3.1Model construction and mesh division
In this paper, 94 ~ 03 Ford Powerstroke 7.3L GTP38 turbocharger impeller is selected. The structure is composed of three parts: disc, large blade and small blade. The large and small blades are evenly distributed, the three-dimensional structure of the high-performance impeller is shown in Fig. 1a), and name some parts of the impeller. The size of the high performance impeller is shown in Fig. 1b), unit: mm.And the physical image of the workpiece is shown in Fig. 3c).Mesh division graph is shown in Fig. 4d). The number of nodes is 188468, and the number of elements is 885401.
3.3 Parameter settings of numerical simulation
The geometric model preprocessing, solver setup, and result post-processing are three stages of numerical simulation. The solver setup process includes the selection of parameters such as turbulence model, material selection, motion boundary conditions, and discrete format. Only by setting reasonable parameters in the solver can flow field changes be effectively displayed. Based on a three-dimensional calculation model, a 3D dual precision pressure based solver is selected, and the time type is transient. Considering gravity, the gravity direction is consistent with the fluid direction, with a value of 9.81m/s ², The turbulence model is the LES model, and the subgrid model is the WMLES model. The multiphase flow model is the Mixture model. The solid phase is silicon carbide particles, with a density of 3170 g/cm ², The liquid phase is oil with a density of 780g/cm³. The motion boundary conditions are pressure inlet and pressure outlet, and the pressure outlet is standard atmospheric pressure, with a turbulence intensity of 5%. The solution algorithm selects a pressure velocity coupled SIMPLEC algorithm. Firstly, the flow field characteristics in the direct flow path and the gradual flow path are simulated and analyzed to select a reasonable flow path structure. Then, based on the flow path structure, the effects of different influencing factors (such as pressure, velocity, abrasive viscosity, etc.) on abrasive flow machining results are explored.
3.4 Numerical analysis of high performance impeller machining by solid liquid two phase abrasive flow
Based on actual abrasive flow machining conditions and relevant research experience, it can be seen that process variables such as inlet pressure, volume fraction, abrasive viscosity, abrasive speed, and abrasive particle size have important effects on abrasive flow machining results. Therefore, inlet pressure (3MPa, 4MPa, 5MPa, 6MPa) is selected in this section The inlet velocity (25m/s, 30m/s, 35m/s, 40m/s) and abrasive viscosity (0.1Pa · s, 0.3Pa · s, 0.5Pa · s, 1Pa · s) were used as processing variables for numerical simulation. The impact of abrasive flow on the surface processing quality of high-performance impeller was investigated by analyzing the characteristics of flow field dynamic pressure, wall shear force, energy, and vortex.
3.4.1 Numerical analysis of high performance impeller machined by solid liquid two phase abrasive flow under different inlet pressures
In actual abrasive flow machining, the pressure pump provides the power for abrasive motion, and under the combined action of inlet pressure and other factors, it achieves the finishing of the workpiece surface. Therefore, it is of great significance to explore the impact of different inlet pressures on the machining effect of abrasive flow polishing high-performance impellers. According to the pressure range and impeller structure types available for actual abrasive flow machining machines, the abrasive inlet pressures of 3MPa, 4MPa, 5MPa, and 6MPa were selected for numerical analysis of abrasive flow machining high-performance impellers.
1) Analysis Of Dynamic Pressure On The Surface Of High Performance Impeller Under Different Inlet Pressures
The cloud diagram of the dynamic pressure change on the impeller surface under different inlet pressures is shown in Fig. 2. By observing the change in the color of the impeller surface, it can be seen that under different inlet pressures, the distribution trend of the dynamic pressure on the impeller surface is the same, both of which are smaller at the inlet of the flow path, and then gradually increase. The dynamic pressure is gradually increasing from the inlet to the outlet. Under the condition that the inlet pressure gradually increases, the dynamic pressure on the surface of the high-performance impeller is also increasing. The characteristics of the stepped distribution of the dynamic pressure nephogram along the impeller axis, i.e., from the inlet to the outlet of the flow path, are more obvious. However, there are some areas with low dynamic pressure in the middle of the blade root where the impeller suction surface and hub surface intersect. The above indicates that the processing effect at the lower end of the impeller is optimal, and the processing effect on the upper end of the blade suction surface The processing effect at the middle part of the blade root is the worst.
