In this study, experimental/numerical investigations on the effect of axial offset of rotor offset in a final stage model of a three-stage centrifugal pump are carried out. The axial offset is intentionally given to the shaft system to both of discharge/suction side. As a result, the axial thrust is significantly affected by the axial offset in the flow rate range below 50% of the design flow rate. From the flow field computed in CFD analysis, the flow mechanism induced by the axial offset leading to the change in the axial thrust is elucidated; the interaction between the back flow form the diffuser and the impeller exiting flow is affected by the axial offset, which are responsible for the change in the angular momentum brought into side gaps and therefore in the pressure distribution inside the side gaps.
To improve the cavitation performance of a centrifugal pump, the high-pressure liquid in the front pump cavity is introduced into the cavitation area on the back of the blade by perforated holes at the front cover of the impeller. Based on the RNG k-ε turbulence model and Zwart-Gerber-be1amri cavitation model, the numerical calculation and analysis of the cavitation flow field in the model before and after perforation under different cavitation numbers is carried out by FLUENT. The results show that: The perforation at the front cover plate can effectively improve the pressure value of the cavitation area on the back of the blade. To some extent, the change of pressure gradient restrains the development of cavitation. The perforation at the front cover plate can effectively reduce the integral value of the cavitation bubble in the passage, improve the flow passage conditions in the impeller passage, and reduce the blockage degree of the cavitation to the passage. At the same time, the fluctuation range of the cavitation volume in the impeller is small in a rotation cycle after the perforation. It can be seen that the way of perforation at the front cover plate can effectively improve the cavitation performance of the centrifugal pump.
In recent years, the fog gun trucks for outdoor dust removal have been widely used and developed in China. In this paper, the spray flow field simulation of FLUENT is carried out for the spray nozzle's mass flow rate and the structure of air supply with circle radius decreasing. The analysis shows that the decreasing degree of the reducing air supply outlet produces the corresponding shear flow, which makes the air supply outlet of the fog gun obtain more energy distribution in the position of the nozzle, and it is more conducive to the divergence and uniformity of the fog droplets by sending the air generated by the nozzle to a longer distance. With the increase of nozzle mass flow rate and wind speed flow rate, it is more conducive to the air supply and divergence of droplets. The effect of adding nozzle around the ejector is better than that in the center of the ejector. A design of flat ejector with higher efficiency is proposed.
The impeller-volute interaction flow in centrifugal pump is influenced by the flow in impeller-tongue gap. In order to completely understand the mechanism of interaction between impeller-volute flow and impeller-tongue gap flow, a transient three-dimensional numerical simulation of the flow in a single stage centrifugal pump was carried out by applying sliding mesh approach and the standard k-ε turbulence model in CFD, and the derived transient flow data were time-averaged over a period of one blade passing the tongue. The analysis of the flow in the pump revealed that under off-design conditions, a reversed flow with lower pressure at small flowrates below the dutypoint or a stagnation region with higher pressure at high flowrates above the dutypoint appeared in the near tongue region in volute, which enhanced the asymmetric flow in impeller channels. It was consequently considered that the flow in impeller-tongue gap was a superposition of a drag flow by impeller and a pressure leakage flow driven by pressure difference between two sides of the tongue, and the pressure difference was zero at design condition, but increased with the deviation degree of the flow in impeller from the dutypoint. Under smaller flowrates, the gap leakage flow has direction opposite to that at higher flowrates, and affects much the volute flow. In the end, based on the analytical results, a semi-empirical model for the volute flow was deduced by one-dimensional flow continuity, and it may supply a reference for optimizing the flow in volute.
The hydropower plants under Himalayan basins are mostly characterized by heavy sediment load due to geographical and metamorphic constraints. Run-off-river projects with limited size of the desilting basins allow suspended sediments to get carried into the turbine components causing wear due to sediment erosion. In the case of high head power plants consisting of Francis turbines, a large portion of the hydraulic energy is transformed into kinetic energy within the guide vanes. This causes various instabilities in the flow due to high acceleration and velocity. Some recent studies have shown that due to the combined effect of the secondary flow around the guide vanes and sediment carrying flow, the size of the clearance gap increases, which further aggravates the performance of the turbine. This study takes a reference of one of the power plants in Nepal containing high head Francis turbines. An in-depth analysis of the effect of the sediment in this power plant and sediment erosion in the turbine components has been performed. A CFD analysis of the guide vanes and runner blades corresponding to the same turbine has been conducted and the results are used to analyze the erosion pattern on the actual turbine. The detailed erosion analysis is made possible with a 3D scanner, such that the eroded regions can be captured and classified based on the flow behavior at those regions. Guide vanes and runner blades are found to be the predominant components affected by erosion. It has been seen that most of the erosion affected regions are originated from increasing clearance gaps between guide vane and facing plates caused due to continuous leakage flow within the two sides of the guide vanes.