An analysis of empirical data on low-frequency oscillations in the inlet flow of a turbopump has led to the conclusion that these oscillations could be reduced or eliminated with suitable design of a jet pump to alter the inlet swirl. The oscillations in question are of fluid-dynamic origin and occur in the absence of externally supplied excitation, are related to cavitation, and are of approximately constant frequency and amplitude. They are undesired because they give rise to instabilities that can be transmitted to the thrust-chamber stage of the turbopump.
Heretofore, such oscillations have been suppressed by ad hoc measures determined by trial and error; e.g., incorporating holes into inducer blades, increasing inducer tip clearances, or prewhirl injection. The present concept involves a combination of (1) a more systematic approach in which one seeks to avoid the flow conditions that favor the oscillations and (2) recognition that jet pumps upstream of inducers or impellers have been found to increase the suction performances of turbopumps. Accordingly, one tries to suppress oscillations by designing either an axial jet pump or an angled-nozzle jet pump to improve suction performance and to alter the inlet flow conditions (in particular, the inlet swirl) in such a way and to such an extent as to prevent the oscillations.
Experiments have shown that the oscillations occur within restricted ranges of net positive suction head (NPSH), speed, flow rate, and angle of incidence of flow on inducer blades. Thus, according to the present concept, one could suppress the oscillations by, for example, designing a jet pump to inject prewhirl to bring the angle of incidence out of the range for oscillation at a given NPSH. The figure illustrates the results of an experiment in which oscillations in a liquid-oxygen turbopump were suppressed in this way.
This work was done by Sen Y. Meng and Laura A. Brozowski of Boeing North American, Inc., for Marshall Space Flight Center.