Session: 8.2 - Fluid Machinery

Paper Number: 169505

169505 - Numerical Analysis of Cavitating Flow Fields in a Rocket Inducer With a Backflow Restriction Step 

Abstract: 

In liquid propellant rocket engines, turbopumps are used to pressurize fuel and oxidizer. In a turbopump, an axial pump which is called an inducer is used just upstream of a centrifugal impeller. Since inducers are driven under low inlet pressure conditions, cavitation occurs at the blade tip and backflow region. Cavitation causes unsteady oscillation of turbopumps, which is known as cavitation instabilities.

Cavitation instabilities can be classified into two types, which are rotating cavitation and cavitation surge. Rotating cavitation causes shaft whirling due to the oscillation of cavities in different phases on each blade. Rotating cavitation can be classified into three types, which are super-synchronous rotating cavitation (Super-S RC), synchronous rotating cavitation (Sync RC), and sub-synchronous rotating cavitation (Sub-S RC) based on the relative speed of cavity propagation to blade rotation. In the stationary coordinate, Super-S RC rotates at 1.1~1.3 times faster than the rotating speed of the inducer. Sync RC has 1.0 times, and Sub-S RC has 0.8~0.9 times of the rotating speed.

On the other hand, cavitation surge causes pulsation of working fluid due to the oscillation of cavities in a same phase. Cavitation surge can also be classified into two types. A phenomenon in which multiple backflow vortex cavities oscillate to the direction to upstream in a same phase is called deep cavitation surge (Deep CS), and the phenomenon in which tip leakage vortex cavities on each blade oscillate to the direction according to the blades in a same phase is called mild cavitation surge (Mild CS).

These cavitation instabilities cause serious damage to turbopumps. Therefore, various countermeasures have been proposed. One example is the Backflow Restriction Step, or BRS. BRS is a circumferential groove at the inlet of the inducer, and it aims to suppress Deep CS by pushing back the backflow through the tip clearance. By using BRS, it was shown that Deep CS can be suppressed, but the Sub-S RC occurs. Also, it was shown that the length of BRS to axial direction is important in terms of suppressing Deep CS.

Although some analyses of flow field around an inducer with BRS have been conducted, the detailed flow field has yet to be analyzed.

With BRS, the pre-whirl flow to the direction of rotation of the inducer is known to be larger. It is considered that this is due to the backflow with a swirling velocity being pushed back into the main flow by BRS while maintaining its swirling velocity. When the axial length of the BRS differs, the position where the backflow is pushed back into the main flow changes, which may result in a change of the pre-whirl flow. The pre-whirl flow affects the incidence angle and changes the pressure difference between the pressure and suction sides at the leading edge of the blade. Since this pressure difference causes backflow, there is a possibility that BRS affects backflow not only by pushing it back but also changing the incidence angle.

Therefore, we investigated its effect on pre-whirl flow and backflow by changing the axial length of BRS by numerical analysis. In addition, we conducted experiments on one geometry of BRS and captured the cavitation with BRS using a high-speed camera for the first time. We also discuss the differences of appearance of cavitation with and without BRS.

Presenting Author: Takeru Yoshino Department of Mechanical System Engineering, Graduate School of Engineering, Tohoku University

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