Investigation of flow through centrifugal pump impellers 1

With increasing computer capability and rapid development of computational fluid dynamics, numerical simulation of the three-dimensional turbulent flow through impellers or even pump sta

INTRODUCTION

1.1 Background

A centrifugal pump consists of a set of rotating vanes enclosed within a casing The vanes impart energy to fluid through centrifugal force The advantages of the centrifugal pumps over the reciprocating pump are based on the high and consistent flow rate, easy and effective operation control and low manufacturing and maintenance costs The main disadvantage is its relatively low efficiency

Traditionally, centrifugal pump design depends on too many factors based on designer’s experience The designer can not accurately predict the pump performance before it is tested The pattern cost, manufacturing of prototype and testing are quite expensive and it may take many trials to obtain satisfactory or desirable results To overcome this problem, Computational Fluid Dynamics (CFD) analyses are being increasingly used as an alternate tool in the design of centrifugal pumps With increasing computer capability and rapid development of computational fluid dynamics, numerical simulation of the three-dimensional turbulent flow through impellers or even pump stage is becoming possible, and the complex internal flows in the impeller and pump stage are being well predicted to speed up the pump design procedure Thus CFD is an important and useful tool for pump designers

Of a centrifugal pump stage, the impeller is the most important part It rotates the liquid mass with the peripheral speed of its vane tips, thereby determining the head produced or the pump working pressure However, the internal flow of the pump impellers is usually very complex and characterized by diffusion and strong swirl

Adverse pressure gradient and secondary flow always occur under off-design operating conditions, sometimes even under the design point Therefore, it is necessary to investigate the internal flow of pump impellers by using some CFD approaches

It is also well known that the flows in centrifugal pump are three-dimensional turbulent flows, and a clear understanding of turbulent flow structures is essential for the optimization of the performance of centrifugal pumps Recently as the rapid development of the technologies, more and more computational fluid dynamics (CFD) approach and turbulent models have been proposed and used frequently as a tool to simulate the turbulent flows numerically However, it is found that most of researchers focused on the use of the standard k −ε model to simulate water flow through pump impellers; few of them took effort to apply different turbulence models

on their simulation work and made comparison among these turbulence models Therefore, studying the influences of different turbulence models to the pump performance is also an important aim of the present investigation

On the other hand, the pumped liquid always contains undissolved gas inside the pump impellers in a wide range of pump applications Presently, the accurate prediction of the performance drop caused by two-phase flow is still a problem Therefore, the knowledge of the performance of centrifugal pumps under gas/liquid two-phase flow conditions is of increasing interest in a wide range of industrial applications, especially in the chemical industries, the offshore oil production, and in relation to safety analyses in nuclear reactors

It is well known that the performance of standard centrifugal pumps decreases rapidly in the existence of entrained air The deterioration begins as the gas fraction is

only about 2-3% of the total volumetric flow and the total breakdown of the flow may occur when the gas fraction reaches about 8-10% of the total volumetric flow rate For two-phase flow applications of centrifugal pumps, it is crucial to predict the influence

of the gas fraction on the pump performance However, this has not been fully done yet Therefore, the accurate prediction of the flow characteristics in centrifugal pumps operating under gas-liquid two-phase flow conditions is imperative

1.2 Literature Review

Many researchers have been involved in the experimental studies on the internal flow of pumps Murakami et al (1980) measured the flow patterns in centrifugal pump impellers with three and seven blades respectively using a cylindrical yaw probe and an oil surface flow method They concluded that the measured distributions of velocities and pressures for the seven blade impeller at the design flow rate coincided well with the numerical solution, whereas the flow patterns

of the three blade impeller deviated largely from those of the seven blade impeller both at the design and off-design conditions Liu et al (1994) measured the flow in the inlet pipe, impeller, volute and outlet pipe of a centrifugal pump with laser-Doppler velocimetry (LDV) The flows in the impeller passages were found to depart from the curvature of the blade surfaces at off-design conditions with separation from the suction surface and from the shroud Poor matching of the impeller and volute at off-design conditions caused swirl and separation in the inlet and exit pipes Yang et

al (2002) used Particle Image Velocimetry (PIV) to measure blade-to-blade flow fields on three planes along the shaft of a transparent centrifugal pump They concluded that the flow fields in the centrifugal pump were asymmetric, and there was

a lower velocity zone near the pressure-side of the blade at low or design flow rate Bwalya and Johnson (1996) made measurement of the 3-D velocities, total and static pressures in a 0.89 m diameter commercial pump impeller by using a 5 hole pressure probe and air as the working fluid Their results at the peak efficiency operating point showed separation of the flow near the shroud on the pressure side of the blade at the leading edge A region of reversed radial velocity was also observed at the outlet

