The objective of this paper is to study the effect of the exit blade angle on the performance of a centrifugal pump when the pump handles fluids of varying viscosity. To achieve the said objective, one radial centrifugal pump was selected. The pump has to supply the fluid at the head of 40 m having a flow rate of 288 cum/h. The speed is 1,450 rpm. The data required by the software for model generation are calculated manually using the method suggested by Stepanoff. A model is prepared in ANSYS for exit blade angles of 24.5°, 26°, 28°, 29.5° and 31°. The said models of different blade angles are simulated in ANSYS at various operating conditions. For each exit blade angle, the fluid of different viscosity is taken to study its effect on pump performance. The fluids taken are water, castor oil, engine oil and methanol. It is found that as the exit blade angle increases, the performance of the pump improves. Also as the viscosity of the fluids increases, the efficiency decreases and the power consumption increases. It is found that as the pump handles high viscous fluids then the Best Efficiency Point (BEP) is obtained at a higher exit blade angle, because as the exit blade angle increases, the wake/turbulence at the exit of the impeller decreases.

When sizing a pump for a new application or evaluating the performance of an existing pump, it is often necessary to account for the effect of the pumped fluid's viscosity. We are all aware that the head-capacity curves presented in the pump vendor catalogs are prepared using water as the pumped fluid. These curves are adequate for use when the actual fluid that we are interested in pumping has a viscosity that is somewhat less than or equal to that of water. Heavy crude oils can have viscosities high enough to increase the friction drag on the pump's impeller significantly. When a fluid of high viscosity such as heavy oil is pumped by a centrifugal pump, the performance is impaired in comparison to the service with water due to increased losses.

Stepanoff (1940) and Telow (1942) have tested the performance of the centrifugal oil pumps as a function of the viscosity without knowing the phenomena of viscous fluid flow within the pump. They have proposed some correction factors when the pump handles the viscous fluid for determining the performance. Wen-Guang Li (2000) has tested the performance of a centrifugal pump using water and viscous oil as working fluids whose kinematic viscosities are 1 and 48 mm²/s, respectively. The flows in the centrifugal pump impeller are also measured accurately by using a two-dimensional Laser Doppler Velocimeter (LDV) in best efficiency and part-loading points while the pump is handling two kinds of working fluids. The effects of viscosity on the performance and flow pattern within the impeller are established based on the experimental results. Wen-Guang Li (2004) has also carried out an experimental investigation of various liquid viscosities while the original impeller is trimmed four times. The trim exponents of flow rate, head, shaft power and efficiency as well as trim curves at best efficiency point, the trim exponents of head and power at shut-off point have been worked out. The other experimental trim exponents are affected heavily by both liquid viscosity and impeller diameter trimmed and have shown very different variation trend with viscosity for each diameter. (Stepanoff, 1992), while the existing analytical trim curves show a great difference with the experimental data, especially efficiency curve. Thus, the performance parameters of centrifugal oil pumps cannot be evaluated exactly by using the existing equations for trim curves in case of impeller diameter reduction at best efficiency point when handling viscous oils; the effect of viscosity on the parameters must be considered in this evaluation.

Mechanical Engineering Journal, Exit Blade Angles, Centrifugal Pump, Laser Doppler Velocimeter, ANSYS Turbo Grid, Power Consumption, Engine Oil, Software ANSYS, Kinematic Viscosities, Pump Vendor Catalogs.