AVS 45th International Symposium
    Vacuum Technology Division Tuesday Sessions
       Session VT-TuM

Paper VT-TuM3
Pumping Mechanism of Helical Grooved Molecular Drag Pumps

Tuesday, November 3, 1998, 9:00 am, Room 329

Session: Molecular Drag Pumping
Presenter: T. Sawada, Akita University, Japan
Authors: T. Sawada, Akita University, Japan
W. Sugiyama, Akita University, Japan
Correspondent: Click to Email
The flow on a rotor of molecular drag pumps varies from viscous to slip to free molecule flow according to the decrease in pressure. As the first step, the flow through a groove facing a wall moving along the groove is analyzed. On the assumption that the flow in the groove is steady, isothermal, incompressible and laminar, the Navier-Stokes equations are simplified in the viscous and slip flow regimes and can be solved numerically with relative ease. In the free molecule flow regime, the drag (or friction) is caused by the momentum carried to a wall-piece by gas molecules colliding with the wall-piece, and the drag must be equated to the force exerted by the pressure on the two cross-sections sandwiching the wall-piece. The weighted linear combination of the two equations for slip and free molecule flows can describe the flow through the three flow regimes. The flow in ridges which leads a leak is treated in the similar way to the flow in grooves. Then, the flows in grooves and ridges hitherto treated separately are connected by the continuity condition of mass flow rate normal to the groove-ridge interface. The pressure gradient which is discontinuous at the groove-ridge interface is smoothed by Boon and Tal's "Narrow groove theory ". The pressure difference or the pressure ratio across the pump is obtained from the relationship between the smoothed pressure gradient and the axial mass flow rate derived above. The calculated results suggest that the radial clearance can be enlarged by factors of 20-100, compared with that of a conventional Holweck-type pump.