Numerical Simulation of Cavitation Performance of Low Specific Speed ​​Centrifugal Pump

Numerical simulation of the cavitation performance of centrifugal pumps with low specific speed. (The Department of Environment and Municipal Engineering, Henan Institute of Urban Construction, Pingdingshan 467001, Henan, China) The study of internal air bubble distribution shows that the pump is already on the back of the blades when the inlet pressure is high. When generating cavitation, critical cavitation margin and allowable cavitation margin, bubbles are distributed on the surface of the blade and the flow channel occupy part of the impeller flow path, affecting the internal energy exchange of the impeller, and may cause cavitation damage to the pump. . The experimental study of the real pump verified the accuracy of the numerical calculation.

Fund Project: 11th Five-Year National Science and Technology Support Program (20C8BAF34B10) -) Female, Ye County, Henan, lecturer, master, engaged in chemical machinery research uE-mail:dimI-pghlIc.edu.cn Centrifugal pump is widely used In the design and use of centrifugal pumps, the cavitation performance is usually one of the factors that must be considered.When the centrifugal pump is in operation, when the pressure in the local part of the pump is lower than the vaporization pressure of the transport liquid, there will be Air bubbles are generated and cavitation occurs.When serious cavitation occurs inside the centrifugal pump, it will cause changes in the external characteristics of the pump, resulting in vibration noise and corrosion damage caused by the overflow parts. FD technology has been able to accurately predict the cavitation performance of the pump. And the analysis of gas-liquid two-phase flow during the internal cavitation of the pump is performed by FD meshing. The suction chamber flow path is divided by structural mesh. Due to the complexity of the model, core hexahedron non-structural is adopted for the impeller and volute passage. In the grid, local encryption was used in the area with a small radius of curvature, and the number of grids for the suction chamber, impeller, and volute was 119408, 436147, and 316061, respectively. 877029. To calculate the area for the grid.

The clear water of the grid region in the calculation area, the turbulence model uses the standard Ke turbulence model, the cavitation model is the Rayleigh Plesset model, and the vaporization pressure of water at 25°C is 3169 Pa. The standard wall function is used near the wall surface, and the boundary condition of the wall surface It is set as adiabatic non-sliding wall surface and the wall roughness is set to 10Mm. The second-order discrete format is used to discretize the calculation region, the calculation accuracy is set to high accuracy, and the convergence accuracy is set to 10-5. The liquid density will change when air bubbles occur. Therefore, the flow inside the pump is set to a compressible flow 183, and heat conduction is taken into account during the calculation.

The analysis type is set to steady state, the inlet boundary condition is set to the total pressure inlet, and the internal cavitation of the pump is caused by adjusting the total inlet pressure. The volume fraction of water at the inlet is set to 1 and the volume fraction of bubbles is set to zero. The outlet boundary conditions are set to the flow of the quality exit control model. The interface between the impeller and the volute and the suction chamber is connected with a frozen-mtor interlkce.

3 Analysis of numerical results The critical cavitation margin NPSHc is used to calculate the effective cavitation margin M9 when the pump head is reduced by 3%. The effective cavitation margin NPSHA is calculated according to formula (1): 31 Cavitation The Distribution of Bubbles on the Surface of a Blade During the Initial Cavitation under Optimum Conditions . It can be seen from this that, when cavitation occurs, bubbles are distributed on the back of the blade inlet near a small low pressure area of ​​the rear cover. The initial position of cavitation is the lowest pressure point of the centrifugal pump impeller. When cavitation was born, the NPSHA of the pump was 17.53m, and the total inlet pressure was 175000Pa. The numerical calculation of the pump under the optimum operating conditions had a NPSHc of 273m, and the total inlet pressure of the pump was 30000Pa. This indicates that the pump At the time of high import pressure, cavitation has already occurred. However, since the air bubbles are only generated and broken in a small local area on the back of the blade inlet, it will not cause cavitation damage of the impeller, and will not affect the energy exchange process inside the impeller due to the air bubbles blocking the flow channel, causing the outside of the pump. Changes in characteristics. According to the results of numerical calculations, cavitation has occurred in all pumps under normal operating conditions.

The distribution of bubble volume on the surface of the blade at the initial stage of cavitation cavitation (NP5HA=1753m) 3.2 The bubble distribution in the impeller during the critical cavitation margin is the blade surface and impeller at the critical operating point cavitation margin. Internal bubble volume fraction. The internal bubble volume fraction of the impeller is the equal volume fraction of the volume fraction of the bubble when 20% of the bubble volume was created by Iarsutface in the CED-Post.

