Performance of small centrifugal pumps with water and mixed polar liquids

Abstract

This research deals with the performance of small, automotive type centrifugal pumps handling both water and Ethylene glycol-water mixtures at temperatures up to 120°C.

Poorly designed hydraulic flow channels of these pumps render them highly inefficient machines. This research looks into the factors which may improve their performance, and examines the various power and scale-up losses. Reynolds number effects due to surface finish, pump speed and liquid viscosity are examined in the light of the numerous non-cavitating test data.

Because of the unfavourable inlet conditions arising at least in part from space limits, most of the small size pumps have a poor suction performance. This research looks into the various design and operational aspects which may influence the suction performance. This includes the inlet pipe design, the type of impeller shroud, the size of the axial tip clearance, and the shape and number of the blades. Also of importance is the test flow ratio Ϙcav/Ϙopt which is of special importance here because of the Re Number influence of the inlet hub vortex at low flows. [The shaft passes through the inlet chamber].

The use of Ethylene glycol-water mixture necessitates the measurement of the gas content during pump cavitation because of the strong gas content effect of the mixture on pump cavitation performance. An amperometric oxygen monitor was used to measure the oxygen content directly and continuously during the test. Gas content effects on pump cavitation were analysed with respect to rig and pump geometry, fluid properties and initial percentage saturation.
Gas filled bubbles of appreciable numbers were found to be recirculating around the system when EG/W mixture is being handled. These bubbles act as cavitation nuclei and increase the cavitation tendency of these mixtures.

Thermodynamic effect of cavitation for the test pumps is investigated in the range 20 - 120°C for the mixture 50/50 EG/W and compared to some data on water. Several influences such as impeller geometry and test flow ratio Ϙcav/Ϙopt are investigated.

Correlation of the thermodynamic effect test data making use of the B-factor is attempted. Suggestions are made to improve the correlation for the mixture to include the mass and heat transfer properties of binary mixtures as well as the number of cavitation nuclei involved in the cavitation process.

Finally, visual observation of the flow field at impeller inlet and the fixed cavity was made and some photographs taken to support the discussion.

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