Floating foil seal is a new non-contact seal structure, which is proposed to meet the sealing requirements of the rocket turbine pump and the main bearing of aero-engines. Specifically, elastic foil with adaptive deformation ability is designed based on the cylindrical gas film seal, with the rigid sealing surface being replaced. In this way, the anti-interference ability of the system can be improved. Research in this regard mostly takes the sealing friction pair as a smooth surface, but in practice, a completely smooth surface does not exist, as any friction interface is composed of many different micro convex bodies and micro pits. On the surface of the friction pair, the dynamic pressure pits or dynamic pressure grooves are usually designed to improve the dynamic pressure effect and friction performance. After re-processing, the working surface tends to exhibit varied surface characteristics, meaning that the accurate control of the groove shape and roughness remains a topic of exploration for high-precision machining technology.
In fact, whether it is a gas seal or other mechanical parts, the true shape of the friction pair surface should not be completely ignored under certain circumstances. In order to investigate how the surface topography of the sealing pair affecting the floating foil sealing performance, this work studied the textured floating foil gas film seal of one end fixed bump foil. The detailed experiments were as follows: the rough flat foil surface with triangular texture was characterized by the three-dimensional W-M fractal function. The pressure control equation for the synchronous rotation of the wedge-shaped gas film and the moving rotor was established, with the micro-channel scale effect, the fluid-solid interface step effect, and the elastic surface deformation had been taken into account. The finite difference method was used to solve the coupling problem, then, the effects of fractal parameters on the lubricating state of the flow field and sealing characteristics were obtained. By changing the characteristic scale coefficient and fractal dimension, this work further analyzed how the real surface characteristics were correlated to the static and dynamic sealing performances.
The research results showed that changing fractal parameters within a certain range did not affect the overall distribution of gas film thickness and pressure, but the influence of fractal parameters on sealing performance could not be ignored in the following three cases: the average thickness was small, the amplitude of surface roughness peak was intense, or the fluctuation was dense. The effect of surface roughness had a monotonic relationship with the sealing performance, while the effect of surface density was irregular. The increase of the characteristic scale factor G and the decrease of the fractal dimension D within a certain range could improve the sealing dynamic pressure effect and reduced the leakage rate, and vice versa, which helped to reduce friction and stabilize the sealing system. The influence of the real surface of the friction pair on sealing characteristics could not be ignored, but the degree of effect could be gradually weakened as the gas film thickness increased. The dynamic characteristic coefficient showed a relatively complex correlation with the thickness change, but with the increase of gas film thickness, the system tended to be more unstable, and the best dynamic characteristics might be the best at 20 μm. This paper brought the theoretical research on the floating foil gas film seal closer to the actual working conditions, and also provided reference for later manufacturing of the elastic foil.