Agricultural Engineering Information Technology Heilongjiang Agricultural Sciences 2011 (3) Comparative Analysis of Flow Field Simulation under Different Flow Conditions of Centrifugal Pump Li Guowei, Sun Qi, Guo Renning, Li Chunfang (College of Mechanical Engineering, Liaoning Technical University, Fuxin, Liaoning 123000) Quasi-method Conduct research to guide practical applications and provide a reliable basis for improved pump design. The commercial CFD software Fluent is used to simulate the whole flow field of the IS14 (M25400 volute centrifugal pump). In the dynamic system coordinate, the Reynolds-averaged Navier-Stokes equation is discretized by the finite volume method, and the standard Are turbulence model is used. The SIMPLEC algorithm is used to solve the data, and the speed and pressure distribution maps of the centrifugal pump under different flow conditions are obtained respectively to truly reflect the internal flow conditions of the pump and analyze and compare them. It is concluded that the pump is close to the rated working condition. When operating, the flow is the most stable.

The pump is a general-purpose machine with a wide range of applications. It can be said that in every department of the national economy, almost all pumps are at work where there is liquid flow. Moreover, with the development of science and technology, the application of pumps is rapidly expanding. Its main application areas include farmland irrigation and drainage, urban water supply and drainage, power industry, petrochemical industry, mining and shipbuilding industry. In addition, the pump in the rocket fuel supply, ship promotion first: Li Guowei (1978-) male, Gaizhou City, Liaoning Province, Ph.D., lecturer, engaged in fluid machinery and engineering professional teaching and research work. E-side has also been applied. According to statistics of different countries, the power consumption of the pumps accounts for about one-fifth of the total power generation in the country. It can be seen that the pumps are currently large consumers of energy. Therefore, it is of great significance to improve the level of pump technology to save energy.

The traditional research and design of the pump is based on experiments, and the research plan is determined by empirical judgment. This is a repetitive work that requires constant design and experimentation. Therefore, a longer period and higher costs are required, and the dependence on empirical judgment is also stronger. In the past decade, with the rapid development of computer technology, numerical simulation has begun to be more widely used in pump design and flow field analysis. Fluent is a dedicated CFD software for simulating and analyzing fluid flow and heat exchange problems in complex geometries. 131. Fluent can be used for solving engineering problems involving fluids, heat transfer, and chemical reactions. It has a wealth of physical models, advanced numerical methods, and powerful front-end processing functions. It has a wide range of applications in aerospace, automotive design, oil and gas, and disaster turbine design. 14-51. Therefore, the centrifugal pump is assisted with Fluent. Design becomes a reliable method. CFD technology has now become one of the most important tools for pump development, and is an important means to improve the core competitiveness of pumps such as hydraulic performance. 6.1 Research objects and modeling Centrifugal pumps are turbines that convert the mechanical energy of prime movers into liquid energy. Mechanically, it consists of a rotating impeller and a stationary volute. The impeller in the volute is driven by the prime mover to rotate at a high speed. The fluid in the impeller rotating at a high speed obtains energy through the action of the blades and is thrown out of the impeller to form a vacuum inside the impeller. At the same time, external fluid flows into the impeller along the center of the impeller. In this way, the cycle works continuously, working on the liquid, increasing the potential energy, pressure energy and kinetic energy of the fluid (high-speed liquid flow) so that the required amount of liquid is transferred from the suction tank to the required height or required pressure via the pump's flow-through part. The place 171. Research object, the basic parameters: the number of impeller blades is 6 impeller inlet and outlet diameters are 140mm and 400mm, the impeller outlet width is 20mm, the blade outlet angle is 25° rotation speed is 1450r-min1, rated flow is 0.09m3V, The inlet pressure of the pump is 101325 Pa. The liquid is transported under standard conditions with a density of 998. m3. The fluid viscosity is modeled as a geometric model, and the passage part of the flow passage part is meshed. Due to the complexity of the model, the paper mainly adopts unstructured grids with better adaptability in the discrete process. And encrypt the mesh at the blade and the tongue to ensure the accuracy of the calculation.

2 The flow involved in the simulation calculation method is an incompressible steady-state turbulent flow, and a non-coupled implicit solver is selected. Considering the situation of computational resources, control equations and various turbulence models and calculation methods, the Reynolds-time-averaged NS equations are discretized in the calculation using the finite volume method. The standard ke turbulence model is adopted and a separate SIMPLE algorithm is selected.

2.1 Control equations The fluid dynamics control equations are a set of differential equations consisting of a continuity equation, an equation of motion, and an energy equation. These equations reflect the conservation properties of physical quantities in unit time and unit volume. Its general form can be expressed as: item. Yr, s have a specific form for a specific meaning. The equations are in turn unconstrained items, convection items, diffusion items, and source items. According to Einstein's summation agreement, the i-corner is called a summation in a medium weight and represents a summation of three terms, i may have a value of 1,23. Due to irregularity, mixing and dissipating of turbulence For other characteristics, statistical methods must be used when dealing with turbulence problems. The time average method (abbreviated as time average method) is often used in engineering to describe and solve turbulence problems. The turbulent flow time-average equation can be obtained by using the continuous equation and the Reynolds time-average rule, that is, the turbulent flow Reynolds time-averaged NS equation. 181 2.2 Boundary conditions According to the characteristics of the centrifugal pump inlet flow channel, it is assumed that the inlet velocity is evenly distributed in the axial direction and the flow is axisymmetric. Without rotation, the speed import boundary conditions are adopted, and the specific value is given by the ratio of the flow rate to the inlet area. Before the calculation, the speed and pressure of the fluid at the outlet of the volute were unknown. Therefore, free outlet boundary conditions were used in the outlet boundary conditions. In viscous fluid calculations, Fluent uses slip-free boundary conditions. When the wall has translation or rotation, a tangential velocity component can be defined as a boundary condition, and shear stress can also be defined as a boundary condition. For impeller fluids, the vanes are rotated about the central axis, so the vane surfaces are set to move the walls, but the speed of movement relative to the adjacent fluid areas is still zero.

