Proceedings of the 8th JFPS International Symposium on Fluid Power, OKINAWA 2011 Oct. 25-28, 2011

1C1-3

CFD ANALYSIS OF INLET FLOW AROUND AN IMPELLER OF AN OIL-HYDRAULIC PUMP Kazushi SANADA* and Naohiro YAMAGUCHI** * Professor, Department of Mechanical Engineering, Graduate School of Engineering, Yokohama National University 79-5, Tokiwadai, Hodogaya, Yokohama, Kanagawa, 240-8501 Japan (E-mail: [email protected]) ** Master Student, Department of Mechanical Engineering, Graduate School of Engineering Yokohama National University 79-5, Tokiwadai, Hodogaya, Yokohama, Kanagawa, 240-8501, Japan

ABSTRACT An impeller is used to prevent cavitation by increasing pressure in a suction port of an axial piston pump. Oil flow around the impeller is analyzed using CFD. A 3D model of an inlet chamber, an impeller disk and the outlet port of the impeller is build. Pressure/velocity distributions and pressure increase across the impeller are calculated. The pressure increase well agrees with experimental results. Visualization of stream lines around the impeller gives important information on the impeller characteristics, such as reverse flow which occurs both at the inlet chamber and at the downstream of the impeller. Modification of the flow path is proposed to prevent the reverse flow. KEY WORDS Oil hydraulics, CFD, Axial piston pump, Impeller, Visualization

valve plate, visualization of flow have been investigated by Oyama and Tanaka [2]. Computational fluid dynamics (CFD) is a powerful tool in order to investigate flow inside oil-hydraulic components. Park and Hwang applied CFD to study effects of groove shape on lubrication characteristics of a spool valve [3]. Tsukiji et. al. proposed a useful method to improve an oil-hydraulic ball valve using visualization techniques including CFD [4] . J. Watton wrote a paper summarizing progress of servo vales using CFD [5]. Cavitation in a plunger pump was studied using CFD by K. Edge et. al. [6]. In addition, a trial to predict the characteristics of the pump was performed by CFD [7]. One method to prevent cavitation at the inlet port of the pump is to increase the inlet pressure by use of an impeller disk. Inlet pressure may be increased by installing an impeller disk at the inlet port just before the valve plate. In

NOMENCLATURE p t

: pressure (Pa) : time (s)

INTRODUCTION Construction machineries are used at severe locations, such as open-air mining of highlands. Pumps and motors are required to be reliable when used at difficult conditions. Cavitation is one of important technical subjects to be solved. Low suction pressure may cause cavitation in the pump and may damage some parts of the pump, for an example a valve plate of an axial piston pump. Tsukiji et. al. studied about jet flow from an orifice of a valve plate with a groove in an axial piston pump [1]. For estimation of flow characteristics of a notch of a

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this paper, an impeller disk installed in the inlet port of an axial piston pump is studied by CFD analysis. At first, a 3D CFD model is build focusing on the impeller disk. Through CFD analysis, flow pattern around the impeller disk is visualized and pressure/flow rate distributions are obtained. Pressure increase due to the impeller disk is discussed comparing CFD analysis and experimental results. In order to improve pressure increase, flow path around the impeller disk is modified. Finally, results of this paper are summarized. AXIAL PISTON PUMP An axial piston pump is treated in this paper. The pump has a rated displacement of 180cc. The maximum pressure is 30MPa. An impeller disk is installed in the inlet port (suction port) of the pump just before a valve plate. It has 13 impellers located in radial direction. The impeller disk diameter is 150mm. The impeller disk is directly connected with a drive shaft of the pump and it rotates at the same speed as the pump.

Fig. 2 Top view of the 3D model

CFD ANALYSIS A 3D CFD model of the impeller disk is shown in Fig. 1. An inlet port (suction port), a top cover (an inlet chamber), an impeller disk, and a flow path to a valve plate are modeled. An inlet port is the left side section of the model. A valve plate is located at the bottom of the model. Therefore hydraulic oil comes into the pump from the left-side inlet port. It is twisted in a right angle and goes downward to the valve plate. A top view of the 3D model is shown in Fig. 2. A transparent model is shown in Fig. 3. These figures are provided for better understanding of the 3D model.

