Abstract
Bulk flow modeling of circumferentially grooved seals uses simplified physics models to predict leakage and rotordynamic coefficients more efficiently than full computational fluid dynamics (CFD) simulations, but at the expense of accuracy. Reducing bulk flow modeling inaccuracies, mainly a result of uncertainties in empirical quantities like friction factors and loss coefficients, requires a more fundamental and physical understanding of the flow features and their dependencies. This study utilizes CFD and an effective film thickness, a physical boundary between the jet and recirculating flows, to investigate Reynolds number effects on grooved seal flow fields and shear stresses. Simulations were run using ANSYS CFX for a single groove seal model assuming fully developed flow for a range of pressure differentials and rotor speeds. The effective film thickness boundary was defined by generating streamlines in the jet flow region that traversed the entire axial length of the model. Radial averaging across the local film thickness yielded bulk flow quantities used in processing and analysis. Flow structures, film thicknesses, shear stresses, and net flow expansion into the groove are found to be described completely by the ratio of circumferential to axial Reynolds number and the total resultant Reynolds number. Decreases in leakage with rotor speed are found to be dictated by increased land shear stresses and a decreased role of the groove at inducing pressure drop. An optimal groove aspect ratio between 0.07 and 0.19 is presented based on maximizing expanded film area while retaining a main groove recirculation region. This is the first paper to analyze circumferentially grooved seal flow from the standpoint of the effective film thickness, and by doing so, provides physical insight needed for a full description of the flow behavior. The results presented highlight bulk flow analysis areas where an effective film thickness approach could lead to new, physics motivated model development and the elimination of particular empirical coefficients. The effective film thickness approach thus provides a strong foundation on which substantial improvements in bulk flow modeling accuracy can be achieved.
