A survey of instabilities within centrifugal pumps and concepts for improving the flow range of pumps in rocket engines

tt_A SURVEY OF INSTABILITIES WITHIN CENTRIFUGAL PUMPS ANDCONCEPTS FOR IMPROVING THE FLOW RANGE OF PUMPS IN ROCKET ENGINESJoseph P. VeresSpace Vehicle propulsion BranchNASA Lewis Research CenterCleveland, OhioABSTRACTDesign features and concepts that have primary influence on the stable operating flow range of propellant-feedcentrifugal turbopumps in a rocket engine are discussed. One of the throttling limitations of a pump-fed rocket engine is thestable operating range of the pump. Several varieties of pump hydraulic instabilities are mentioned. Some pump designcriteria are summarized and a qualitative correlation of key parameters to pump stall and surge are referenced. Some of thedesign criteria were taken from the literature on high pressure ratio centrifugal compressors. Therefore, these have yet to bevalidated for extending the stable operating flow range of high-head pumps. Casing treatment devices, dynamic fluid-damping plenums, backflow-stabilizing vanes and flow-reinjection techniques are summarized. A planned program has beenundertaken at NASA Lewis Research Center to validate these concepts. Technologies developed by this program will beavailable for the design of turbopumps for advanced space rocket engines for use by NASA Lewis in future space missionswhere throttling is essential.INTRODUCTIONNew rocket engines in the 20 000 to 50 000 lb thrust class will be utilized by NASA Lewis for future space-vehiclepropulsion and launch-vehicle upper stages. The current candidate is a rocket engine fueled by liquid hydrogen andoxygen. 1 Pressurization of the propellants to high combustion chamber pressure imposes stringent requirements onturbopumps to provide a high performance wide operating range that is free from stall and cavitation. Anticipated spacemissions could require that engines be throttled to levels as low as 5 percent of design thrust.Cryogenic pump design historically has focused on meeting the performance goals at a single point or within a narrowoperating range. Designs were driven by the performance at the design point and, to some extent, by producibility.Throttling over a broad stable operating range is an additional requirement that may require somewhat of a tradeoff withthe performance at the design point.System analysis has shown that high degrees of engine throttling require the turbopumps to operate stably down tonearly 20 percent of their design flow coefficients. Figure l(a} illustrates an engine throttle line superimposed onto a pumpoperating map of head versus flow. The pump is sized to operate at its design point (point 1) for design thrust of theengine. However, throttling the engine typically causes the pump to move away from its design flow coefficient andultimately intersect with the surge line, point 2 in Figs. l{a} and (b). The change in flow coefficient results from therelationship between the pump surge line and the engine operating line, or throttle line. These two lines always intersect atsome throttled condition: This intersection can be a limiting factor of engine throttling range and appears to be indepen-dent of engine cycle. The amount of throttling range gained by system optimization of the engine cycle is limited by thestable flow range of the pump. The most effective way to improve engine throttling range is to provide a pump with a widerstable performance map. As point 3 illustrates, a pump with an improved surge margin can provide more engine throttlingcapability before the point of intersection of the system and surge lines. The required surge margin of the turbopumpimposed by the engine throttling range influences the conceptual design of the pump. To achieve a wide stable operatingrange, the pump configuration must have less susceptibility to hydraulic instabilities.Conceptual-design ideas to obtain improved pump operating range, particularly higher surge margin, are summarized.Several of the design concepts have been under development for use in gas turbine compressors and industrial pumps, buthave not yet been applied to turbopumps in rocket engines. The goal of stable operation of compressors is the same as forpumps, with the exception of cavitation. In some cases, cavitation appears to be the result of local flow separation and maybe a precursor to pump surge. Using compressor technology to prevent, or minimize local separations may delay cavitationand, as a consequence, delay pump surge. Design concepts to minimize or delay local separations include various casingtreatment devices, recirculation and the limits on hydraulic Ioadings within the blading. Because of the many similaritiesbetween compressors and pumps, similar improvements in flow range can be expected if these technologies are developed forcryogenic turbopumps.