Effect of Tip Clearance on Cavitation Performance and Flow Characteristics in a Turbopump Inducer

Abstract

In a modern liquid-rocket-propulsion system, a turbopump-feed cycle to produce high pressure fluid for feeding a combustion chamber is preferred to a pressure-feed cycle due to relatively lower system weight and better performance. An inducer is employed in a turbopump upstream of the inlet of the impeller, which causes a rise in the inlet pressure before the impeller to prevent substantial performance degradation and strong unsteady forces acting on the pump due to severe cavitation. Many researchers have been motivated to study the flow physics in an inducer including cavitation experimentally and numerically. It is known that an optimum value of tip clearance exists in a turbopump inducer. In general, tip clearance flows have a deleterious effect on the performance in turbomachines and large tip clearance leads to high tip clearance loss. However, an adverse effect of small tip clearance on the performance is not explained clearly yet. The objectives of the present study were to investigate the effect of tip clearance on cavitation performance and flow characteristics in a turbopump inducer using the computational fluid dynamics (CFD). Three different sizes of tip clearance were analyzed under noncavitating and cavitating conditions at design (Qd) and off-design (0.8Qd, 1.2Qd) conditions. Rayleigh-Plesset model implemented in ANSYS CFX 13.0 based on the rate equation controlling vapor generation and condensation in the context of two-phase one-fluid analysis was adapted to calculate the cavitating flows. Numerical results were validated by comparison with experimental results for noncavitating performance and suction performance. Under noncavitating condition, backflow penetrates upstream farther for large tip clearance at low mass flow rate, and therefore, performance is declined rapidly. On the other hand, for small tip clearance, extent of backflow is reduced but development of the hub separation in the middle of the passage causes the performance degradation. It is confirmed that a certain extent of tip clearance opposes the development of the secondary flow in the passage. Cavitation inception occurs at the leading edge of the blade tip. Under high cavitation number, static pressure under cavitating condition is almost same with that under noncavitating condition because tip vortex cavitation and tip leakage vortex cavitation cannot affect the flow significantly and deteriorate the overall performance. Tip vortex cavitation and tip leakage vortex cavitation tend to become large for small tip clearance at low mass flow rate. However, long blade cavitation is developed for large tip clearance and obstructs the throat between two adjacent blades. It results in head breakdown under high cavitation number for large tip clearance. Further, at high mass flow rate, head breakdown occurs under high cavitation number due to abrupt obstruction at throat by blade cavitation on the pressure side as well as on the suction side.