2) Analysis of shall shear force on the surface of high performance impeller under different inlet pressures
The distribution of wall shear force of the impeller under different inlet pressures is shown in Fig. 3. It can be found that the wall shear force at the upper end of the impeller hub surface and the suction surface of the large blade is smaller, while the wall shear force at the lower end is the largest. The wall shear force gradually increases in the axial direction from the upper end to the lower end. At the same time, the wall shear force at the edge of the impeller is greater than that at the middle and root of the blade, indicating that the machining effect of abrasive flow on the impeller gradually improves from the upper end to the lower end of the blade, and gradually improves from the outer edge to the wall root. Observing the effect of different inlet pressures on the wall shear force of the flow passage, it was found that when the inlet pressures were 3MPa and 4MPa, the uniformity of the wall shear force on the impeller surface was the best, but the numerical value was small and the processing effect was poor; When the inlet pressure is 5MPa and 6MPa, the overall wall shear force of the impeller presents a stepped distribution, and the machining effect is good and the numerical difference between adjacent regions is small, which does not affect the polishing uniformity. When the inlet pressure is 6MPa, the overall effect is better.
3) Vortex And Energy Analysis In Flow Path Under Different Inlet Pressures
Fluid turbulent motion is a complex, unstable and irregular flow, often accompanied by vortices. The existence of vortices in turbulent motion is accompanied by energy transfer and mass transfer. Therefore, studying the distribution of vortices in the flow path can better reveal the law of abrasive flow on micro cutting of the workpiece surface. In order to study the formation of vortices within the abrasive flow machining flow path and the impact of the flow path energy distribution on the machining effect, and further explain the distribution of wall dynamic pressure and shear force, four cross sections were selected from top to bottom along the axial direction of the flow path for analysis, respectively named as section 1, section 2, section 3, and section 4, as shown in Fig. 4.
Select the 6MPa inlet pressure with the best simulation effect for vortex and energy analysis. The energy and vertex distribution diagram is shown in Fig. 5. According to the figure, there is basically no vortex distribution at the suction surface position at section 1. The cloud image is darker in color, with small total energy, dense vortex lines at the pressure surface position, significantly lighter in color, and larger in total energy, consistent with the concept of vortex carrying energy. It can also be seen at section 1 that the vortex line distribution at the upper end of the hub surface is characterized by a concentration in the middle, with a slight decrease on both sides, and the energy distribution is characterized by a large in the middle and a small on both sides. This also reveals the reason why the dynamic pressure and wall shear force distributions are large in the middle and small on both sides. At section 2, there is a vortex between the large blade pressure surface and the small blade suction surface, and only part of the vortex lines exist between the large blade suction surface and the small blade pressure surface, so the energy at the corresponding positions is also significantly different. This is because the small blades hinder fluid movement, exacerbating the irregular movement of the flow, causing severe turbulence, and increasing particle kinetic energy. At section 3, there are vortices in the space between the blades and the energy distribution is relatively uniform, which is mutually verified with the dynamic pressure nephogram and the wall shear force nephogram. Combining section 2 and section 3, it can be seen that along the axial direction of the flow path, there is always a vortex between the pressure surface of the large blade and the suction surface of the small blade, so the suction surface of the small blade presents a stepped distribution from the blade root to the outer edge; A partial vortex line initially exists between the suction surface of the large blade and the pressure surface of the small blade, and then forms a vortex. Therefore, the pressure surface of the suction surface of the small blade presents a gradually increasing trend from top to bottom. The vortex lines at each position of section 4 are evenly distributed and the total energy is relatively large, but there is no obvious vortex, which is due to the influence of the position of section 4 close to the outlet. From the cross-section of the above four positions, it can be seen that the flow path energy is gradually increasing from the upper end inlet to the lower end outlet, and the processing effect on the workpiece surface is also gradually increasing.
3.4.2 Numerical analysis of high performance impeller machined by solid liquid two phase abrasive flow at different inlet velocity
During abrasive flow machining, the movement of abrasive particles is accompanied by the collision between abrasive particles and the wall surface, which completes the finishing of the workpiece surface. Therefore, it is also important to analyze the impact of abrasive inlet velocity on the machining effect. In this paper, four speeds, 25m/s, 30m/s, 35m/s, and 40m/s, were selected for numerical simulation and analysis of the abrasive flow polishing process of high-performance impellers.
