Analysis of the Flow in the Helical Grooves of a Molecular Pump
A computaional procedure is described to analyze the flow in the helical grooves of a molecular pump. To fit the irregular boundaries of the flow field curvilinear non-orthogonal coordinates are employed. The system of algebraic equations are solved by the SIMPLE-type algorithm. The work done in this researth is divided into three categories: 1. Use three different groove arrangements along with slip and no slip boundary conditions to analyze the flow in a molecular pump investigated experimentally by Urano and Enosawa. 2. Compare the computational results and experimental data. 3. Analyze the effects of geometric parameters. It can be seen from the calculations that th use of a simplified groove model, termed type A, can yield close results to the other more realestic groove models. With the slip boundary condition, the pressure difference between inlet and exit is improved, though not significantly, leading to closer agreement with experiments. A number of flow rates are tested. It is shown that at low flow rates the pressure at exit is higher than that at inlet. Due to the adverse pressure gradients, recirculating flow is formed in the downstream region near th exit. Another important contribution of this research is the use of a momentum balance theory to analyze the effects of geometric parameters. According to the theory, the pressure promoted by the pump is caused by the pressure difference exerted on the two side walls of the groove. The geometric parameters chosedn to test are: groove angle, groove numbers, groove height and either setting of the grooves on the rotor or stator. The results show that there exist an optimum groove angle and an optimum groove height to obtain best pumping effectiveness. The tests also show that with less number of grooves leads to lower inlet pressure and, thus, higher performance. The setting of grooves on the rotor is prefered to that on the stator because the former gives higher moving wall speed and better pumping performance.
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