Session: 19-03 Compressors and pumps

Paper Number: 153713

Insights Into the Cavitation Behaviour of a Boundary Layer Pump 

A boundary layer pump’s rotor consists of a stack of parallel, flat, thin discs with small gaps between them. The fluid enters the gaps radially from the bore and is then tangentially accelerated by the viscous forces from the rotating discs. Based on its working principles and the absence of blades, several researchers have suggested that this type of pump might indicate a smoother increase of pressure inside the rotor. On the contrary, centrifugal pumps indicate local pressure drops and hence possible cavitation near the leading edge of the blade. The few studies that discussed boundary layer pump cavitation performance modelled only the rotor flow with simple analytical one-dimensional techniques. They evaluated the suction specific speeds and found that they are 10-20 times higher than that of a centrifugal pump which suggests a significant cavitation advantage. More advanced three-dimensional CFD approaches in literature used single phase flow and did not assess cavitation. The experimental approaches did not decrease the inlet pressure to obtain a drop curve for a boundary layer pump.

The current study examines the cavitation performance of a boundary layer pump based on a water pump manufactured in a previous study. This previous study explored experimentally and parametrically the effect of inter-disc gap size, rotational speed and disc dimensions on hydraulic efficiency. It obtained a configuration with relatively high efficiency (around 40%) and, in the present study, this same geometry of the full pump is assessed with 3D RANS CFD and two-phase flow to investigate the cavitation mechanisms. For this purpose, the initial inlet pressure is set equal to the one of the experimental study and is then gradually reduced so that the differences in the flow field, the vapour formation areas and the NPSH-head drop curve is obtained. A centrifugal pump is designed at the same operating point and is numerically assessed with CFD for the same range of inlet pressures as the boundary layer one. Hence, the flow fields and the NPSH-drop curves of the two can be compared for perspective.

The results indicate that there is a cavitation sensitive region in boundary layer pumps equivalent to the blade leading edge of centrifugal pumps. This region is the entrance of the inter-disc channels closer to the axial inlet of the pump. The vapour formation starts in the first channel, from the pump inlet, and as the inlet pressure drops it expands to the adjacent channels towards the back and in higher radii inside each channel. The conclusions differ from the simple analytical approaches which assumed the same flow field in each gap and could not model the flow at the rotor inlet interface. The head drop curve is acquired and shows similar behaviour with the centrifugal one. The understanding of the cavitation mechanism can contribute to design guidelines for Tesla pumps with increased cavitation performance.

Presenting Author: Agapi Bakogianni Cranfield University

Presenting Author Biography: Integrated Master in Mechanical engineering from National Technical University of Athens. PhD student in the Centre for Propulsion in Cranfield University

Authors:

Agapi Bakogianni Cranfield University
David John Rajendran Cranfield University
Eduardo Anselmi Palma Cranfield University
Vassilios Pachidis Cranfield University
Chloe Jo Palmer Rolls-Royce plc