Numerical Simulation of Effect of Centrifugal Pump Pressurized Water Chamber on Performance of Micro Electric Pump

A miniature electric pump is a pump with input power less than 1.1 kW. It has characteristics of small flow, high lift, light weight, simple structure, strong versatility, easy use, etc. Wide fund project: National Science and Technology Support Project (2008BAF34B15) in the Eleventh Five-Year Plan and Science and Technology Service Industry Project (BM2008375) in Jiangsu Province.

Pan applied to agriculture, petroleum, chemical industry and other fields. Most of the miniature electric pumps are low specific speed centrifugal pumps. Because the blade outlet width is small, the impeller outer diameter is large, and the axial flow path is narrow and long, resulting in large disc loss and hydraulic loss, so the efficiency of the pump is very low.

The pressure chamber is one of the main flow components of the pump, and its form mainly includes a spiral pressure chamber, an annular pressure chamber and a space guide vane. In general, the spiral pressure water chamber conforms to the flow law of fluid outflow, the flow state is ideal, and the water pump can obtain better hydraulic performance. Most centrifugal pumps use a spiral pressure water chamber. The annular pressure water chamber is mainly used for the slurry pump, because this structure has a large gap between the tongues, and it is not easy to cause the clogging of impurities, and the process is convenient; the final guide vane of the multi-stage pump also uses the annular pressure water chamber, because of this Symmetrical structure, easy to wear through the bar, and uniform thermal deformation.

Most of the miniature electric pumps use spiral pressure water chambers. However, due to the small cross-sectional dimensions of the volute, the flow channels cannot be machined, causing their shape and size, surface finish, etc. to be guaranteed directly by casting, and the difficulty of casting is high. The roughness of the track surface is large, resulting in a large hydraulic loss in the pump body. For miniature electric pumps, the hydraulic loss in the pump body is second only to the friction loss of the impeller disc, which has a decisive influence on the performance of the pump. At present, many scholars have conducted a series of studies in this area. Liu Zailun et al.1 studied the effect of volute shape on the performance of high-speed partial-flow pump, and pointed out that the use of a rectangular spiral volute can improve the lift-off point of the dead-end and improve the efficiency of the pump at the same time. Guo Pengcheng and others studied the effect of volutes with different cross sections on the performance of centrifugal pumps and found that the efficiency of spiral and rectangular spiral volutes is slightly higher than that of horseshoe-shaped volutes under large flow conditions, and slightly lower than that of horseshoes at design operating points. some. It has been mentioned that when the specific speed is lower than 40, the pump efficiency may be higher than that of a spiral pressure chamber that is not processed because the annular pressure water chamber is easy to machine and grind.

Based on this idea, based on the spiral pressure chamber, according to the design theory of the annular pressure chamber and the ease of machining, three types of annular pressure chambers with rectangular sections were designed, and these four pressures were calculated. The water chamber is combined with the same impeller for three-dimensional constant-value simulation. By comparing with the prediction of the performance of the micro-electric pump in the traditional spiral pressure chamber and the analysis of the internal flow, it provides the rationale for the performance optimization of the micro-electric pump. The company's XCm158 centrifugal pump is used for numerical simulation of the research object. The relevant parameters are: impeller's inlet diameter A = 38.5mm, outlet diameter D2 = 162mm, number of blades z = 6, blade outlet width 2 = 2.2mm, blade outlet mounting angle = 26° volute base diameter A = 164mm The 8th section area Ai = 102.5mm2, the inlet width of the chamber 63 = 10.5mm; the pump rated flow CL = 4m3 / h, rated head Hi = speed n = 29. It is defined as No. 1 pump.

The XCm158 miniature electric pump uses a spiral pressure water chamber, on the basis of which it is changed to a rectangular cross-section of the annular pressure water chamber, and to ensure that the area of ​​the 8th section of the two is equal. In addition, on the basis of the annular pressurized water chamber, further improvement is mainly followed by the following principles: 1 the diameter of the base circle is constant; 2 the inlet width of the pressurized water chamber is not changed; 3 the outlet diameter and relative position of the diffusion section are not changed.