1) Dynamic Pressure Analysis Of High Performance Impeller Surfaces At Different Inlet Velocity
The velocity of abrasive flow polishing fluid near the impeller surface directly affects the impact of abrasive particles on the impeller surface. Therefore, the dynamic pressure distribution nephogram of the impeller at different speeds is selected for analysis, as shown in Fig. 6. It can be observed that when the inlet speed is 25m/s, the dynamic pressure distribution at the upper end of the suction surface of the large blade of the impeller is relatively uniform, and the dynamic pressure is relatively large, and decreases from the middle position of the upper end to the surrounding area, but there is no obvious delamination phenomenon, indicating that the abrasive flow has a good processing effect on the upper end of the suction surface of the large blade. When the inlet speed is gradually increased, the dynamic pressure at the middle of the blade increases significantly, and decreases radially from the middle to the surrounding area. This is higher than the dynamic pressure at other locations, resulting in a significant delamination phenomenon. This is because the abrasive flow polishing fluid is injected into the flow path at a certain speed in a straight line direction and directly collides with the middle of the blade. The force on this part is relatively large, and the direction changes after the fluid collides with the blade, The kinetic energy of particles has also increased. Due to the complex shape of the blade, the abrasive movement direction is chaotic, the position of the blade subjected to force changes, and the greater the inlet velocity, the more severe the fluid turbulence phenomenon. Through comparison, it can be found that when the inlet speed is low, the overall dynamic pressure is small, and the machining effect is uniform. As the inlet speed increases, the overall dynamic pressure of the impeller is also increasing, which indicates that increasing the inlet speed can improve the cutting ability of the abrasive flow on the overall impeller, thereby improving the machining effect.
2) Wall Shear Force Analysis Of High Performance Impeller Surfaces At Different Inlet Velocity
Figure 7 is a nephogram of the distribution of wall shear forces on the impeller at different inlet speeds. It can be seen from the graph that the distribution of wall shear forces on the impeller presents the same distribution trend as the distribution of dynamic pressure. The wall shear forces on the upper and middle ends of the suction surface of the large blade are the largest, and gradually decrease in a circular and radial shape toward the periphery, indicating that the processing effect of abrasive flow particles on the upper and middle ends of the blade is optimal, and this phenomenon becomes more apparent with the increase of inlet speed, When the inlet speed increases, the kinetic energy of the fluid increases, and when there is a collision with the blade surface, the fluid turbulence phenomenon intensifies, improving the cutting ability of the abrasive flow on the impeller surface, thereby improving the machining effect.
3) Vortex And Energy Analysis In Flow Path At Different Inlet Velocity
The flow path extracted by Boolean operations for the gradational flow path structure and high-performance impeller belongs to a thin layer and complex curved flow path. The movement of abrasive flow in this flow path is very complex, so the four cross sections of the flow path are selected to further analyze the movement process of abrasive flow.
Figure 8 shows the cloud diagram of vortex and energy distribution in the flow path at an inlet velocity of 40m/s. There are vortices at the suction surface of the large blade in section 1, but no obvious vortices exist on the pressure surface. Therefore, the energy at the suction surface is significantly greater than that at the pressure surface. On section 2, it can be observed that there are vortices in the interval between the blades. This is because the presence of small blades changes the original direction of fluid motion, making fluid motion irregular, and increasing energy between the blades. Therefore, the energy at section 2 is greater than that at section 1, which also explains the larger suction surface and pressure surface of the large and small blades, It also explains the characteristics of radioactive distribution in the upper and middle portions of the suction surface of the large blade. The vortex and energy distribution on section 3 and section 4 exhibit the same distribution phenomenon as on section 2. The energy distribution between the large blade suction surface and the small blade pressure surface is greater than that between the large blade pressure surface and the small blade suction surface. Combining the four cross sections to observe the wheel hub surface, it can be found that the energy distribution of different cross sections of the wheel hub surface gradually increases from left to right, and the energy distribution along the wheel hub surface gradually increases from top to bottom, which is consistent with the dynamic pressure distribution and wall shear force at this speed, explaining why the machining effect of abrasive flow on the right side is better than that on the left side.