Tsinghua Science and Technology Tsinghua University, China ISSN: 1007-0212 Vol. 6, Num. 5, 2001, pp. 492-496

Tsinghua Science and Technology, December 2001, 6(5), pp. 492-496

Failure Analysis and Design Changes of Oxygen Pump Inducers*

LIANG Hengli  , CHEN Zuoyi 

Department of Thermal Engineering, Tsinghua University, Beijing 100084, China

 * Supported by the State Key Developments Plan Project of China (No. G1999022304)

Received: 2000-07-07

Code Number: ts01103

Abstract:

The  failure of an oxygen pump inducer during a test run was found to be the result of flow induced vibration. Oscillating fluid mechanics theory was used to determine the oscillating flow field around the inducer for various external oscillating perturbation frequencies. Enormous pressures can occur at some frequencies, which are sufficient to break the inducer. Some design changes were analyzed to improve the flow induced vibration characteristics.

Key  words: inducer;  flow induced vibration; paremetric polynomial method (PPM); numerical calculation

Introduction

In the aerospace fields, flow induced vibrations within a rocket engine have a significant effect on rocket reliability. However, the development of modern high-performance rocket pump inducers has focuses mainly on the effects of rotating cavitation. In a recent experiment of a new type of rocket engine, an oxygen pump inducer was destroyed. Analysis of the pump showed no pitting caused by cavitation on the surface, but oblique fractures in the inlet blade. Figures 1 and 2 illustrate the seriously damaged inducers from the tests. The sample in  Fig.1  was made of aluminium alloy, and is the same size as the one in  Fig.2  which was made of alloyed steel with much greater strength than the aluminium alloy. However, both inducers were destroyed at the same flow conditions. Analysis of the inducer showed that the inducer damage was due to low-cycle fatigue caused by tremendous pressure oscillations. One objective of this paper is to analyze the characteristics of the flow induced vibration on the inducer to determine whether the damage was really caused by flow induced vibration. The other objective is to design an improved inducer structure and to reduce the flow induced vibrations.

This research work is composed of two parts. The first part is to analyze the flow induced vibration characteristics of the present inducer to identify the cause of the inducer damage. The second part is to change the geometric design of  the inducer to reduce the flow induced vibration characteristics.

Four oxygen pump inducers were analyzed to study the effects of the flow induced vibration, see Table 1.

The  rotation speed was 13 000 r/min, the design flow was 0.0613 m³/s, and the pressure in front of the inducer was 0.29 MPa. The flow induced vibration characteristics were analyzed for various external perturbation frequencies from  20 Hz  to  1  000 Hz  and flow rates 0.8, 0.9, 1.0, 1.1, and 1.2 m³/s of design flow. The analytical results showed that serious flow oscillations occurred with the low frequency perturbations and intensive flow induced vibrations as the flow rate increased. The design was improved using numerical tests with various inducer inlet angle and pitches. The results showed that increasing both the inlet angle and the pitch could reduce the flow induced vibration. 

1 Mathematical Method

Flow induced vibration characteristics in an oxygen pump inducer are related to the propagation of external perturbations within the inducer and whether the oscillation grows or decays. The propagation of the oscillations is an unsteady three-dimensional viscous flow phenomenon. However, analysis of all kinds of geometries, frequencies, and flows would require a large number of calculations. For example, to analyze four types of inducers with five perturbation frequencies and five flow rates would need more than one hundred analyses. If the inlet angle were analyzed for three different conditions, then three hundred analyses would be needed. Therefore, it is impractical to solve the problem with conventional three-dimensional unsteady viscous flow numerical methods. In this paper, the oscillating fluid mechanics method and the parametric polynomial method are applied to quickly solve the problem. The oscillating fluid mechanics theory transforms the three-dimensional unsteady viscous flow equations into amplitude equations which are solved using the parametric polynomial method (PPM).

The fundamental equations for the oxygen pump inducer use turbine circular cylindrical coordinates. The steady-state equations must be solved before solving amplitude equations from the oscillating fluid mechanics theory. Both the steady-state equations and the amplitude equations are solved using the parametric polynomial method.

In a three-dimensional flow, the basic unsteady equations are[1]:

The amplitude parameters can be represented in parametric polynomial form:

which are then substituted into the governing equations to be solved for the flow field characteristics.

2 Results and Discussion

Of the four types of inducers analyzed here, 0-04 and 0-04J had different size hubs and  0-04J2  and  0-04J3  had different pitch lengths. The analyses showed that large flow oscillations could be induced by external perturbations. For example, for an external perturbation frequency of 40 Hz,  0-04J  has large pressure oscillations at a circumferential position of about 60°, where the perturbation pressure amplitude increases almost 60 times, see Fig. 3. Since very low external perturbation frequencies may occur during the tests, the pressure oscillations were analyzed for external frequencies of 1 Hz to 1000 Hz. The maximum pressure amplitudes for 0-04J shown in Fig. 4 for frequencies from 1 Hz to 60 Hz show that the most serious flow induced vibration occurs at an external perturbation frequency of about  20 Hz . The pressure amplitude is strong enough to damage the inducer. All four inducers experienced large pressure amplitudes at the frequency of  20 Hz , see Figs. 5 and 6. The characteristic of the flow induced vibrations with 0-04J for five kinds of flow rates at the external frequency of 20 Hz are shown in Fig. 7. When the flow rate exceeded the design flow rate, large pressure oscillations occurred in front of the inducers, which may be the main cause of the inducer damage. The increasing external frequencies of 500 Hz and 1000 Hz did not produce any large flow oscillations. Therefore, the external low frequency perturbation must be avoided to eliminate the flow induced vibration damage of the inducers, so low frequency perturbation sources must be eliminated if possible. An alternative method is to improve the inducer geometry. Three inlet geometries were analyzed with inlet angles of 90°, 120°, and 150°. The increasing inlet angle weakened the intensity of the flow induced vibration for all types of inducers, ( Fig.8 ), as was proven in later experiments. However, the lower amplitude is not enough to completely eliminate the flow induced vibration damage to the inducers. Therefore, design changes are needed to improve the inducer structure. The results in Fig. 9 show that 0-04J3 with a larger pitch has smaller pressure oscillation than the other inducers, which lead to the numerical analysis of different pitches. Two types of inducers with pitches of 80 mm and 100 mm are compared in  Fig. 10 . With a pitch of 80 mm, the maximum pressure amplitude did not reach the order of megapascal (MPa), which can not destroy the inducer. With a 100 mm pitch the oscillations were attenuated which provides a better design to eliminate flow induced vibrations in inducers.

3 Conclusions

The flow induced vibration characteristics in inducers were analyzed numerically:

(1) The oxygen pump inducer damage is caused entirely by serious flow oscillations induced by external low frequency perturbations;

(2) The external perturbations will generate large flow induced pressure oscillations at the inducer inlet;

(3) Increased inducer inlet angles will reduce some of the pressure amplitudes;

 (4) Increasing  the inducer pitch will significantly reduce the flow oscillation amplitudes.

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