Fig. 3 Transparent model CFD software Commercial software was used for CFD analysis. As a flow model, mixed-phase flow was assumed, that is, liquid flow including air bubbles including a small amount of oil vapor. When pressure is decreased, the gas bubble is expanded according to circumferential pressure. It may suggest outbreak of cavitation. However in this study, pressure stays over the certain level and cavitation is not important for the CFD analysis. As a friction model, k   turbulent model was used. Computational grids For CFD analysis, an ideal computational grid is sphere shape. However it is impossible to make grid pattern using the sphere grid. Instead of the sphere, cubic grid and tetrahedron grid are used in practice. The tetrahedron grid is useful to model complicated shape. However the tetrahedron grid may result in increase of the number of meshes. On the other hand, cubic grid is suitable for simple shape. Usage of grid type is important in terms of accuracy of CFD analysis. In this model, flow around the impeller disk was modeled by the tetrahedron grid system and other parts were modeled by cubic grid system as shown in Fig. 4.

Fig. 1 3D CFD model of an impeller disk

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Fig. 5 An example of stream lines

Fig. 4 Tetrahedron grid system for the impeller disk Boundary conditions Pressure increase across the impeller disk is one of the targets of the CFD analysis. In order to calculate the pressure increase, it is necessary to set boundary conditions at the inlet and at the outlet port of the model. A boundary condition of “pressure inlet” was applied to the inlet port. This means that pressure in the cross sectional area of the inlet port is uniform and that it is specified as a condition of calculation. Another boundary condition of “velocity inlet” was applied to the outlet port. This condition corresponds to determination of uniform velocity distribution at the cross sectional area of the outlet port. When pump speed is specified, flow rate of the pump is known and flow rate at the outlet of the impeller model is the same as that of the pump. Therefore velocity at the outlet port is calculated by dividing the flow rate by the cross sectional area of the outlet port.

(a) Side view

RESULTS OF CFD ANALYSIS A number of computations by the CFD analysis were carried out for various conditions using a desktop personal computer. One case of computation took eight hours to get animation for flow visualization. In the following analysis, pump speed was set as 1800rpm. Rotation of the impeller disk was considered in the CFD analysis. The CFD software provided animation video files for visual understanding of flow pattern.

(b) Top view Fig. 6 An example of velocity distribution Pressure distribution Pressure distribution calculated by the CFD is shown in Fig. 7. Red color indicates pressure level of 0.1MPa and dark blue is 0MPa. In the figure, the impeller rotates in a counterclockwise direction. Pressure is increased according to the rotation of the impeller disk. At the outlet of the impeller indicated by a red arrow in the figure (b), pressure reaches the highest value. In the downstream path in the figure (a), the highest pressure was observed in the middle of the flow path between the impeller disk and the outlet port (valve plate). As an ideal design, pressure may reach the highest value at the valve plate.

Velocity distribution An example of stream lines calculated by the CFD analysis is shown in Fig. 5. Velocity distributions are shown in Fig. 6. Velocity of the flow is classified by colors. Red color is 15m/s and dark blue is 0m/s as indicated in the left color bar. The oil flow has the highest velocity of about 14m/s at the outer edge of impeller dsik.

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Pressure increase across the impeller disk Inlet and outlet pressure obtained by the CFD are plotted as a function of time in Fig. 8 (a). Pressure increase across the impeller was about 60kPa. An example of experiments carried out on a test bench is plotted in the figure (b). The port side pressure and the kidney side pressure of the experiments correspond to the inlet pressure and the outlet pressure of the model, respectively. The CFD results agreed with the experimental results. MODIFICATION OF FLOW PATH Reverse flow in the inlet chamber When looking at flow pattern in the inlet chamber at the top of the impeller disk, reverse flow is observed, as indicated by red circles in Fig. 9. Around the reverse flow region, oil flow may lose velocity and pressure may increase locally. Flow path of the inlet chamber was modified in order to prevent the reverse flow, as shown in Fig. 10. The top of the inlet chamber was rounded and the corner connecting the chamber with the impeller disk housing was tapered.

(a) Side view

(b) Top view Fig. 7 An example of pressure distribution

Fig. 9 Reverse flow at the inlet chamber

(a) Calculated results

(b) Experimental results Fig. 8 Inlet and outlet port pressures

Fig. 10 Modification of inlet chamber

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CFD results of stream lines of the modified inlet chamber are shown in Fig. 11. Because of the modification, the hydraulic oil flows smoothly along the rounded flow path and the reverse flow is significantly reduced.

