Computational Analysis of Circumferentially Grooved Seals Using Effective Film Thickness

Modern turbomachine analysis seeks designs that maximize efficiencies and push operational extremes, highlighting a continual need for accurate and efficient component level performance prediction methods. Annular seals are particular components found in a wide variety of turbomachinery applications that serve to reduce leakage flow across a region with a large pressure differential. Circumferentially grooved seals further reduce leakage through the use of grooved sections on the rotor or stator surfaces that serve to dissipate kinetic energy through the formation of vortices, though the addition of grooves adds significant complexity to the fluid dynamic response of the sealing component. Computational analysis of grooved seals includes simplified, one-dimensional bulk flow models and full Navier-Stokes computational fluid dynamics (CFD) studies. Bulk flow models, while efficient and easy to use, lack accuracy due to numerous assumptions, while full CFD studies display higher accuracy but are expert knowledge driven and computationally intensive.

This work seeks to address the limited accuracy of bulk flow models for circumferentially grooved seals through the use of a novel modeling approach based on an effective film thickness, a physical flow boundary that separates the jet flow in the seal clearance region from the recirculation flow within the seal grooves. A simplified, single groove CFD model is employed to establish the effective film thickness analysis framework, providing insight into the flow mechanisms dictating leakage performance and illustrating the potential for reduced empiricism in bulk flow modeling. This framework is then applied to shear stress modeling within the groove region, where the additional shear stress contribution is isolated and directly quantified as a correction to a traditional bulk flow shear stress definition. These shear stresses are termed here as form shear stresses (FSS) based on the close relationship between their behavior and the expansion and contraction of the effective film thickness. Models for the FSS are developed as functions of local Reynolds number and implemented into a simplified bulk flow method, demonstrating the ability of the novel modeling approach to capture physical flow behavior and eliminate the need for an empirical groove loss coefficient. Finally, a rigorous sensitivity study examines the impact of the upstream and downstream regions, rotor centrifugal growth, and modeling whirl amplitude on the prediction of physical flow phenomena and rotordynamic coefficients from a quasi-steady full CFD method. The results highlight the careful consideration needed during model setup in order to realize the accuracy advantages assumed by the use of these higher fidelity methods. This work presents the first use of an effective film thickness in bulk flow analysis and shear stress modeling for grooved seals. The bulk flow developments of this dissertation set forth a new modeling approach that can be applied to many sealing scenarios for reduced empiricism and increased prediction accuracy, while the contributions to the knowledge base of full CFD rotordynamic prediction enable more widespread and appropriate use of higher fidelity methods for seal analysis.