Application of the hybrid RANS/LES method on the hydraulic dynamic performance of centrifugal pumps *

Zheng-chuan Zhang, Hong-xun Chen, Zheng Ma, Qun Wei, Jian-wu He, Hui Liu, Chao Liu

1. Shanghai Institute of Applied Mathematics and Mechanics, Shanghai University, Shanghai 200072, China

2.Chinese Ship Scientific Research Center, Wuxi 214082, China

3. College of Civil Engineering and Architecture, Nanchang University, Nanchang 330031, China

4. Shanghai Marine Equipment Research Institute, Shanghai 200031, China

Abstract: The nonlinear hybrid RANS/LES method has been developed considering the simulation efficiency and accuracy to simulate the hydraulic dynamic characteristics of the centrifugal pump. It has been proved that the nonlinear hybrid RANS/LES method could effectively capture the unsteady flow structure and pressure pulsation in 2-D centrifugal pumps. To further investigate the hydraulic dynamic performance of a redesigned 3-D-gap drainage centrifugal pump, the nonlinear hybrid RANS/LES method is employed. Both numerical simulation and experimental results show that the hydraulic performance and pressure pulsation characteristics of the 3-D-gap drainage impeller centrifugal pump are significantly enhanced.

Key words: 3-D-gap drainage blades, nonlinear hybrid RANS/LES method, pressure pulsation

Centrifugal pump as a kind of hydraulic machinery has been widely used which takes water or other liquid as working mediums and energy conversion carrier to achieve the mutual transformation between liquid energy and mechanical energy. The internal flow of hydraulic machinery is extremely complex and its characteristics can be summarized as high Reynolds number, multi-scales, inhomogeneous and vortex-dominant unsteady turbulence, which interact with rotating dynamic boundary (rotor blade)[1-2]. Accurate analysis and prediction of complex internal turbulent flow in centrifugal pumps is the precondition and foundation in order to study internal flow mechanism and to propose effective flow active/passive control method for its reasonable application.

In recent years, we have focused on the prediction and control of unsteady flow in centrifugal pumps.Based on the idea of flow control, a 2-D gap drainage blades structure was proposed and applied to centrifugal pump impellers[3]. It was proved that the 2D-gap drainage blades could significantly improve the hydraulic performance and anti-cavitation of centrifugal pumps under certain working conditions based on numerical simulation and experiments[4]. By considering the simulation efficiency and accuracy,Wei developed a Non-linear Hybrid RANS/LES method[5]to investigate the pressure fluctuation characteristics of the 2-D centrifugal pump based on the result of experiment and DNS[6-7]. The Non-linear Hybrid RANS/LES turbulence model was adopted to simulate the internal flow of the 2-D centrifugal pump.The expression of the eddy viscosity and smoothing function are given as follows:

As shown in Figs. 1, 2, the nonlinear hybrid RANS/LES model on unsteady internal flow structure and pressure pulsation have great ability and matches very well with the experimental results.

Fig. 1 (Color online) The mean velocity distributions of 2-D traditional pump

Fig. 2 (Color online) The mean velocity distributions of 2-D-gap pump

The pressure pulsation waves characteristics comparison between the experiments and numerical simulation in different pumps as seen in Figs. 3, 4, the nonlinear hybrid RANS/LES model not only can catch the periodic characteristics, but also predict the pressure pulsation waveform and amplitude.

Fig. 3 The pressure pulsation distribution in 2-D-con pump

Fig. 4 The pressure pulsation distribution in 2-D-gap pump

Fig. 5 The variation of the pressure pulsation amplitudes in 2-D impellers

According to the numerical simulation results,the pressure pulsation amplitude of gap remarkably decreases in the whole conditions as shown in Fig. 5 in contrast with 2-D tradition pump. Moreover, the domain frequency of 2-D-gap pump pressure pulsation also reduces as seen in Fig. 6.

Fig. 6 Spectrum distributions of pressure pulsation in 2-D volute

To apply the gap drainage blades technology in a low specific speed 3-D centrifugal pump, the original blades have been retrofitted. Comparing with the conventional impeller (referred to as con), the difference of the 3-D-gap drainage impeller (referred to as gap) is that the 3-D-gap drainage blades make the main blades shortened slightly and a small gap between the drainage blade and the main blade, as illustrated in Fig. 7.

Fig. 7 (Color online) The 3 -D model of two impellers

The Non-linear Hybrid RANS/LES turbulence model is adopted to simulate the internal flow of the 3-D centrifugal pump.

Taking the standard deviation of pressure fluctuation signalas the quantity of pressure intensity, the mean value of the pressure pulsation signal can be expressed asto represent the average level of the pressure signal over a period of time, and the pressure pulsation coefficient and the standard deviation of dimensionless (STDD) pressure fluctuation are respectively expressed as

The pressure pulsation amplitude of 3-D-gap is remarkably reduced by a factor about 37.5%-71.4%underat impeller near the entrance of blade (at R/3 ) as seen in Fig. 8.

Fig. 8 The variation of the pressure pulsation amplitudes with flow in impellers

As illustrated in Fig. 9, the amplitude of the main frequency pressure pulsation in the volute of 3-D-gap has been significantly reduced by about 30% under. It is worth noting that the main frequency components of pressure pulsation may be transferred to high frequency components, which could be inferred that the 3-D-gap drainage blades not only reduce the amplitude of the main frequency pressure pulsation at the measuring points in the volute, but also improve the distribution of the pressure pulsation.

Fig. 9 The pressure pulsation spectrum distributions in volutes

Fig. 10 (Color online) Pressure and vortex dissipative distributions at

A micro jet flow between the vice blade and the main blade at the inlet reduces the pressure gradient and makes the separation points backward, which significantly improve the pressure distribution of the impeller. Besides, the viscous dissipation in the con is obviously stronger than the gap, which causes the greater energy loss and leads to the poor hydraulic performance as shown in Fig. 10.

Fig. 11 Experimental comparison of hydraulic performances

The experiment results of the hydraulic performance are shown in Fig. 11. The experimental comparison indicates that the head and the efficiency of gap-pump is higher than that of the con-pump in the whole condition.

In summary, the nonlinear hybrid RANS/LES method could effectively capture the abundant flow structures and pressure pulsation in centrifugal pump.The numerical and experimental results show that the 3-D-gap blades based on the flow control can not only effectively improve the hydraulic characteristics of the centrifugal pump to have a wider range of operations,but also reduce the intensity of pressure pulsation.