Development of a MEMS-Scale Turbomachinery Based Vacuum Pump

Abstract

This study forms part of a larger study to develop a MEMS scale turbomachinery based vacuum pump. This would allow very high vacuum to be drawn for handheld mass spectroscopy. This thesis concentrates on the roughing portion of the turbo pump where flow can still be treated as a continuum but the no slip boundary condition is not accurate. The first portion of this thesis investigates flow at Knudsen numbers ranging from 0.001 to 0.1. By using a first order analysis, the wall shear stress can be specified in a commercial computational fluid dynamics code allowing slip flow to occur. This method was validated against a basic Poiseuille flow at these higher Knudsen numbers where slip flow was present. This demonstrated that it was possible to use a commercial code to model Knudsen number flows between 0.001 to 0.1. The second part of the thesis focused on the design of a roughing pump stage consisting of three blade rows a stationary inlet and outlet surrounding the rotor blade row. The no slip condition was not imposed as the simulated stage was assumed to be the outlet stage, and thus operating at a very low Knudsen number. A two dimensional analysis was developed to define the initial blade shape to achieve a maximum pressure ratio. A three dimensional simulation was developed to investigate the effects of tip leakage losses. The simulations are able to predict pressure ratio and power consumption of a particular stage of a MEMS scale turbopump. The final predicted pressure ratio of a stage with tip clearance is 1.0722 with power consumption of 0.4648 watts.