Performance Testing on Transonic Rotors

This article discusses the performance testing of transonic rotors at the Turbopropulsion Laboratory at the Naval Postgraduate School. The Mach number is one of the most important parameters in the case of high-speed compressors. In order to limit power consumption in a test machine, the simplest change is to scale down the machine. A second concept to reduce the power consumption of the machine once it has been scaled down is to throttle the flow before the rotor rather than after it. As a high-speed rotor compresses the incoming air by around 1.4–1.6 times, the air leaving it is appreciably denser than that coming in. If one throttles upstream of the rotor, the exhaust air leaves the machine at atmospheric pressure, which means that the incoming air is below atmospheric pressure. With upstream throttling, care has to be taken to provide long enough ducting ahead of the test compressor to present as uniform as possible flow after the flow rate measuring nozzle.