Centrifugal pump delivery head and flow rate drop effectively during the pumping of viscous fluids. Several methods and correlations have been developed to predict reduction rate in centrifugal pump performance when handling viscous fluids, but their results are not in very good agreement with each other. In this study, a common industrial low specific speed pump, which is extensively used in different applications, is studied. The entire pump, including impeller, volute, pipes, front and rear sidewall gaps, and balance holes, is simulated in Computational Fluid Dynamics and 3D full Navier Stokes equations are solved. CFD results are compared with experimental data such as pump performance curves, static pressure in casing, and disk friction loss. Dimensionless angular velocity and leakage rate are investigated in sidewall gap and efficiency variation due to viscosity is studied. The results demonstrate that the behavior of the fluid in sidewall gap is strictly sensitive to viscosity. Increasing viscosity improves the volumetric efficiency by reducing internal leakage through wear rings and balance holes, causing, however, a significant fall in the disk and overall efficiency. Results lead to some recommendations for designing centrifugal pumps which may be used in transferring viscous fluids.
Centrifugal pumps are usually capable of transferring liquids with viscosities lower than 520–760 cSt. The viscosity can be increased to 1000 cSt by using specific impellers. However, for a pump to be economically efficient, the maximum recommended liquid viscosity is 150 cSt [
Performance curves of centrifugal pumps which are presented in manufacturer documents are related to test with cold water. In addition, predicted performance of pumps for handling a viscous fluid is usually calculated by correction charts of some companies such as [
For a low specific speed centrifugal pump, some research on disk friction loss such as [
Simple model of disk friction loss.
In recent years, some experimental and numerical investigations into viscosity effect on pump performance have been performed in real centrifugal pumps. Li [
The power consumption of a pump can be defined as
With growing the friction factor, the internal leakage through wear rings decreases.
With increasing Reynolds number, hydraulic efficiency increases.
Disk friction losses on the impeller sidewalls grow along with the increasing viscosity.
The mechanical losses are independent of the viscosity of the fluid.
The wall shear stress occurring on surfaces of a rotating disk in a casing full of fluid can be written as follows:
Theoretical head of a centrifugal pump is the sum of the useful head,
The investigated pump (Figure
Main dimensions of the investigated pump.
Impeller rated diameter (mm)  200 
Impeller full diameter (mm)  209 
Impeller outlet width, 
4.2 
Blade outlet angle, 
20 
Number of blades  6 
Number of balance holes  6 
Shroud thickness at impeller tip (mm)  3.7 
Volute base circle diameter (mm)  210 
Suction nozzle diameter (mm)  50 
Discharge nozzle diameter (mm)  40 
Impeller suction eye diameter (mm)  61 
Wear ring radial clearance (mm)  0.4 
Diameter of balance holes (mm)  7 
Hub thickness at impeller tip (mm)  4.1 
Cutaway drawing of the pump parts.
The commercial CFD code ANSYS CFX was employed for the numerical simulation of the pump fluid domain (Figure
CFD model of the pump (fluid domain).
To achieve an improved mesh quality, for the regions which are located near walls, the structured mesh was used, whereas unstructured mesh was employed for areas away from the wall to properly cover the complex geometry (Figure
Mesh configuration used for flow analysis.
Orthogonal quality, aspect ratio, and skewness were inspected during the grid generation process, to be in appropriate range. The grids between rotating and stationary parts such as impeller and volute or suction pipe and impeller were adjoined by means of frozen rotor interface. Mass flow rate with flow direction and constant pressure were implemented for inlet and outlet boundary conditions, respectively.
A closed loop test rig fulfilling the requirements of ISO 9906 [
Centrifugal pump test setup.
To determine the pressure field in the sidewall gap and validate numerical results, peripheral distribution of static pressure is measured by means of pressure transducer with accuracy of 0.25% of the full scale. The signals from the transducers are digitalized by a data acquisition device and, with capturing enough samples, the data are averaged arithmetically. The uncertainties of flow rate, head, power, and efficiency are approximately 0.5%, 0.3%, 0.5%, and 1%, respectively.
To validate the CFD simulation, in Table
Results of averaged static pressure
Sector  CFD  Exp.  Error% 

