Keywords: centrifugal, double entry, pump, pressure pulsation, CFD, low flow.

The design of both the impeller and volute in centrifugal pumps are complex, with numerous geometrical parameters being required to identify a design. It is well known that the operation of rotodynamic pumps can result in the generation of instabilities and pressure pulsations, which can affect the mechanical integrity of the pump, resulting in component fatigue or excessive vibration and noise. Currently, accurate predictions of the magnitude of pressure pulsations are not easily achievable by numerical means and experimental/empirical results are still used in the evaluation of new designs. However, continuing advances in computational power and CFD capabilities have provided new options in analyses that allow complex interactive effects between multiple components to be investigated.

The current work aims to improve the scope of previous work related to understanding flow and pressure pulsations by performing simulations involving the complete hydraulic pump geometry. The numerical model of a reduced scale, high energy, double entry, single stage pump incorporates all of the major flow paths encompassing the suction inlet, impeller, leakage pathways and the volute casing. Key parameters in the design process that have an effect on the pressure variation in the pump and which are studied here are, the cutwater clearance gap, the snubber clearance gap, sidewall clearance and blade clocking or stagger. The analysis covers three flow rates namely, the design flow condition (1.00Qn), a reduced flow condition (0.50Qn) and the lowest continuous pump operating point (0.25Qn).

Twenty-seven transient analyses were conducted using the commercial CFD code CFX-TASCflow with a standard k-epsilon turbulence model. Special attention was paid to developing a robust stable analysis in a reasonable timescale. Detailed grid convergence tests were conducted on the impeller and care was taken in matching the individual components of the numerical model. Parallel experimental work available from industrial tests is briefly described.

A comparison of the CFD and available experimental results has been conducted at all three flow rates. The CFD analyses show reasonably good agreement with the experimental data at the majority of the pump locations, with the magnitude of the RMS normalised pulsations being of a similar order. The comparison is generally better at the best efficiency flow point than at the reduced flows. The relative pulsation magnitudes at different locations within the pump are also well predicted.

Typical results from the analyses, condensed from an enormous data set, are presented. A number of interesting flow characteristics have been identified which are consistent with earlier published work, although the present work is more comprehensive. Pressure related data is also shown for a number of locations around the pump that indicate that the impeller outlet location experiences the largest variation in pressure. A summary of typical pressure pulsation values, gained through a parametric study using the Taguchi method, is provided alongside percentage contributions of a number of pump geometry features to the pressure pulsation. This information indicates that the cutwater gap and vane arrangement are the dominant geometrical influences on the pressure pulsation magnitude.

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £75 +P&P)