(a) Leaf surface surface nucleus critical cavitation margin The bubble volume fraction at the surface of the blade and inside the impeller (NPSHfc=2 from which it can be clearly seen that the bubble has been distributed only on the back of the blade when the bubble has been vacant from the initial cavitation margin The local low pressure region expands to the working face of the blade, occupies part of the flow channel, and the distribution of bubbles on the blade surface and inside the impeller is not symmetrical.

Due to the low specific speed centrifugal pump impeller, the flow path is long and the internal liquid flow rate is high. Therefore, when the bubble is generated, it rapidly spreads to the impeller flow path and the blade surface and breaks in the high pressure area. With the reduction of the effective cavitation margin, the amount of bubble generation will increase, and its degree of diffusion to the exit site will also increase, thus changing the internal flow state of the impeller, resulting in blockage of the flow channel and affecting the energy exchange process inside the impeller. The characteristic performance is the decline of the external characteristic curve. The asymmetrical distribution of bubbles on the surface of the blade and the impeller is due to the fact that the internal flow of the centrifugal pump is a low-speed flow, and the flow state in the impeller will be affected by the flow state of the flow-through components after the impeller such as the volute. Due to the asymmetry of the volute and the pressure pulsation caused by the vortex and back flow between the impeller and the volute, the pressure distribution of the liquid inside the different flow channels of the blade is asymmetric. According to the distribution of air bubbles inside the impeller, it can be seen that for low specific-speed centrifugal pumps, if the pump is operated under a critical cavitation excess condition for a long time, the impeller will generate cavitation.

3.3 Allowable cavitation allowance impeller bubble distribution law often k take 0.0.5m). In this paper, when the allowable cavitation margin of the pump is 5m), the internal flow of the impeller is studied, and the distribution of gas-liquid two-phase inside the impeller is obtained when the allowable cavitation allowance is allowed.

The air bubble distribution inside the impeller is allowed when the cavitation allowance is allowed.

The bubble volume fraction allows the cavitation allowance when the bubble distribution on the surface of the blade and the impeller can be seen from the vacancy allowance, when the allowable cavitation margin, the volume fraction of bubbles on the surface of the blade and the impeller has been reduced much more than the critical cavitation margin. However, bubbles still occupy part of the flow path and have a small amount of distribution on the surface of the blade. Since cavitation is a complex process involving mechanical, physical, chemical, electrolytic, etc., cavitation occurs long before the allowable vacancy margin, And the value of k is not certain. According to the numerical calculation results, if the low specific-speed centrifugal pump is operated for a long time under the allowable cavitation margin, it may cause cavitation to the impeller. The concept of APSH4Gh has been given in foreign countries in the hydraulic design of the impeller. It is recommended that NPSH40000h is the product of F and NPSHc, F is the safety factor, and F1. 5 (or 20) (F is the large value at large flow, and it is small at small flow). This is the cavitation in the pump for the domestic fluid machinery industry. In-depth research has certain implications.

4 Verification test for accuracy of numerical calculation In order to verify the accuracy of the numerical calculation, a cavitation test was performed on the pump according to Standard 9. The test was carried out using a closed test bench. The vacuum level at the suction inlet was controlled by a vacuum pump to allow the pump to cavitation, and test data such as inlet and outlet pressure, water temperature, shaft power, and rotational speed were recorded.

For the test device schematic.

The schematic diagram of the test device (a) is the comparison of the curve between the effective cavitation heading and the head resulting from the test and numerical calculation under optimal operating conditions. (b) is the critical cavitation allowance obtained from the experimental and numerical calculations at different flow rates. Comparison curve with the relationship between the flow rate. It can be seen from the above that the critical cavitation margins obtained by the test and numerical calculation of the pump at optimal operating conditions are 2.82m and 273m, respectively. The maximum relative error is 6 The cause of the error may be the pump's casting error and numerical dissipation. Error, wall roughness settings, etc. From the results of experiments and numerical calculations, numerical calculations have higher accuracy.

5 Conclusions Through the cavitation performance test and numerical study of a low specific speed centrifugal pump, the accuracy of the numerical simulation method to predict the cavitation of the pump was verified, and the internal gas-liquid two-phase flow field during the occurrence of pump cavitation was analyzed. The distribution of air bubbles inside the impeller when the pump is cavitation is obtained.

Based on the study of the distribution of air bubbles inside the impeller when the cavitation primary, critical cavitation headroom and allowable cavitation margin of the pump are used, it is pointed out that the low specific speed pump has already generated cavitation when the inlet pressure is high; the pump is critical When the cavitation allowance and allowable cavitation allowance are in operation, bubbles are distributed in the inner part of the impeller, which may cause cavitation damage to the impeller. Research results have a certain degree of in-depth study of centrifugal pump cavitation

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