Developing an Optimal Helix Angle As a Function of Pressure for Helical Groove Seals

Helical groove seals are non-contacting annular seals used in pumps between impeller stages and at the balance drum. These seals have helically machined grooves on the surface of the rotor and/or stator. They work to sustain a pressure difference given a mass flow rate of the impeller through two flow phenomena which can be characterized by their flow direction. Fluid flowing axially dissipates kinetic energy through turbulent mixing as fluid is pushed through the jet stream region and mixes in the larger groove region, thus producing a pressure differential. Fluid flowing in the groove direction rotates with the rotor wall and is positively displaced toward the high pressure region, essentially acting as a screw pump. Previous work with optimization of helical groove seals has shown that the ideal helix angle of the seal is steeper for lower pressure applications and shallower for higher pressure applications. This is due to lower pressure applications having higher circumferential velocity in the grooves. In high pressure applications, the groove circumferential velocity has even been shown to be negative, and therefore the fluid leaks out the end of the grooves. The objective of this study is to use computational fluid dynamics simulations to find the optimal helix angle of the seal given the pressure differential. To accomplish this goal, simulations were run in ANSYS CFX for various inlet pressures, given zero gauge outlet pressure, and the helix angle of the grooves are varied. The helical grooves seals in this study have grooves on only the stator surface. The number of grooves is varied with the angle to keep the axial cross section of the seal consistent. By doing this, the study is able to focus in on the pumping mechanism of the helical groove seal without substantially changing the energy dissipation. The mass flow rates from each simulation for a given inlet pressure are plotted and quadratic regression was used to calculate an optimal helix angle as a function of inlet pressure. This study also answers the question of whether is there a limit where circumferentially grooved, i.e. labyrinth, seals outperform helical groove seals for very high pressures. Results comparing the powerloss of helical groove seals versus labyrinth seals and the effect of helix angle on powerloss are also given.