Effects of stable clearances on backflow in the variable-pitch screw vacuum pump

Effects of various clearances of variable-pitch screw vacuum pump on gas backflow of internal flow field of the pump cavity is studied. The theoretical tooth surfaces of screw rotor are optimized by radial, normal equidistance modifications, and four types of stable clearances of rotor circumferential, radial, tooth shape and tooth sides are obtained. The backflow calculation model in clearances considering Couette backflow and orifice backflow is improved. The three-dimensional model of variable-pitch screw vacuum pump is designed, and the experimental prototype and test device are developed. The internal backflow of vacuum pump is analyzed by using the commercial software Ansys-Fluent® and the calculation model. The results show that the smaller the clearances, the smaller the backflow and the higher the vacuum degree. Among the four types of clearances, the circumferential clearance plays the primary roles. The backflow is directly proportional to the inlet pressure and rotating speed. Predictions are validated by the experimental data with satisfied agreement.