Gas foil bearings have many advantages such as low cost, non-pollution, high speed, high stability, etc., which are widely used in high-speed turbo machines. In order to meet the demands of higher power density rotating machinery, gas foil bearings usually work at high-speed condition. At this time, due to the high-speed shear dissipation of heat generated by the viscosity of the gas, the bearing operating temperature rises rapidly. Without timely heat evacuation, the lubricating gas film temperature is too high, which may lead to bearing thermal failure and other problems. Therefore, the thermal analysis and the load carrying capacity characteristics of foil bearings are very important in the bearing performance study. Deformation of the foil affects the flow of gas in the lubricating gas film. In turn, the pressure of the gas film interacts with the foil. There is a complex fluid-solid coupling relationship involved. In addition, the heat generated by the viscous shear of the lubricating gas in the gas film affects the physical properties of the gas and the foil. Therefore, it is essential to establish a three-dimensional thermal-fluid-structure coupling model of foil bearings to analyze the performance of bearings.
In order to deeply analyze the influence of gas film temperature on the load-bearing characteristics of foil bearings, a three-dimensional flow-solid-thermal coupling model of the multi-leaf foil bearing was established considering the actual flow characteristics of the cooling air using the finite element commercial software COMSOL in this paper. The effects of thermal conductivity in the circumferential and radial directions of foils, and the actual flow characteristics of the cooling gas were considered. The accuracy of the numerical model was verified by comparison with experimental results. The load carrying capacity characteristics of the foil bearing under the conditions of variable bearing clearance and variable rotational speed were analyzed. The pressure and temperature distribution of lubricating gas film as well as the load carrying capacity were obtained. The contact deformation of the foil was studied. In addition, the influence of cooling gas flow rate on bearing heat evacuation and load carrying capacity characteristics was analyzed. The results showed that the air film surface temperature gradually increased as the bearing clearance decreased and the bearing speed became higher. As the bearing speed increased and the bearing clearance decreased, the area of the high-pressure region and the peak pressure of the gas film gradually became larger. The high-pressure zone of the air film was mainly located at the position of the trailing edge. The temperature in the high-pressure area of the gas film rises more rapidly. The air film temperature exhibited a linear trend with the bearing clearance and a non-linear increase with the rotational speed. Compared with the bearing clearance, the effect of increasing bearing speed on the temperature of the gas film was greater. When the rotational speed was increased from 80 000 r/min to 140 000 r/min, the maximum value of the air film temperature increased from 85 ℃ to 345 ℃, with an improvement of about 305%. Thermal failure might occur when the air film temperature was too high. Therefore, it was necessary to study the influence of ambient temperature on bearing performance. With the increase of cooling gas flow rate, the film temperature and bearing load decreased gradually. When the cooling air flow rate increased to a certain value, the cooling effect no longer changed significantly. Therefore, the cooling gas flow rate should be appropriately controlled so that the cooling gas consumption could be reduced while ensuring the cooling effect of the bearings.
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