A screw-type centrifugal pump with a wide flow passage has been widely used for drainage of rainwater as well as slurries and mud to avoid the flow passage blockage with the congestion of solids. Due to the complicated configuration of this pump, the design method of this pump has not been established yet. The authors succeeded in predicting the internal flow numerically by using our own grid-generation system and a commercial 3-D N-S code, TASCflow, as a solver. In this study, the internal flow has been predicted numerically at the design point on the five impellers with different hub cone shapes in order to clarify the influence of meridian shape on the pump performance. In particular, the relationships among the pump characteristics, the back flow at the blade pressure surface and the back flow from the volute casing to the impeller exit have been discussed in the numerical results on the velocity and pressure distributions.
The authors have been investigating the various characteristics of screw-type centrifugal pumps, such as pressure fluctuations in impellers, flow patterns in volute casings, and pump performance in air-water two-phase flow conditions. During these investigations, numerical results of our investigations made it clear that three back flow regions existed in this type of pump. Among these, the back flow from the volute casing toward the impeller outlet was the most influential on the pump performance. Thus the most important factor to achieve higher pump performance was to reduce the influence of this back flow. One simple method was proposed to obtain the restraint of back flow and so as to improve the pump performance. This method was to set up a Ring-like wall at the suction cover casing between the impeller outlet and the volute casing. Its effects on the flow pattern and the pump performance have been discussed and clarified to compare the calculated results with experimental results done under two conditions — namely, one with and one without this Ring-type wall. The influence of wall’s height on the pump head was investigated by numerical simulations. In addition, the difference due to the wall’s effect was clarified to compare its effects on two kinds of volute casing. From the results obtained it can be said that restraining the back flow of such pumps was very important to achieve higher pump performance. Furthermore, another method was suggested to restrain back-flow effectively. This method was to attach a wall at the trailing edge of impeller. This method was very useful for avoiding the congestion of solids because this wall was smaller than that used in the first method. The influence of these factors on the pump performance was also discussed by comparing simulated calculations with actual experiments.
A screw-type centrifugal pump is a kind of non-clog type pump and has been widely used, for instance, for drainage of rainwater as well as slurries and mud in sewage control systems (Reference to Fig. 1). In order to avoid choking the impeller passage with the congestion of solids, the impeller is designed to have a wide passage whose configuration is screw formed by a three-dimensional spiral blade added to a conical hub cone. The prediction of radial and axial thrust is important and useful information for the safe operation and the impeller design, to say nothing of the internal flow prediction and pump performance prediction. Particularly, the fluctuation of radial and axial thrust in this pump is large because of one blade. Therefore, to predict the thrust correctly is important. In this paper, the radial and axial thrust has been numerically predicted. Especially, the numerical result on the radial thrust is compared with the experimental results. The validity of thrust prediction has been discussed.
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