Optimizing a Helical Groove Seal With Grooves on Both the Rotor and Stator Surfaces

Abstract

Helical groove seals are non-contacting annular seals commonly used in pumps within the impeller stages to sustain a pressure differential for a given leakage. Helical groove seals have continuously cut grooves, like the threads of a screw, on the surface of the rotor, the surface of the stator, or both. The two main components of the flow within helical groove seals are axial flow and groove flow. The axial flow serves to reduce the leakage by dissipating kinetic energy as the fluid expands in the grooves and then is forced to contract within the jet stream region. The groove flow serves to reduce the leakage by acting as screw pump. The fluid within the grooves is displaced towards the high pressure region as it spins with the rotor. Previous work has shown that seals with grooves on both the surface of the rotor and the surface of the stator can sustain higher pressure differentials for a given leakage than seals with grooves on only one surface. The goal of this study is to optimize the leakage performance of a double surface helical groove seal for a given set of operating conditions. To accomplish this goal, simulations are run in ANSYS CFX. A sufficient mesh with appropriate boundary layers is determined from the mesh independence study. The turbulence model is k-ε turbulence for water at 25°C. This is the first paper to present numerical results for the performance of helical groove seals with grooves on both the rotor and the stator. The design parameters used in the optimization are inner (rotor) groove size, inner helix angle, outer (stator) groove size, and outer helix angle. A Kennard-Stone algorithm, which optimally spaces the simulations within the design space, is used to select the designs to be simulated. A multifactor quadratic regression is derived. Backward regression is used to reduce the performance function to only statistically significant terms. Finally, the optimal seal design is derived from the performance function and is simulated to demonstrate the predictive power of the performance function. Interaction terms for the rotor and stator design parameters will be used to explore the mechanism whereby helical groove seals with grooves on both the rotor and the stator surfaces are able to have lower leakage than helical groove seals with grooves on just one surface. The end result of this study is a seal design which minimizes leakage and therefore improve machine efficiency.