Pressure Recovery in Hypersonic Engine Test Facilities

Hypersonic testing of airbreathing engines with true flight simulation will require unusually large and costly air supply and exhaust systems. Since the pressure recovery obtained from the flow which bypasses the test engine influences the size and cost of the exhaust system, model tests of free-jet engine test facilities were conducted at Mach 7 and 11 to provide pressure recovery data at hypersonic velocities. At both Mach numbers, running pressure recoveries in excess of 50% of freestream normal shock recovery were attained with an adjustable bypass flow diffuser. Starting pressure recoveries varied from 75% down to 30% of the running values depending upon the configuration and the exhaust handling technique. A thorough analysis of the pressure-recovery data was made to define trends and to arrive at a more basic understanding of the diffusion process. The study led to a new definition of diffuser efficiency which compares the actual pressure recovery to the recovery which would be obtained if the bypass flow were expanded to the diffuser throat area and then subjected to a normal shock deceleration.