Lubricating Grease Flow in a Double Restriction Seal Geometry: A Computational Fluid Dynamics Approach

Abstract

In this paper, numerical simulations of lubricating grease flow in the grease pocket of a double restriction seal geometry using computational fluid dynamics are presented. The grease is treated as a singlephase Herschel-Bulkley fluid with different rheological properties corresponding to NLGI grade 00, 1 and 2. The numerical code and rheology model have been validated with a semi-analytical solution based on flow measurements using microparticle image velocimetry. The flow has been modelled for low and high rotational speeds driving the flow, and elevated temperatures. Also, the evolution of contaminant particles in the grease pocket is investigated. It was found that the flow and velocity distribution in the pocket-and consequently the contaminant particle concentration evolution, is characterized by the shear thinning rheology of the grease. With higher shear rates in the grease and higher temperatures, the grease approaches a more Newtonian type of behaviour leading to a reduced yield and shear thinning characteristics directly affecting the grease ability to transport contaminant particles.

Contaminant Particle Motion in Lubricating Grease Flow: A Computational Fluid Dynamics Approach

“…The flow is driven by the rotating shaft ( Figure 1). One of the benefits with the DRS geometry is that it enables a combined axial (leakage) flow; see Westerberg et al [14], where the evolution of an introduced particle concentration is modeled. As the centrifugal force increases with a larger radius, a particle in the flow migrates to a larger radius, i.e., they travel in the positive radial direction from the rotating shaft.…”

“…Considering the leakage flow velocity in the axial direction to be significantly lower than the velocity induced by the rotating axis, only the radial velocity component is considered. For the influence of a leakage flow on a contaminant particle concentration inserted into the domain, please see Westerberg et al [14]. Further, the particle acceleration is set to zero as the grease viscosity is very high and the contaminant particles are very Lubricants 2018, 6, 10 4 of 13 small, resulting in long migration times.…”

“…Then, from a given start position for a particle with a given density and radius (a spherical shape is assumed), the acting forces by means of the drag force and centrifugal force are calculated using the Comsol particle tracing module with a time-dependent solver. The time-dependent solver uses as the [14]. …”

“…To extend this approach, a numerical model of the particle model in the Double Restriction Seal (DRS) grease pocket is considered in the present paper. The model calculates the full 3D velocity field in the pocket, analogous to the model developed by Westerberg et al [14], resulting in the contribution from the three spatial components on the forces acting on the particles; and hence, the effect on the particle migration. The numerical model also uses a fourth order Runge-Kutta scheme to solve the governing equations, which generally is considered more accurate.…”

“…F1-3 and B1-3 symbolize possible measurement planes. The thick red arrow indicates the direction of the leakage particle motion [14] (not covered in the present study), while the dashed black line is the rotating shaft, and the solid black lines are the stationary housing. The solid black lines also denote the lateral boundaries.…”