Unstable or flat head-flow curves can cause problems in parallel operations or in flat systems. Despite the considerable efforts that have been devoted to the study of head-flow curve instability in single-stage centrifugal pumps with volute casing, the cause of such phenomenon is not sufficiently understood. In this study, we investigated the variation of hydraulic losses based on the relationship between velocity distribution and entropy generation fields. Steady-state and unsteady simulations were obtained for a pump with an impeller outlet diameter of 174 mm, and the unsteady results are more coincided with the experiments. Results showed that the losses mainly focused on the blade suction surface and volute tongue, as well as in the region of the volute discharge at high flow rates. The entropy generation rate of the pump casing at partial flow rates changed slightly with a decrease in flow rate, whereas the energy losses in the impeller increased steeply when the flow rate dropped to 35 m 3 /h (the design flow rate was 60 m 3 /h). The losses in the impeller were mainly concentrated on the region near the inlet and outlet and were lower near the impeller inlet than near the impeller outlet, where a counter-rotating vortex was developed near the blade trailing edge. The vortex caused a drastic increase in the entropy generation rate on the pressure surface and in the flow passage. Such increase was the main cause of the head-flow characteristic instability.
The effects of complex vortex structure on the internal flow and performance of a centrifugal fan with inclining symmetrical volute tongue were investigated by numerical simulations. The comparison between experimental results and numerical results on performance of a centrifugal fan is presented. To provide a quantitative analysis on the vortex structure in the internal flow of fan, Q criterion as a rule of vortex decision is implemented. Effects on vortex structure and X-velocity of the volute outlet are analyzed by modifying clearance and radius. It is analyzed to provide insight into the performance of the centrifugal fan. Special attention is devoted to the influence of the static pressure and efficiency of the fan by increasing radius of the volute tongue, changing tongue clearance and inclining volute tongue in this paper. The results also show that the static pressure of model B rises as much as 10.59 Pa and the efficiency can be improved by more than 4% compared with the original configuration due to the reduction of flow loss. It is further found that the static pressure efficiency increases with decreasing Q value distribution in the internal flow of the fan.
Process valves are responsible for regulating and controlling the rate and direction of flow in pipeline systems. The V-port ball valve is one kind of process valve with a regulating performance influenced by V-angle. In this article, a DN50 V-port ball valve is taken as the research object. This work therefore aims to investigate the effect of and relationship between the V-angle on valve performance and internal flow properties via experiments and numerical simulations. Results indicate that an increase in either V-angle or valve opening causes a large-pressure fluctuation near the valve outlet, thus leading to a long pressure-stable distance. Meanwhile, the flow coefficient increases exponentially with valve opening, and the value of the exponent remains at 2.5 for different V-angles. Furthermore, the stable position of internal energy loss along the downstream pipe is well-matched with the stable position of external pressure fluctuation. This inspires a new method for controlling the pressure stability downstream from the valve. These results may facilitate improvements in the design and optimization of the process valve, thus benefiting the development of fluid transport techniques in energy industries.
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