Numerical Simulation for Vortex Structure in a Turbopump Inducer: Close Relationship With Appearance of Cavitation Instabilities

Abstract

Unsteady cavitation phenomena such as rotating cavitation and cavitation surge are often observed in a turbopump inducer of a rocket engine, sometimes causing undesirable oscillation of the system. Investigation of their mechanism and prediction of such unsteady phenomena are, therefore, crucial in the design of inducers. As many experiments have shown, the appearance of cavitation instability is highly related to the flow rate as well as to the inlet casing geometry. Experimental observations have shown that a very complex flow structure, including such phenomena as backflow and vortices, appears upstream of the inducer. In this work, therefore, we conducted 3D unsteady computational fluid dynamics simulations of noncavitating flow in a turbopump inducer, mainly focusing on the vortex structure, for three types of inlet casing geometry with various flow rates. Simulation results showed that the vortex structure for the geometry of the inlet casing and that for the flow rate differed. Especially, it was found that development of the tip leakage vortex was dependent on the inlet casing geometry and the flow rate. This tendency is analogous to that observed between the appearance of rotating cavitation and the casing geometry and flow rate in cavitation tunnel tests. This result strongly implies that the tip leakage vortex is responsible for the appearance of rotating cavitation. By adding a gutter to the inlet casing, it was found that backflow was completely confined to the gutter regardless of flow rates. This numerical result implies that the volume of cavity generated in the backflow region should be stable despite a change of the flow rate, resulting in the suppression of increase of the mass flow gain factor. This result also supports the experimental result that cavitation surge was effectively suppressed using such a casing with a gutter.

Characterization of Rotating Cavitation in a Four Bladed Inducer

“…The transition to RC is not always observed at the same conditions in the experiments and the tip vortex position cannot be accurately predicted by the numerical calculations. ABC in four bladed inducers yields a symmetric and quasi-stable operating condition of the turbopump and the trajectory of the tip vortex is similar to that of the non-cavitating case as shown by Kimura et al [13]. In the present study, numerical calculations indicate that at conditions for which RC was observed the tip vortex cavity interacts with the adjacent blade.…”

“…Kimura et al [13] combined experiments and single-phase computations to investigate the root cause of RC in three bladed inducers. The authors observed that at flow coefficients where RC is observed, the tip vortex cavity of one blade interacts with adjacent blades.…”

“…In inducers with an even number of blades (four), the cavities can occur on alternate blades, forming Alternate Blade Cavitation (ABC). Watanabe et al [12], Kimura et al [13] and Tsujimoto [2] showed that the AC is a precursor of RC in three bladed inducers. Tsujimoto et al [14] and Kang et al [15] conducted experiments to characterize RC, suggesting that the instability is likely to occur just before the head fall-off and at flow coefficients higher than design condition.…”

Numerical and experimental study of cavitating flow through an axial inducer considering tip clearance

“…It can also have a very important effect on cavitation instabilities. 11,15 The typical flow structure around an inducer is characterized by a backflow region and vortices. The backflow region is created by leakage flow from the tip clearance.…”

“…The interaction of cavitation with these complex flow structures is thought to trigger the cavitation instabilities often observed in actual inducers. 11 In this study, a two-bladed inducer was modeled in steady state. Tip clearance was at first not considered; later, numerical simulations were carried out taking tip clearance into consideration.…”