (minimum) depending on the Reynolds number desired. After being heated, the air passes into the flow conditioning section and is then expanded through the nozzle. The flow properties remain approximately constant through the test section at the design Mach number of 8 until reaching the diffuser. At the diffuser inlet, the flow accelerates briefly before being compressed through a complicated shock system (there is no second throat) and then cooled to approximately ambient temperature in the heat exchanger. It then passes through the ejector system before exiting via a silencer outside. The inlet flow conditions are limited by the maximum pressure and temperature specified in the design of the heater assembly. The capacity of the air storage facility limits possible run times due to the large mass flow demands of the ejectors.
A high-strength material was needed to withstand the combination of high stagnation temperatures and pressures in the upstream sections of the facility. Inconel was considered, but 316L stainless steel was selected as the fabrication material because of its relatively low cost in comparison with Inconel. 316L stainless is a material used in high-temperature applications. It is readily available and has good strength and corrosion properties. 316L was used to fabricate the heater coil, nozzle, test section, expansion joint, and diffuser.
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Figure 1. Schematic of the Hypersonic Facility
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Figure 2. Schlieren Photo of Flat Plate Model taken in the Facility