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Research Papers: Flows in Complex Systems

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

[+] Author and Article Information
Toshiya Kimura

Japan Aerospace Exploration Agency, Kakuda Space Center, Kakuda, Miyagi 981-1525, Japankimura.toshiya@jaxa.jp

Yoshiki Yoshida

Japan Aerospace Exploration Agency, Kakuda Space Center, Kakuda, Miyagi 981-1525, Japanyoshida.yoshiki@jaxa.jp

Tomoyuki Hashimoto

Japan Aerospace Exploration Agency, Kakuda Space Center, Kakuda, Miyagi 981-1525, Japanhashimoto.tomoyuki@jaxa.jp

Mitsuru Shimagaki

Japan Aerospace Exploration Agency, Kakuda Space Center, Kakuda, Miyagi 981-1525, Japanshimagaki.mitsuru@jaxa.jp

J. Fluids Eng 130(5), 051104 (May 05, 2008) (9 pages) doi:10.1115/1.2911678 History: Received December 18, 2006; Revised December 17, 2007; Published May 05, 2008

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.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Computational grid around inducer

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Figure 2

Three types of casing geometries: the casing with a step (solid line), the casing without a step (dotted line), and the casing with a gutter (dashed line)

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Figure 3

Velocity distribution and vortex core lines. (a) shows the tip leakage vortex, which emerges near the tip end. White thick lines and arrows represent vortex core lines and velocity vectors, respectively. (b) shows backflow vortices (black thick lines) and velocity distribution (white arrows) on the plane normal to the rotational axis.

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Figure 4

Distribution of the axial velocity component is plotted on the plane, which includes the rotation axis by a gray scale contour map. The brightest area represents the region with positive velocity, i.e., the backflow region. Thick red lines show vortex cores. The surface of the inducer is colored to show pressure. These figures show the results for the casing with a step with four different flow rates: (a) Q∕Qd=0.9, (b) Q∕Qd=1.0, (c) Q∕Qd=1.1, and (d) Q∕Qd=1.2.

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Figure 5

Distribution of axial velocity component and vortex cores for the casing without a step with three different flow rates: (a) Q∕Qd=0.9, (b) Q∕Qd=1.0, and (c) Q∕Qd=1.1

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Figure 6

Distribution of axial velocity component and vortex cores for the casing with a gutter with three different flow rates: (a) Q∕Qd=0.9, (b) Q∕Qd=1.0, and (c) Q∕Qd=1.1

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Figure 7

Distributions of vortex cores for three types of casing are plotted by three different kinds of symbols. Open and solid circles represent the tip end of each blade. Open circles show the tip end of the blade, whose radius is less than the maximum radius, i.e., swept back-region.

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Figure 8

Water flow diagrams of pressure fluctuations for casings without a step (a) and with a step (b) with the flow rate Q∕Qd=0.9. The axis labeled f∕f0 represents the frequency normalized by the rotation frequency. The nonlabeled axis shows the change of the cavitation number. The vertical axis labeled ΔP represents the intensity of pressure fluctuation.

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Figure 9

Water flow diagrams of pressure fluctuations for casings without a step (a) and with a step (b) with the flow rate Q∕Qd=1.0. The fluctuation in the area marked by an oval is due to supersynchronized rotating cavitation.

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Figure 10

Water flow diagrams of pressure fluctuations for casings without a step (a) and with a step (b) with the flow rate Q∕Qd=1.1. The fluctuation in the area marked by an oval is due to supersynchronized rotating cavitation.

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Figure 11

Water flow diagrams of pressure fluctuations for casings without a step (a) and with a step (b) with the flow rate Q∕Qd=1.2. The fluctuation in the area marked by an oval is due to supersynchronized rotating cavitation.

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Figure 12

Schematic diagram of PIV system

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Figure 13

Comparison between PIV visualization (a) and CFD velocity map (b) for the casings with a step (left) and without a step (right)

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Figure 14

Photographs taken in tunnel tests for the casing with a gutter (left) and the casing with a step (right) (from Tomaru )

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