Fig. 12 Stagnation at the outlet port

(a) Side view

Fig. 13 Reshaping of the outlet port

(b) Close-up view Fig. 11 Streamlines of the modified inlet chamber Modification of outlet flow path At the downstream of the impeller, oil flow also has reverse flow region indicated by a red allow shown in Fig. 12. To prevent the reverse flow, the outlet flow path was modified as shown in Fig. 13. A part of the flow path at downstream of the impeller disk was cut out diagonally. A CFD example of stream lines is shown in Fig. 14. The reverse flow region is diminished and the oil flows along the modified flow path smoothly.

Fig. 14 Stream lines of modified outlet flow path Combination of the modifications Combining both modifications at the inlet chamber and at the outlet port, a revised model is proposed as Fig. 15. An example of CFD results of stream lines is shown in Fig. 16. Reverse flow regions at the inlet chamber and at the outlet port are not observed. Calculated results of pressure increase of the original model, the modification of the inlet chamber (modification 1), the modification of the outlet flow path (modification 2), and the combined

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modification are summarized in Table 1. An experimental result of the inlet port pressure was -0.6kPa. For CFD analysis, pressure at the inlet port was set as the same value of -0.6kPa. Calculated results of the outlet pressure are listed in the table and pressure increase is obtained from the difference between the inlet and the outlet pressures. Compared with the original model, the modification 1, 2 and combined modification improve the pressure increase. The pressure increase of the modification 2 is larger than that of the modification 1. Furthermore the pressure increase of the modification 2 is similar to that of the combined modification. Among the modifications, the modification 2 is enough effective to improve pressure increase.

Table 1 Summary of modifications Pressure (kPa) Inlet (fixed) Original Mod. 1 Mod. 2 Combined

-0.6

Pressure Increase Increase Outlet Rate (calculated) (kPa) 60.4

61

100%

62.6

63.2

104%

68.6

69.2

113%

69.3

69.9

115%

Particle trace analysis When hydraulic oil flows in the reverse direction inside the model, a certain part of oil may stay inside the model. The CFD software used for this study has a function of particle trace. As shown in Fig. 17 (a), a certain amount of tiny particles is put at the cross section of the inlet port. Mass of the particles is negligibly small. The total number of the particles is known. The particles may move along the stream lines. Fig. 17 (b) shows the particle distribution entering to the impeller disk. In Fig. 17 (c), the particles are driven by rotation of the impeller disk. In Fig. 17 (d), some part of the particles has gone out of the impeller disk and reaches at the outlet port. When the number of particles that have passed through the cross section of the outlet port is counted, the number of particles remaining in the model between the inlet port and the outlet port is obtained. The ratio of the number of particles remaining in the model for the initial total number of particles is shown in Fig. 18. The graph shows how many particles remain inside the impeller model. Original, modification 1, modification 2 are indicated by red, light-blue, and dark-blue lines, respectively. It takes about 0.15s for the first particles passed the cross section of the outlet port in all three cases. Then the ratios of remaining particles began to decrease. In the case of the original model, even after 0.8s, some part of the particles still remains inside the model. In the cases of the modifications 1 and 2, almost all particles pass the outlet port within 0.8s. In the case of the modification 2, the remaining particles inside the impeller decreased the fastest among them. This means that the modification 2 is the most effective in terms of smoothing stream lines.

Fig. 15 Combined modification

Fig. 16 Stream lines of the combined modification

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(a) Initial position Fig. 18 Ratios of particles remaining inside the model as a function of time CONCLUSIONS CFD was applied to analysis of inlet flow around the impeller disk installed between the inlet port and the valve plate of an axial piston pump. The rotation of the impeller disk was considered when computing the flow pattern by the CFD software. Velocity and pressure distribution were calculated and animations of the flow dynamics gave us visual understanding of the flow characteristics of the impeller disk. CFD results of pressure increase across the impeller disk agreed with experimental results. From investigation of stream lines, reverse flow regions were observed in the inlet chamber and in the flow path downstream of the impeller disk. Modifications of flow path both in the inlet chamber and in the outlet port were proposed. Pressure increase was improved by the modifications. A promising modification is the modification of the outlet port. Investigation by use of the particle-trace function of the CFD software supported the study of improvement of pressure increase. The CFD analysis is very useful to improve the impeller performance and efficiency.

(b) To the inlet chamber

(c) Through the impeller disk

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3. (d) Out of the outlet 4.

Fig. 17 Particle trace

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