The basic working principle of hydraulic seals is to prevent contaminants from entering the system or hydraulic oil from leaking by providing a proper seal for the hydraulic system. Hydraulic seals are an important part of numerous disciplines such as the aerospace, automotive, medical, and natural gas industries. Although hydraulic seals are an inexpensive consumable item, they have a vital role in the proper function of hydraulic systems. In the aviation field, with the development of lightweight aircraft, the performance requirements for hydraulic sealing structures such as aircraft actuators have become more stringent. For instance, the relative speeds at hydraulic seals are growing, working temperatures are rising, and maximum working pressures are increasing. Under harsh temperature and pressure conditions, sealing components are easily worn, thereby leading to sealing failures and possibly resulting in accidents. Therefore, it is essential to study the failure mechanism of hydraulic seals in harsh environments and to propose a set of schemes to optimize their sealing performance..
| 1. Fluent Simulation Model |
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| 1.1 Simplified Physical Model of the Flow Field |
| 2. Meshing |
| 3. Analytical Model |
| 4. Sealing Material |
| 5. Results and Discussion |
| 6. Effect of Texture Depth on Bearing Capacity and Leakage |
| 7. Effect of Texture Width on Bearing Capacity and Leakage |
| 8. Effect of Texture Area Ratio on Bearing Capacity and Leakage | 9. Conclusion |
Figure 1 presents a schematic diagram of the sealing structure and the annular flow field generated by it. The key friction pair in the dynamic seal is an annular flow field generated between the shaft and the sealing ring. The size of the texture is generally between several microns and several hundred microns, and the diameter of the annular flow field is much larger than the texture size. Therefore, the annular flow field can be simplified to a plane flow field.
The study of surface texture in this paper is a regular array of the same basic texture elements. Therefore, fluid lubrication analysis of the entire flow field can be performed by establishing one micro-texture element and applying periodic boundary conditions. Groove type is a typical surface texture treatment variable that affects tribological properties, and the groove-type texture has the same cross-section along the groove line direction. In this study, we select groove-shaped texture elements with rectangular, arc-shaped, and triangular cross-sections as the research objects, and explore their hydrodynamic pressure effects. Figure 2 shows the three analysis element models and cross-sectional dimensions of each analysis element, where h is texture depth, w represents texture width, c signifies oil film thickness, and L is unit length.
The method of meshing the model is based on the division of additional blocks. First, create a whole block, then divide the block according to the geometric model. Next, associate the divided block with the entity and determine the node distribution on the block. In this way, a high-quality structured mesh that has good convergence and high precision in the subsequent analysis is produced. Since the groove depth is between a few microns and tens of microns, it is necessary to properly densify the grid in the direction of the film thickness and groove depth. On the premise that the aspect ratio of the hexahedral element is controlled within 5, the longest distance between two adjacent nodes is set to 0.001 mm.
The flow field simulation calculation is carried out using FLUENT software. A preliminary calculation indicates that the Reynolds number is small, so the viscous physical model of the flow field is assumed to be laminar flow. As Figure 4 shows, the left and right sides of the flow field are set as periodic boundary conditions, so the parameters such as pressure value and flow velocity corresponding to each point on the left and right boundaries are the same. This is equivalent to the fluid flowing in from the left wall and out from the right wall, then directly flowing in from the left wall again, which effectively simulates an infinitely long flow field area. The upper wall is set as a static wall and the lower wall is set as a moving wall, which moves to the right at a speed of v = 10 m/s. Besides, the fluid density is set to 860 kg/m3 and the viscosity is 0.021 Pa·s
Due to its excellent properties such as low friction and self-lubrication, PTFE is widely used in the sealing systems of high-end aviation equipment. Therefore, we use PTFE as the sealing material for our research. The sealing system of high-end aviation equipment often works under high temperatures and pressures. The temperature has a certain impact on material properties, so the influence of temperature on sealing performance cannot be ignored.
the uniaxial tensile/compression mechanical properties test of PTFE sealing material at a certain temperature is conducted using a high and low-temperature tensile/compression testing machine. After obtaining the stress-strain curve at a certain temperature, the Young’s modulus of the PTFE sealing material is calculated. the Young’s modulus of PTFE sealing material is 300 MPa and the Poisson’s ratio is 0.4. 3.
