A study of brush seal oil lift through long bearing analysis

Recent advances in brush seal gas turbine engine applications show the promise that carefully designed bristle packs can also function as oil and oil mist seals. Because brush seals are primarily contact seals, oil temperature rise and coking become major concerns in addition to leakage performance. Although individual bristles form very small bearing surfaces, hydrodynamic forces generated by viscous sealing medium combined with high surface speeds may easily lift the compliant bristle pack off the shaft surface. Amount of lift affects seal operating clearance, which - in return - determines oil temperature rise and leakage rate. Balancing these two conflicting performance criteria depends on good understanding of the bristle hydrodynamic lift. This work presents an analytical solution to bristle lift forces based on long bearing assumptions. Starting with general 2-D Reynolds Equation, formulation is simplified taking advantage of seal geometry. A long bearing analytical solution linking bristle lift force to oil viscosity, shaft speed, seal clearance, and bristle geometry has been provided.