In this study,the unsteady flow and heat transfer characteristics of a laminar slot jet at low Reynolds numbers impinging on an isothermal plate surface in a two-dimensional confined space are numerically investigated...In this study,the unsteady flow and heat transfer characteristics of a laminar slot jet at low Reynolds numbers impinging on an isothermal plate surface in a two-dimensional confined space are numerically investigated.The investigations are performed at Reynolds numbers of 120,150 and 200 based on the nozzle width and mean inlet velocity of the jet.Results show that the Reynolds numbers of 120,150 and 200 correspond to different flow features,namely,a steady flow,an intermittent flapping motion of jet column and a continuous sinusoidal flapping state,respectively.Based on some time snapshots of the flow field,the dynamic characteristics and driving mechanism of the intermittent flapping motion of the jet column and the continuous sinusoidal flapping state are explained.When the jet flaps at the Reynolds number 150 and 200,there are other Nusselt number peaks outside the stagnation zone,which are related to the interference between the vortices shedding on both sides of the jet and the boundary layers of the plate surface.Furthermore,the dynamic mode decomposition is implemented to accurately extract flow modes with characteristic frequencies.For a Reynolds number of 150,there is a flapping mode,which describes the lateral flapping motion of the jet column.When the Reynolds number is 200,there are multiple modes related to the flapping motion of the jet,as well as a low-frequency mode,which reflects the periodic changes of the boundary contour and position of the recirculation zone.展开更多
To improve the cavitation performance of the centrifugal pump, a new kind of centrifugal pump impeller with slot is proposed. The slot is on the impeller shroud near the suction side of the blade leading edge. So, the...To improve the cavitation performance of the centrifugal pump, a new kind of centrifugal pump impeller with slot is proposed. The slot is on the impeller shroud near the suction side of the blade leading edge. So, the fluid with high energy in the impeller front side chamber is drained to the lowest pressure area. The jet flow would compensate the blade inlet flow with a certain energy. With numerical simulations, the pump's inner flows for 5 different slot sizes and their hydraulic performances are compared with those of the prototype pump. The slot jet would result in the increase of the impeller flow rate and the volute area ratio, leading to the head decrease within the whole flow rate range. The slot jet can suppress the reverse flow remarkably at a low flow rate, and the pump efficiency is improved. The slot jet improves the pump cavitation performance effectively, especially at a low flow rate. To some extent, the smaller the slot size is, the better the pump cavitation performance is. The cavitation performance of the slot impeller with a size of 2 mmx 1.75 mm is much better than that of the prototype impeller. Compared with the prototype impeller, the lowest pressure near the slot impeller blade inlet and even the pressure in all forepart regions are improved significantly. The slot impeller is shown to be effective in suppressing the cavitation, and the available net positive suction head is improved.展开更多
基金the support for the research from the National Key R&D Program of China(2018YFB0604404)。
文摘In this study,the unsteady flow and heat transfer characteristics of a laminar slot jet at low Reynolds numbers impinging on an isothermal plate surface in a two-dimensional confined space are numerically investigated.The investigations are performed at Reynolds numbers of 120,150 and 200 based on the nozzle width and mean inlet velocity of the jet.Results show that the Reynolds numbers of 120,150 and 200 correspond to different flow features,namely,a steady flow,an intermittent flapping motion of jet column and a continuous sinusoidal flapping state,respectively.Based on some time snapshots of the flow field,the dynamic characteristics and driving mechanism of the intermittent flapping motion of the jet column and the continuous sinusoidal flapping state are explained.When the jet flaps at the Reynolds number 150 and 200,there are other Nusselt number peaks outside the stagnation zone,which are related to the interference between the vortices shedding on both sides of the jet and the boundary layers of the plate surface.Furthermore,the dynamic mode decomposition is implemented to accurately extract flow modes with characteristic frequencies.For a Reynolds number of 150,there is a flapping mode,which describes the lateral flapping motion of the jet column.When the Reynolds number is 200,there are multiple modes related to the flapping motion of the jet,as well as a low-frequency mode,which reflects the periodic changes of the boundary contour and position of the recirculation zone.
基金Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0200901)the National Natural Science Foundation of China (Grant No.51469014)the Longyuan Young Innovative Talents Program.
文摘To improve the cavitation performance of the centrifugal pump, a new kind of centrifugal pump impeller with slot is proposed. The slot is on the impeller shroud near the suction side of the blade leading edge. So, the fluid with high energy in the impeller front side chamber is drained to the lowest pressure area. The jet flow would compensate the blade inlet flow with a certain energy. With numerical simulations, the pump's inner flows for 5 different slot sizes and their hydraulic performances are compared with those of the prototype pump. The slot jet would result in the increase of the impeller flow rate and the volute area ratio, leading to the head decrease within the whole flow rate range. The slot jet can suppress the reverse flow remarkably at a low flow rate, and the pump efficiency is improved. The slot jet improves the pump cavitation performance effectively, especially at a low flow rate. To some extent, the smaller the slot size is, the better the pump cavitation performance is. The cavitation performance of the slot impeller with a size of 2 mmx 1.75 mm is much better than that of the prototype impeller. Compared with the prototype impeller, the lowest pressure near the slot impeller blade inlet and even the pressure in all forepart regions are improved significantly. The slot impeller is shown to be effective in suppressing the cavitation, and the available net positive suction head is improved.