Data Processing Method By integrating the pressure data, the data of the simulation results for the pressure distribution of the flow field are derived and the bearing capacity is obtained. Figure 5 shows the pressure distribution cloud map when the texture depth is 5 µm. A high-pressure region appears in the textured convergent wedge, with a maximum value of approximately 9.90 × 104 Pa. A negative pressure region forms at the divergent wedge of the texture, and the minimum value is around −9.90 × 104 Pa. Cavities are generated at the divergent wedge, so to prevent the development of negative pressure, the Half-Sommerfeld cavitation boundary condition is used for processing mathematical calculations.
To determine the boundary conditions, after the iterative calculation is completed and the oil film pressure distribution is obtained, all the negative pressures in the pressure data are reset to zero and the processed data are integrated to obtain the oil film bearing capacity. Due to the varying texture widths and total lengths of texture elements under the same area ratio, the bearing areas corresponding to the bearing capacity obtained from the integral are different. Thus, it is impossible to compare and judge sealing performance. Consequently, the bearing surface areas are unified, and the bearing capacities are compared by calculating the bearing capacity with an area of 1 mm2 . The oil film bearing capacity is obtained through subsequent calculations to compare the influence of texture parameters on oil film bearing capacity.
When the sealing material is PTFE, the strain of the sealing material must be considered. The solution process is relatively complicated, and the calculation results are close to those of a rigid body sealing material. Therefore, setting the sealing material as a rigid body has little effect on the qualitative evaluation of the influence of varying texture pa‐ rameters on sealing performance. Thus, in the simulation calculations of this paper, the sealing material is set as a rigid body to calculate the oil film bearing capacity and leakage. Figure 5. Pressure cloud map of rectangular texture flow field with texture depth of 5 µm. For sealing materials, texture treatment has a certain impact on sealing performance,
Therefore, it is necessary to consider the effect of surface texture treatment on leakage. The leakage amount is the flow rate at the inlet and the outlet. In addition, the size of the leakage is visually observed, and the leakage amount is converted into a leakage rate per 1 mm2 at a certain point in time. Due to the setting of the symmetry plane, the width of the texture element does not affect the calculation results. Thus, to reduce the number of calculations, the element width should be as small as possible. Among the surface texture parameters of the texture treatment, parameters such as texture shape, depth, size, and area ratio may have a significant impact on oil film bearing capacity and leakage. Therefore, in this paper, we mainly study the influence of depth, width, and area ratio of the groove texture on oil film bearing capacity and leakage. The three-groove texture cross-sectional shapes selected for this study are rectangular, triangular, and arc shaped.
Figure 6 shows the stress distribution cloud diagram and pressure distribution curve on the section using either PTFE or a rigid body as the sealing material and with constant texture parameters. The calculated oil film bearing capacities are 16.856 N and 17.365 N, respectively, displaying a difference of less than 5%. Additionally, the calculated leakage rates are 4.75057 g/s and 4.74997 g/s, with a difference much lower than 1%.
The oil film thickness is set to 10 µm, the texture width is 100 µm, and the area ratio is 50%. Figure 7 presents the results of the simulation analysis on the effect of different texture depths on bearing capacity and leakage.
An increase in texture depth, the bearing capacity first increases and then decreases. However, the extremes generated by different texture types vary significantly. When other conditions are constant, there is an optimal texture depth value that results in a peak dynamic pressure effect. In addition, different texture types have dissimilar optimal values. Leakage also initially increases and then decreases with a rise in texture depth. However, leakage with the triangular texture and the arc-shaped texture falls rapidly as texture depth increases, and the leakage is much smaller than that of the rectangular texture at the same texture depth.