However, experimental study is usually time-consuming and expensive Hence, Computational Fluid Dynamics (CFD) analyses are being increasingly used in the design of centrifugal pump impellers With the aid of CFD approach, the complex internal flows in the pump impellers, which are not fully understood because of the strong secondary flows caused by the flow passage geometry, Coriolis force, centrifugal force, the tip leakage flow and so on, can be predicted well to accelerate the pump design procedure Thus a CFD analysis is an important tool for pump designers

Many studies on the complex flow in all types of centrifugal pumps have been reported using CFD analyses Oh and Ro (2000) used compressible time marching method, traditional SIMPLE (Semi-Implicit Pressure-Linked Equations) method and a commercial program of CFX-TASCflow to simulate flow pattern through a water pump and compared the differences between these methods in pump’s performance prediction They concluded that the time marching method showed about 5-10% higher performance than the pressure correction methods This is mainly due to different velocity fields and lower level of the impeller exit flow angle

Tatebayashi et al (2000) also used CFX-TASCFLOW code with a standard

ε

−

k two-equation turbulence model to simulate internal flow of a screw-type

centrifugal pump with tip clearance They concluded that when the calculated results are compared with the experimental results, the trends are very closely related The results showed that not only the flow patterns and the pressure distributions but also the pump performance could be predicted reasonably well by simulation with the effect of the tip clearance Similar results were also presented by Han et al (1998) Goto (1992) presented a comparison between measured and computed exit flow fields of a mixed flow impeller with various tip clearances, including the shrouded and unshrouded impellers, and confirmed the applicability of the incompressible version of the three-dimensional Navier-Stokes code developed by Dawes (1986) for mixed-flow centrifugal pump Zhou and Ng (1998), Ng et al (1998) also developed a three-dimensional time-marching incompressible Navier-Stokes solver with the pseudo-compressibility technique to study the flow field through Goto’s mixed-flow water pump impeller The applicability of the original code was validated by comparison with Goto’s well published experimental and computational results

Wang et al (2000) used CFD techniques to analyze the flows through a pump-turbine runner The SIMPLEC algorithm with body-fitted coordinates and staggered grid system was used for the solution procedure of the discretized governing equations It was found that the relative velocity values on the suction side were larger than those on the pressure side And the efficiency of the runner was higher in the pump operation condition than that in the turbine operation conditions

Huang et al (2000) presented preliminary results obtained from theoretical, experimental and CFD analyses of a new updraft free-exit-flow hydropower turbine system In their CFD analysis, the commercial Star-CD code was selected as the

modeling platform and a quasi-three-dimensional numerical model was adopted to minimize the lengthy computational time requirement and cost It was found that the CFD results on the model turbine flow behavior and operational characteristics were better than an initial trial prototype design involving a totally new hydropower turbine technology could anticipate

Wu et al (1998) investigated the three-dimensional turbulent flow through the semi-open impeller of centrifugal pump for two-phase flow under the design operation condition by numerical flow analysis They used the SIMPLEC algorithm with body-fitted coordination and staggering grid system for the solution procedure of the discretized equations based on the Navier-Stokes equation and the standard k−ε

model The calculated results were compared with experimental data for pressure distribution, and good agreement was achieved between the calculated result and experimental data The flow behaviour in the impeller was also presented in their paper

Recently, many researchers began to use numerical approaches to study pump off-design performance For example, Potts and Newton (1998) presented their work

on numerically predicting the pump shut-off behavior using the Fluent CFD package Rotor/stator interaction was modeled by using sliding mesh approaches, and the time-dependent simulation was conducted in a two-dimensional domain However, the measured shut-off head rise was overpredicted by approximately 25 per cent

Kaupert et al (1996) and Sun and Tsukamoto (2000, 2001) studied pump off-design performance using commercial software CFX-TASCflow and STARCD respectively Although these researchers predicted reverse flow in the impeller shroud region at small flow rates numerically, some contradictions still existed For example,

Kaupert’s experiments had shown simultaneous appearance of shroud side reverse flow at the impeller inlet and outlet, but his CFD results failed to predict numerical outlet reverse flow Sun and Tsukamoto (2001) validated the predicted results of the head-flow curves, diffuser inlet pressure distribution and impeller radial forces by the experimental data over the entire flow range and predicted back flow at small flow rates, but they did not show exact back flow pattern along the impeller outlet either

Wu et al (2000) simulated the three-dimensional turbulent flows through a hydraulic pump-turbine runner based on standard k−ε turbulence model They adopted SIMPLEC algorithm in the numerical procedure and indicated the velocity distribution and pressure distributions through the impellers They found the vortex occurs in off-design point and it causes the decrease of the velocity momentum