According to the above principles and the design theory of the annular pressurized water chamber, the area of ​​the 8th section of the annular pressurized water chamber is 102.5mm2. In the diffuser section, the outlet size adopts the standard nominal diameter of 24mm, which is defined as the No. 2 pump. The height of the axial surface of the 8th section of the chamber is 9.7mm. Based on this model, the height of the axial surface of the section of the annular pressure chamber is increased by 5 and 10mm, respectively, as comparison models No. 3 and No. 4 pumps. The main geometry of the pressure chamber is shown in Table 1.

Model Number Base circle Diameter A3/mm Section 8 Height 8/mm Inlet width Section 8 Area 1 Pump 2 Pump 3 Pump 4 Pump Table 1 Main geometric parameters of the pressure chamber Tab. 2 Model establishment and algorithm 2.1 The establishment of the model is modeled by PRO/E and then imported into ICEM to mesh the model. In modeling, in order to avoid the influence of inlet vortex zone on flow field and flow rate Pfi, add an inlet pipe at the inlet section of the impeller, the length of which is 3 times of the inlet diameter; take into account the export boundary conditions on the outlet flow field and convergence of the volute outlet. The effect is to add an outlet pipe to the outlet section of the volute, and its length is 5 times the diameter of the outlet. The inlet and outlet pipes adopt a structured hexahedral mesh; while the impeller and volute passages are complex in shape, an unstructured tetrahedron adaptive body-fitted mesh is used.

2.2 Numerical calculation methods Numerical simulation uses ANSYSCFX 12.0 to solve the Reynolds time-averaged equations, in which the Reynolds stress terms are solved using the standard e-turbulence equation model and the equations are closed. In ANSYSCFX12.0, the finite-volume method was used to discretize the system of equations. The convection items in the discrete process were analyzed using high-resolution grids, design-point operating conditions, and large-flow conditions (1.4-fold operating conditions). Static pressure cloud under operating conditions.

Comparing the static pressures of the two types of pumps, it can be seen that the static pressure at the outlet of No. 1 pump and No. 3 pump is substantially the same at the operating condition of 0.6, and the static pressure change between the annular pressurized water chamber and the impeller of No. 3 pump is relatively uniform. The pressure gradient of the spiral pressure water chamber of No. 1 pump near the diaphragm is large, and the impeller has a high pressure area near the exit surface of the diaphragm pressure surface of the diaphragm. This is because the No. 1 pump does not flow at a small flow rate. Uniform, the direction of the velocity vector is chaotic, resulting in backflow.In the 1.0Qi working condition, the static pressure distribution in the annular pressurized water chamber first increases and then decreases and then increases, which may be due to the annular structure of the pressure chamber and If the clearance between the impellers is too large, there will inevitably appear a backflow phenomenon of the static pressure cloud diagrams of the two pumps under different working conditions, and part of the fluid re-enters the pressure chamber at the partition tongue, but it is due to the diversion effect of the reflux. The flow rate at the outlet section of the pressurized water chamber is greatly reduced, and the conversion of the kinetic energy of the pump outlet to the pressure energy is realized. This result is different from that of the spiral pressurized water chamber.

At 1.4 (3, operating conditions, there is a significant difference in the outlet static pressure, the outlet pressure of the annular pressure water chamber is significantly higher than the spiral pressure chamber. The reason may be that the larger the flow, the pressure chamber The greater the proportion of friction losses in the process, the more prominent the advantages of the smooth internal wall of the annular pressure water.In addition, the difference between the static pressure of the two pump outlets and the difference in the head curve is plausible.

It can be seen that the pressure and speed of the spiral pressure water chamber are only distributed evenly at the highest efficiency point of the pump, and the pressure and velocity distributions are not even when the pump is operating in a partial working condition.

