Increased demand for power-per-unit of volume for modern centrifugal pumps has made cavitation the main limiting factor in pump design.
Demand for higher power per volume has changed the limits of modern pump design. The traditional approach tries to avoid cavitation if possible because of the damage it may cause in the impeller.
Pump designers increasingly consider incipient cavitation and the effects of the flow’s three-dimensional behaviour when they define the blade shape because one-dimensional design rules are no longer sufficient.
Improving suction capabilities
Employing numerical tools to design and optimise the hydraulics of pumps is standard in industry. Up to now, this optimisation has focused largely on the efficiency and the stability of the head curve through a better control of the recirculation and a reduction of secondary flows. Increasingly, cavitation is also a major limiting factor in pump design.
However, only a few attempts have been made to improve cavitation in pumps using numerical approaches.
They were based upon results obtained from cavitation-free flow analysis using methods of computational fluid dynamics (CFD). While this approach usually helps to delay cavitation inception, it does not support the designer in improving the suction capabilities of the pump.
An optimisation of the suction capabilities is only possible if one is able to control the effect of a modification of the vane shape on the cavitation development and to predict when this cavitation development will impair the head developed by the pump. Hence there is a strong need for an accurate and rapid method to predict the three-dimensional cavity development as well as the associated performance drop. This is especially true for the design of high-suction specific speed pumps for which a smooth operation at part load can only be obtained by optimising the vane shape.
Fast numerical method
Some commercial CFD codes offer two-phase flow models. These methods are able to model the phenomena involved in the cavitation development, but their application needs an unsteady approach. Due to the long computation time, this technique doesn’t meet the requirements during a design process.
To predict the cavitation in pumps, Sulzer Pumps uses a simplified version of a cavity interface tracking method developed at the Laboratory of Hydraulic Machines of the Swiss Federal Institute of Technology in Lausanne (LMH-EPFL). In its original form, this method iteratively adapts the cavity shape in order to reach a given condition at its boundary.
Experiments show that the shape of the cavity can be determined with a fast, non-iterative formula, as long as the cavity development does not affect the main flow. This condition is approximately met in most cases if the cavity doesn’t reach the throat of the blade-to-blade channel.
With the aid of this new tool, Sulzer engineers can calculate the evolution of the length of an attached cavity with the NPSH (net positive suction head) value taking into account the viscous, turbulent and three-dimensional nature of the flow in a pump and predict the head impairment due to the cavitation development.
The cavitation development can take place on the suction side or on the pressure side of the vanes, depending on the flow rate. It has to be noted that, at part-load, the head impairment is mostly caused by the cavity reaching the blade-to-blade throat at the hub even if the cavity starts to develop at the shroud much sooner than at hub.
Reducing the risks of vibration and erosion
This is due to the fact that the throat area is positioned more downstream at shroud than at hub, allowing a larger cavity to develop before reaching the blade-to-blade throat. It indicates the necessity to calculate accurately the cavity length along the span of the vane as a function of the cavitation coefficient to be able to predict the associated head impairment.
Such a procedure can also significantly improve the reliability of high-suction specific speed pumps, allowing to better forecast the effect of possible inlet recirculation at part-load on cavitation behaviour. Cavitation associated with inlet recirculation is one of the possible causes of vibration at partload. If cavitation can be avoided during the design process, it is possible to extend the operating range of a pump.