Iino et al (2002) and Kato et al (1999, 2002) investigated numerically the internal flow of a mixed-flow pump by using commercial CFD code, CFX-TASCFLOW and large eddy simulation (LES) method Kato et al (2002) predicted the instability of head-flow characteristics at low flow rate ratios and showed good agreements between the computed and measured profiles Iino et al (2002) studied the discontinuous characteristics and the hysteresis on the performance curve of the mixed-flow pump They also reported that the large backflow and recirculation occurred in two places at pump partial flow operation, one of which was near the casing at the impeller inlet and the other was near the central part of the stator hub

Lu et al (2002) numerically analyzed a centrifugal pump with different tip clearance working at the design and off-design flow rates Commercial CFD code FLUENT with standard k−ε turbulence model and MRF (multiple reference frames)

model were used within the entire stage of the pump The numerical results were

compared with experimental data given by Liu et al (1994) with LDV It was found that the relative flow on the mid-plane of the passage followed the curvature of the blade at the design flow rate whereas at the off-design conditions the flow between impellers became irregular and separation happened The existence of low pressure area at outlets of passages and the vortices in the passage close to the shroud at small flow rate increased the loss of the flow and reduced the efficiency of the centrifugal pump

Kosyna and Kecke (2002) investigated flow structures within the pump impeller and the overall pump performance by numerical simulation and experiments The single-phase flow (water) was first calculated using CFD code CFX-TASCFlow One impeller channel was simulated by considering rotational symmetrical boundary condition and applying the standard k−ε turbulence model For calculating the

two-phase flow, the CFD-code PHOENICS with the implemented Eulerian multitwo-phase flow model IPSA (Inter Phase Slip Algorithm) was applied The numerical results were compared with the experimental data and showed good consistence for various gas void fractions

Zhou et al (2002) predicted the performances of a centrifugal pump by using a commercial CFD-code FLUENT Several factors that may affect the numerical results were investigated and discussed These factors include the relative position of the impeller and the spiral case, the mesh size and the turbulence models Although their computational results showed great difference with experimental data, they found that small mesh size seemed to have a little improvement over the larger mesh size It was also seen that the RNG k −ε turbulence model gave results more close to the

experimental data than the Realizable k−ε turbulence model, especially for

off-design points

Zhou et al (2003) used a three-dimensional Naiver-Stokes code, with a standard k−ε turbulence model to simulate the internal flow in three different types

of centrifugal pump impellers The finite-volume method and an unstructured grid system were used for the solution procedure of the discretized governing equation Comparison of computational results for various types of pumps showed good agreement for the twisted-vane pumps However, for the straight-vane pump, the computational results were somewhat different from widely published experimental results The calculation also predicted reasonable results in both the flow pattern and the pressure distribution under pump design and off-design conditions

Many investigations on the drop of pump performance due to gas/liquid two-phase conditions were also carried out However, most of these projects focused on the experimental studies Murakami and Minemura (1974) investigated the effects of entrained air on the performance of a centrifugal pump with a low specific speed of

s

n = 23 by measuring the total pump head and using photographic observations Sato

et al (1996) studied the performance of a centrifugal pump impeller with various blade angles, while Kosyna et al (2001, 2002) used eight miniature pressure transducers for the unsteady measurement of the blade pressure, and a telemetric system was mounted on the shaft for transmitting the data from the rotating impeller for various gas fractions A high-speed digital camera was also used to investigate the flow structure in the pump impeller visually The experimental results improved the understanding of the physical mechanism which was responsible for the performance drop of pumps while operating under two-phase flow conditions

Some studies on the two-phase flow in centrifugal pump impellers have been reported using CFD analyses Minemura and Uchiyama (1993) proposed a three-dimensional numerical method on the basis of a bubbly flow model to predict the behavior of gas-liquid two-phase flows in a centrifugal pump impeller The method was applied to a radial-flow pump, and the results obtained were confirmed by experiments within the range of bubbly flow regime

Noghrehkar et al (1995) studied head degradation phenomena based on a one-dimensional two-fluid model and to analyze the gas-liquid interaction within the pump impeller under high pressure, steam-water two-phase flow conditions

Hirschi et al (1998) employed a single-phase Reynolds-Averaged-Navier-Stokes (RANS) methodology to predict the cavitation behavior of a centrifugal pump and to compare this prediction to model tests The proposed method, which allowed the performance drop prediction, consisted of assuming the cavity interface as a free surface boundary of the computation domain and in computing the single phase flow The unknown shape of the interface was determined using an iterative procedure matching the cavity surface to a constant pressure boundary The predicted behaviour

of the cavitating flow in a centrifugal pump, using the proposed method, compared very well with the experiment

Kang et al (2002) used a two-fluid model to predict power law fluid-particle two-phase turbulent flow in a pump impeller Inter-phase drag coefficient was modeled via a Schiller Naumann drag model and solid particle collision forces were also taken into account The results showed that particle volume fraction was highest

at impeller suction surface near outlet and pressure surface