Moreover, when the texture depth exceeds 20 µm, the leakage amount is much smaller than when there is no texture. Furthermore, higher bearing capacities result in less leakage and better sealing performance. Therefore, considering the above results, the triangular texture with a texture depth of more than 20 µm should be selected for the grooved texture. The flow field velocity vector diagram of the arc-shaped texture when the texture depth is 10 µm. It reveals that there is a reverse flow in the flow field at the bottom of the texture, which suggests that the reverse flow leads to changes in the bearing capacity. In addition, variations in texture depth influence leakage rates
existence of countercurrent weakens the influence of the dynamic pressure effect. When the texture depth is small and there is a weak countercurrent, increased texture depths lead to a stronger dynamic pressure effect and larger bearing capacity. However, when the texture depth is large, the countercurrent region fully forms. The reverse flow impairs fluid pressure, exceeding the dynamic pressure effect at a certain depth. Consequently, the bearing capacity decreases, so the bearing capacity first rises and then falls.
Leakage is also related to the dynamic pressure effect and the reverse flow phenomenon. The leakage rate varies with changes in fluid pressure. In addition, the occurrence of reverse flow reduces the outflow of a portion of the fluid and even exceeds the effect of pressure when the reverse flow is strong. As a result, the leakage rate may be lower than the untextured case when the texture depth is large.
Here, the oil film thickness is set to 10 µm, the texture depth is 10 µm, and the area ratio is 50%. Results from the simulation analysis of bearing capacity and leakage with different texture widths are shown in Figure 10.
It can be seen from the figure that larger texture widths result in greater oil film bearing capacities. In addition, the texture width has a linear relationship with the oil film bearing capacity. This is because an increase in texture width expands the size of the flow field convergence area, thereby increasing the oil film bearing capacity. Moreover, larger texture widths lead to greater leakage. The triangular and arc-shaped textures witness a marginal increase, while the leakage of the rectangular texture increases significantly with rising texture widths. In addition, at a constant texture width, the leakage rate of the rectangular texture is much larger than those of triangular and the arc- shaped.
As the bearing capacity rises, the leakage rate falls, resulting in superior sealing performance. Therefore, it is appropriate to select triangular or arc-shaped textures with larger widths for the groove-type texture. Between them, the triangular texture has a weaker bearing capacity but less leakage than the circular arc texture.
The oil film thickness is set to 10 µm, the texture depth is 10 µm, and the total length of the texture unit is 200 µm. Figure 11 displays the analysis results of the bearing capacity and leakage for different texture area ratios.
The figure reveals that with an increase in texture area ratio, the oil film bearing capacity shows a trend of first increasing and then decreasing. After the area ratio reaches a certain value, the arrangement of textures becomes particularly dense and the distance between textures becomes smaller. When the fluid passes through the convergent region of the texture and has not been sufficiently pressurized, it reaches the divergent region of the texture, causing the pressure to sharply drop.
Therefore, after the area ratio reaches a certain value, the bearing capacity becomes smaller. As the area ratio rises, the leakage rate also increases, because the volume of the leakage channel increases along with the area ratio. In addition, the leakage rate of the rectangular texture is much larger than those of the triangular and circular arc textures when the area ratio is constant. Superior sealing performance can be achieved with larger bearing capacities, resulting in less leakage. Thus, with the grooved texture, it is most appropriate to select a triangular texture or a circular arc texture with an area ratio of about 70%.
A fluid dynamics simulation analysis of a material surface texture treatment was conducted. The influence of texture depth, width, and area ratio of different texture types on bearing capacity and leakage were studied, and the influence of different texture parameters on the sealing performance of the sealing system was analyzed. The research background was high-end aviation equipment sealing systems, and the sealing material was PTFE. Since sealing systems often work under various temperatures and pressures, the properties of the sealing material at a certain temperature were studied through experiments to provide parameters for subsequent simulations.