A water-flow test facility has been built to enable measurement of dynamic transfer functions (DTFs) of cavitating pumps and of inducers in such pumps. Originally, the facility was intended for use in an investigation of the effects of cavitation in a rocket-engine low-pressure oxygen turbopump. The facility can also be used to measure DTFs of cavitating pumps in general.
It is necessary to measure DTFs in order to understand the dynamic couplings between a cavitating pump and the rest of the flow system of which the pump is a part. In the case of a turbopump, inducer cavitation dynamics can cause flow and pressure pulsations arriving at the turbopump inlet to become amplified by the turbopump, thereby giving rise to very large flow and pressure fluctuations in the feed system served by the turbopump. If the feed system in question is a rocket-engine fuel or oxidizer feed system, these flow and pressure fluctuations can, in turn, cause large variations in engine thrust, even to the point of pogo instability. Within the turbopump, the cavitation-induced dynamic couplings generate intense dynamic loads on the inducer blades and the rotor. These loads cause blade failures, seal rubs, and rotordynamic instabilities.
The DTF-measurement facility was constructed by integrating DTF-measuring equipment into a prior pump-testing facility. The major pieces of the DTF-measuring equipment, in order of position along the flow starting at the upstream end, are an inlet flow pulser, inlet flow conditioner, inlet bandwidth-enhanced electromagnetic flowmeter, inlet pressure-measurement station, test inducer, discharge collector, exit flow conditioner, exit enhanced-bandwidth electromagnetic flowmeter, exit flow pulser, flow conditioner, loop flowmeter, and throttle valve. A magnetic-bearing-supported test rotor that was part of the original pumptesting facility is used to support and drive the test inducer. A closed reservoir from the original pump-testing facility is retained for supplying fluid to the inlet and receiving fluid from the outlet of the pump or inducer under test. The facility also includes instrumentation and data-acquisition and data-processing systems designed specifically for quantifying dynamic transfer functions of cavitating inducers.
The flow pulsers can be used to superimpose discrete-frequency pressure and flow fluctuations on the mean loop flow. The enhanced-bandwidth electromagnetic flowmeters enable accurate measurement of the time-dependent components of flow. Special-purpose software calculates parameters of a four-terminal transfer function model of the cavitating pump or inducer system from the amplitudes and relative phases of inlet and exit flow and pressure pulsations over a range of perturbation frequencies and cavitation numbers.