Numerical Simulation Of Oil Vapor Flow In The Diffusion Vacuum Pump

Oil diffusion pump is equipment for acquisition of high vacuum, which is widely applied in electronic, metallurgical, mechanical and atomic energy industries and space technology domain. The pump is no mechanical rotation, vibration or voice.and its structure is simple. And it can be easily operated and used circularly. Meantime, easy maintenance and long life are available for oil diffusion pump. So oil diffusion pump is used by many customers. However, the working medium of diffusion pump is diffusion pump oil, how to improve the working stability, reducing diffusion oil backstreaming rate has been a significant problem of diffusion pump to researchers and users.The pumping characteristics and oil backstreaming rate is influenced by the shape of the oil jet, but also with factors such as the inlet pressure, geometry size of pump and working gas species. Based on the theory of oil vapor flow characteristics, Fluent software is employed to simulate the oil vapor flow. The results show the effects of inlet pressure nozzle, geometric parameters, oil physical parameters on the oil stream flow. It has a vital reference value on understanding the pumping mechanism, improving the pumping performance and reducing the oil backsteaming rate.The results show that expansion of the jet steam is mainly affected by inlet pressure and the viscosity of pump oil materials. Expansion decreases with the decrease of the entrance pressure. When the entrance pressure is reduced to a certain value, the expansion area no longer changes. However, the expansion would increase with decreasing of pump oil viscosity. Pump oil with low viscosity can be selected to improve the steam expansion without changing the inlet pressure.The results show the maximum speed of oil jet steam is mainly affected by molecular weight and the viscosity of pump oil materials. The maximum velocity increases with the decrease of molecular weight and the viscosity. The maximum speed isn’t affected by throat structure. So pump oil with smaller molecular weight and smaller viscosity is good for higher speed vapor.The results nozzle exit velocity is affected by pump oil materials and the throat size. The nozzle exit velocity increases with the decrease of molecular weight and the viscosity of pump oil materials. Moreover, it increases with the decrease of throat gap size, also with the increase of nozzle angle and the length of the nozzle. In order to reduce the oil backsteaming rate, expansion rate should be improved by the method changing the nozzle structure, such as increasing the angle of the nozzle throat, reducing the gap size and increasing the pump nozzle length.This article also makes simulation of K200 diffusion pump to obtain the biggest inlet pressure of each jet and the optimal distance between levels. Moreover, compared with two kinds of oil retaining cap existed now, a new oil retaining cap structure which can effectively prevent the oil return is presented, which has a reference value on the structural improvement.