For the volute chamber, it is set as a stationary wall because it is at rest during the entire flow.

(3) Simulation results and comparative analysis show the flow field distribution under rated flow conditions for flow conditions, and the pressure and velocity distribution maps (see below) were obtained and compared and pressure distributions were made. 1 It can be seen from the pressure field distribution that the fluid pressure gradually rises from the pump inlet to the discharge port, with the minimum pressure occurring near the impeller inlet and the maximum pressure occurring near the outlet of the volute. The pressure is essentially the same at the impeller outlet and the volute outlet. However, a pressure gradient appears in the area of ​​the flow path included in the two blades. The pressure gradient at the inlet of the impeller is not large, but the exit is obvious. That is, the pressure gradient gradually increases from the inlet to the outlet of the impeller. Looking at the pressure at the same radius of the impeller, the pressure at the working face of the blade is greater than the pressure at the back of the blade. Since the pressure on the back of the blade is not far from the inlet, it is the place where cavitation is most likely to occur. From the comparison of several different flow conditions, it can be seen that as the flow rate increases, the pressure in the entire flow field of the centrifugal pump also increases. In general, the trend of pressure distribution is basically the same. However, the smaller the flow, the greater the area of ​​negative pressure at the inlet of the impeller, so the smaller the flow, the easier it is to generate cavitation. As the flow rate increases, the maximum pressure of the pump does not appear at the outlet of the volute, but a high pressure area appears along the volute one week, which can cause serious energy loss due to the decrease of the outlet pressure. No matter under large flow conditions or under small flow conditions, it is not conducive to the work of the pump, so it should be ensured that the pump operates under rated conditions.

3.2 Velocity field distribution Velocity distribution diagram It can be seen from this that the speed from the inlet to the outlet of the impeller is increasing. The speed of the working face of the blade is smaller than that of the back face, and the speed is basically the same near the outlet of the impeller. The relative flow between them creates an axial vortex. There is little change in speed in the attachment layer of the working face of the blade, but only a slight increase in the speed at the blade exit, and the speed change in the attachment layer on the blade back face is more obvious. From the figure, it can also be seen that the speed of the impeller inlet to the outlet gradually increases, the inlet speed gradient is relatively small, the outlet speed gradient is relatively large, and the speed at the blade outlet reaches a maximum. The flow velocity in the volute changes little, and at the outlet of the volute, the velocity gradually decreases.

It can be seen from the comparison of several different flow conditions that as the flow rate increases, the centrifugal pump outlet flow rate increases. Under the condition of small flow conditions, due to the small flow rate and high impeller speed, the flow of the fluid becomes unstable and the gradient of the velocity changes quickly. When it reaches the outlet, the speed becomes very small. If the speed continues to decrease, It is very easy to produce backflow, causing a great deal of energy loss. Under conditions of large flow conditions, as the flow rate increases, in local areas of the diffuser tube location, the fluid velocity becomes large due to localized shocks, and the flow is irregular, so that backflow and eddy current phenomena occur. Therefore, the flow of fluid is still the most stable at rated flow conditions.

4 Conclusion Model 125-400 volute centrifugal pump flow field simulation of the whole machine, in the dynamic coordinates, the Reynolds-time-averaged Navier-Stokes equation is discretized by the finite volume method, the standard ke turbulence model and the SIMPLEC algorithm are used to evaluate angles. The solution shows the distribution of speed and pressure of the centrifugal pump under different flow conditions, which truly reflects the internal flow of the pump, and analyzes and compares the results.

From the results of numerical analysis, the results obtained from the simulation calculations are in accordance with the general rules of the centrifugal pump operating characteristics. This shows that it is correct and feasible to use Fluent to perform numerical simulations on the flow field of the centrifugal pump.

After the fluid enters the impeller, it flows out of the impeller through the vane runner.

During the flow of the fluid in the impeller, the rotating blades transfer the energy to the fluid and at the same time the pressure energy and kinetic energy of the fluid increase accordingly. From the pressure and speed distribution map, the overall distribution trend is consistent with theoretical research. It can be seen from the figure that part of the fluid in the volute casing part is a large part of the energy loss, and in the vicinity of the impeller outlet, the fluid pressure and speed of the volute are greatly changed, which is also a factor causing instability of the flow field in the centrifugal pump. one.

Through comparison and simulation of the internal flow field of the centrifugal pump under several different flow conditions, we can see that with the increase or decrease of the flow rate, the pressure and velocity of the internal flow field of the centrifugal pump have been changed to varying degrees. In addition, in the centrifugal pump with small flow and large flow conditions, the flow field inside the impeller shows a very obvious difference, and the pressure and velocity gradients appear to be large in some areas. In contrast, the distribution of the flow field under the rated conditions is relatively uniform, and the impeller and volute have better effect, and the fluid flow is also more stable.

The fluid flow is the most stable under the rated conditions. With the increase or decrease of the flow rate, the fluid flow will often have various unfavorable conditions, which will cause the loss of energy and will not be conducive to the work of the pump. Therefore, various measures should be taken. Measures ensure that the pump is operating near rated conditions.

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