1  0.390  0.379  2.9% 
2  0.409  0.397  3.0% 
3  0.403  0.403  0% 
4  0.407  0.404  0.7% 
5  0.408  0.400  1.9% 
6  0.405  0.393  2.9% 
Figure
Comparison of CFD and test results for water.
Pump performance curve for oil with
Comparison of resultant
The graph published in [
CFD curve is obtained from simulating flow in 6 operating points and as it is shown the agreement between the CFD results and experimental data is acceptable especially in lower flow rates. The largest error as expected has occurred in overload condition which is less than 10% in
It has been shown in Figure
Effect of operating point location on reduction of pump head for three viscous fluids:





0.5  0.07  0.12  0.14 
0.65  0.11  0.17  0.20 
0.8  0.12  0.20  0.26 
1.1  0.2  0.32  0.38 
Figure
Influence of viscosity on efficiency and shaft power.
Figure
Streamline of flow inside pump.
Figure
Influence of leakage flow on impeller suction regime. Right: water; left: viscous fluid.
The dimensionless internal leakage rate (
Leakage rate through front wear rings versus viscosity and Reynolds number.
When the pump is utilized for pumping viscous fluids, use of impeller back vanes or expeller to balance axial thrust is not recommended due to increase in disk friction loss. The best way is to use balance holes and mating wear rings even with a larger clearance in order to minimize repair intervals and extend the operating life of rings. This geometrical optimization diminishes the risk of face contact of wear rings due to shaft deflection or misalignment and thus improves reliability of the equipment which is completely important in specific applications. API 610 Standard [
The circumferential velocity of fluid in sidewall gap is normally described by the dimensionless angular velocity
Effect of Reynolds number on dimensionless fluid rotational angular velocity.
When centrifugal pump handles water instead of oil, Reynolds number and leakage flow rate through rings (
Figure
Efficiency as a function of Reynolds number, based on CFD results.
If pumping highly viscous liquid (
The amount of efficiency data versus oil viscosity is summarized in Table
The effect of oil viscosity on pump efficiency, based on CFD results.
1 cSt  35 cSt  64 cSt  90 cSt  


0.75  0.89  0.92  0.94 

0.77  0.69  0.66  0.63 

0.84  0.58  0.50  0.45 

0.98  0.98  0.98  0.98 

0.47  0.35  0.30  0.26 
In this paper, the effects of decreasing Reynolds number due to change in viscosity on centrifugal pump performance were studied for a low specific speed pump. The results of CFD agreed well with experimental data in BEP region; however, in overload conditions, the accuracy of CFD was limited. Considering the experimental and numerical investigations, the following conclusions can be made:
In partload region, the effect of viscosity on pump performance is smaller than that in BEP and overload regions. For 90 cSt oil, head coefficient reduces by just 14% in constant flow rate of
With decreasing the Reynolds number, the leaked flow through wear rings and balance holes decreases and thus the volumetric efficiency increases remarkably. For wear rings with 0.4 mm clearance, the volumetric efficiency improves by approximately 20% if the impeller Reynolds number reduces to 17 × 10^{3} from 1.5 × 10^{6}.
The dimensionless rotational angular velocity in the sidewall gap drops effectively by decreasing the Reynolds number resulting in greater drag on impeller. 10% and 30% reduction occurs in outer and inner radii, respectively, when decreasing viscosity from 1.5 × 10^{6} to 17 × 10^{3}.
Disk friction power increases from 15% of total shaft power to more than 50% when water is replaced with 90 cst fluid. Therefore, although volumetric efficiency improves, the overall efficiency of pump decreased by 21%.
In case of pumping oils, use of expeller for limiting thrust load and very tight wear ring clearance for improving volumetric efficiency should be avoided. Impeller balance holes with optimum rear and front ring clearance may be utilized to prevent undesirable hydraulic and mechanical effects.
Impeller outlet width
Friction coefficient
Impeller outlet diameter
Viscosity correction factor for head
Viscosity correction factor for flow rate
Viscosity correction factor for efficiency
Delivery head
Rotation of fluid in impeller sidewall gap
Pump specific speed
Pressure
Dimensionless pressure
Disk friction power
Shaft power
Mechanical power
Useful hydraulic power
Volume flow rate
Impeller outer radius
Radius
Dimensionless radius
Reynolds number
Hydraulic friction losses
Hydraulic mixing losses
Dimensionless leak flow
Fraction of friction losses to the head
Angular velocity of the fluid
Kinematic viscosity
Fluid density
Head coefficient
Flow coefficient
Power coefficient
Pump overall efficiency
Pump volumetric efficiency
Pump hydraulic efficiency
Pump mechanical efficiency
Pump disk friction efficiency.
Viscous fluid
Water
Theoretical
Best efficiency point of pump.
The authors declare that they have no competing interests.