Research on the efficiency improvement of pump as turbine (PAT) is inadequate. Blade thickness is an important geometry parameter in blade design. To explore effects of blade thickness on the influence of PAT, numerical research on three different specific speeds of PATs with different blade thickness was carried out. Their performance changes with blade thickness were presented. Besides, the variations of hydraulic loss distribution with increasing blade thickness were performed. Theoretical analysis gives a reasonable explanation for the performance change. Results show thatPAT's flow versus efficiency curve ( Q-η) is lowered; flow versus head ( Q-H) curve and flow versus power ( Q-P) curve are increased with increasing blade thickness. The increase of Q-P is mainly attributed to the increase of theoretical head caused by increasing blockage of impeller inlet area. Hydraulic loss distribution analysis indicates that the total hydraulic loss within PAT is increased with increasing blade thickness. The increase of Q-H curve is a combined effect of the increase in theoretical head and the total hydraulic loss. The decrease of efficiency with increasing blade thickness indicates that the blade thickness of PAT should be as thin as possible if its strength could be met.
The radial gap between the impeller tips and volute tongue is an important factor influencing the overall performance and unsteady pressure fields of the pump as turbine (PAT). In this paper, a numerical investigation of the PAT's steady performance with different radial gaps was first performed. The results show that there is an optimal radial gap for a PAT to achieve its highest efficiency. An analysis of the PAT's unsteady pressure fields indicates that the rotorstator interaction of a rotating impeller and stationery volute would cause high frequency unsteady pulsation within the volute and low frequency unsteady pressure pulsation within the impeller. The high frequency unsteady pressure pulsation would propagate through the PAT's flow channel. Thus, the unsteady pressure field within the impeller is the combined effect of these two kinds of pressure pulsations. The unsteady pressure pulsation within the outlet pipe is mainly caused by the propagation of unsteady pressure formed within the volute. With the increase of the radial gap, the amplitude of high frequency unsteady pressure pulsation within the volute caused by the rotor-stator interaction is decreased, while the amplitude of the low frequency unsteady pressure pulsation caused by the rotor-stator interaction within the impeller remains unchanged.
The use of centrifugal pumps as turbines in the recent years has come as a boon to small and micro power application given its simplicity and robustness. However, attempts are continuously being made to improve the performance by modifying the geometry, and yet to retain its simpleness. This paper proposes a new design to the impeller of an existing pump with forward-shaped vanes in an unchanged volute that is in complete contrast to the conventional backward vanes. Three methodologies are involved in the analysis starting with classical theory, experiment and simulations. The theory entails the focus on optimizing the nozzle shape of the impeller to reduce viscous and eddies. The forward vane having shorter nozzle length has proved to be more efficient compared to the longer backward vane impeller, with an efficiency increase of nearly 5%. The experimental and CFD analysis to study the internal flow saw similitude in the streamline change in Euler moment. It also showed that there were not only increased viscous effects but also enhanced flow separation in the backward vanes at overload flows. The study also found radial clearance losses to be unacceptable for both the shapes. The overall conclusion was to move ahead with the forward design and convince the industry to adopt them for there have been greater strides in cost-effective manufacturing processes. The paper also recommends more study of intermediate blade angles since there was still persistence of small degree of vorticities in the forward blade nozzle. The optimization of volute-impeller interface along with the influence of non-flow zone would be other areas for future investigation. Synergy of academia and industry is also welcome to lend improved understanding in pumps as turbines and better translation to praxis.
In order to predict the axial force of pumps as turbines,computational fluid dynamics software CFX is adopted in the whole flow field numerical simulation of pumps as turbines.The pressure distribution is acquired in all the impellers cover plate and the blades surface.While the angular velocity distribution of liquid is achieved in the cavity.Results show that the axial force of each impellers is not equal at the best efficiency.The value of axial force on first-stage impeller is larger than the next.With the increase in water heads,the axial force of impellers goes up.The average of the liquid angular velocity of all cavities are higher than the traditional calculation value 0.5ω.It ranges from 0.42ωto 0.7ω.
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