Study Of Internal Flow In High-Speed Centrifugal Pump With Semi-Open Impeller

Semi-open centrifugal pump is widely used in petroleum chemical industry, aerospace and other fields owing to several advantages of itself: simple structure, easy processing, good performance without blocking and so on. In order to avoid the collision or friction between impeller and the casing in the op eration process, axial clearance is arranged between them. Scholars at home and abroad have done some researches on axial clearance of the semi open impeller, but there’s few reports on how to value semi-open centrifugal pump impeller axial clearance, how to determine the optimum clearance and what is the law of fluid flow in the clearance. Therefore, This paper do the research about the influence of semi open centrifugal pump axial clearance on the performance of the pump by using numerical simulation meth od, based on a single-stage single suction air fuel centrifugal pump provided by a domestic factory. Main work and research results of this paper are as follows:1. The thickness of boundary layer in the outlet of the impeller was calculated according to the formula ? ?1/5=0.37 Rex?x-? ?, then the study was carried out on 5 kind of tip clearances which were obtained around the boundary layer, the value of the tip clearances was 0.25 mm 、 0.3mm 、 0.35 mm 、 0.4mm 、 0.45 mm. By analyzing the numerical simulation results found that:the efficiency of the pump was highest in the design condition when the tip clearance is 0.3mm. The head and efficiency of the pump decreased with the increase of the tip c learance when the value of tip clearance was bigger than 0.3mm. In order to verify the precision of the numerical simulation, the external characteristics of the experiment(the tip clearance was 0.35mm) was compared with numerical simulation result, it turned out that the difference between head was less than 3%, and the difference between efficiency was less than 4.3%. So the numerical simulation method used in the article is feasible.2. The internal flow in the tip clearances was analyzed in order to study the leakage in tip clearances of semi-open impeller and the results were as follows: The flow in the clearance both have peripheral velocity and radial velocity in the same time. The peripheral velocity varied in 0.8~0.95 times of the impeller peripheral velocity, the smaller the clearance, the bigger the peripheral ve locity. Radial velocity of the flow was negative which indicated that the flow direction in the clearance is opponent to the impeller: which means there’s exist clearance leakage. The leakage amount of the clearance was calculated by averaging the radial velocity and then multiplied the area of the inlet, it was found that the amount of leakage in the clearance increased with the increase of the axial tip clearance.3. There’s some discoveries after analyzed the flow in the impeller with different axial clearances: the pressure of blade near the side of clearance was lower than it near the rear cover plate; Near the back entrance of the blade there has a low pressure zone.With the increase of the axial clearance, the low pressure zone near the back entrance of the blade was gradually disappeared, which indicated that the increase of clearance could improve the anti-cavitation performance of the pump4. In order to find the difference between semi-open and closed impeller with same geometric parameters, a front plate of 2mm was added in the impeller and numerical simulation was carried out in the closed impeller. After analyzing the results of the closed impeller and the 0.3mm semi-open impeller, it turned out that the head of closed impeller was generally 1.5 ~2m higher than the semi-open impeller at same flow rate and the value was 1.7m in the design condition. The ratio of the head in closed and semi-open impeller is 0.876 in design condition and 0.82~0.88 in the range of total flow rate. It remains uncertain ty whether the conclusion is common or not.5. The pressure fluctuation characteristics of the blade leading edge were studied by using non stationary numerical simulation method. After analyzing the pressure fluctuation in the blade tip, it turned out that the number of pressure fluctuation was equaled to the blade’s number in one recycle period of impeller, and the basic frequency of the pressure fluctuation was the pass frequency of the blade. The amplitude of pressure fluctuation in blade tip decreased with the increase of the axial clearance which means that the stability of the internal flow in semi-open impeller was improved by increasing axial clearances.