On the other hand, the pressure in the annular pressurizing chamber is contrary to the distribution of the pump pressure and velocity at the dead point. Once the flow rate is generated, the balance is destroyed. At the highest efficiency, the annular hydraulic chamber has a greater hydraulic loss than the spiral pressure chamber. In the micro-electric pump, because the surface of the annular pressure water chamber flow path can be machined, better hydraulic performance can be obtained, which exceeds the influence of the unstable pressure distribution brought by the annular pressure water chamber on the performance of the pump.

4 Radial Force Analysis When the pump is running, the radial force of the fluid along the radial direction of the impeller will be absorbed, and the radial force will cause the pump shaft to be subjected to the alternating stress to generate the directional deflection, and its size will directly affect the stability of the pump shaft; In addition, the radial force acts to make the seal gap become non-uniform, and the shaft seal gap is too large is the main cause of some pump leakage. Therefore, radial forces need to be properly considered when designing the pump. Numerical simulations of predicted radial forces for No. 1 and No. 3 pumps.

The radial force distribution of the two pumps can be seen from the fact that the radial force of No.1 pump first decreases and then increases with the increase of flow rate, and reaches the minimum value near the design operating point, but it is not 0. The reason is Because of the asymmetric structure of the pump body, the distribution of the flow rate, flow velocity and impeller outlet pressure in each flow path of the pump impeller is asymmetric; while the radial force of the No. 3 pump is the smallest at the small flow rate and increases with the flow rate. The radial force distributions of these two pressurized water chambers are consistent with each other.

In addition, the radial force of No. 3 pump is smaller than No. 1 pump from the small flow rate to the rated flow rate of the pump; in the large flow area, the radial force of No. 3 pump is slightly larger than No. 1 pump. In this way, compared to the No. 1 pump, No. 3 pump with an annular pressure chamber can operate safely and stably in the full flow range.

5 Test verification According to the hydraulic energy of the rotary power pump of the No. 1 and No. 3 pumps (turning to page 88), the turbulent kinetic energy and dissipation rate of the impeller of the centrifugal pump are analyzed. Ye Daxing and Wang Yang will compare the turbulence dissipation rate. There is a very similar pattern to the distribution of turbulent kinetic energy: Under different operating conditions, the turbulent dissipation rate first increases with the increase of radius, reaches a maximum and then begins to decrease, and then begins to increase after a minimum value. Until the impeller outlet (except for 6 (under operating conditions), the turbulence dissipation rate reaches the maximum in the area only = 60mm; under the design conditions, the turbulence dissipation rate is the smallest overall, except only = 55mm to only = 65mm Outside the region, the turbulence dissipation rate is below 400m2/s3.. Under the 6Qd operating conditions, the turbulent dissipation rate distributions that are opposite to those under other operating conditions appear, with small middle and large ends, and other middle working conditions. With a small distribution, it can be seen that the turbulent dissipation rate increases very rapidly in the range of only 65 mm to only 85 mm. This may be due to the fact that under small flow conditions, axial vortices are generated in the impeller flow channel in this region. Causes a drastic increase in turbulence dissipation rate Under operating conditions, although the turbulent dissipation rate is higher than the design condition, it can be seen that it is far below 0.6 (operating conditions; overall, under the design condition, the turbulent dissipation rate is the smallest, and under large flow, turbulence The dissipated energy rate is slightly higher than the design condition. At low flow rates, the turbulent dissipation rate is the highest.

4 Conclusion In this paper, the e double equation turbulence model is used to verify the experiment, and the reasons for the difference between the data of XST standard centrifugal pump head, efficiency, and shaft power measured by numerical calculations and experiments are analyzed, and the reliability of numerical calculation is verified. . ) The distribution of turbulent kinetic energy and turbulent flow dissipation rate along the radius is very similar, ie, the turbulent dissipation rate is also large in areas with large turbulent kinetic energy, and vice versa. In addition to 0.6 small flow conditions, the distribution of turbulent kinetic energy and turbulent flow rate along the radius first increases, then decreases, and finally increases this phenomenon.

In the low flow conditions of 6Qd, the maximum flow energy and turbulent dissipation rate are the most serious, and the fluid energy loss is the most serious. From the aspect of efficiency, the pump should be avoided to run under small flow conditions. □