A bulk-flow model of multiple-blade, multiple-pocket gas damper seals

A bulk-flow model for determination of the dynamic force and sealing characteristics of multiple-blade, multiple-pocket gas damper seals is presented. Zeroth- and first-order equations describe the equilibrium flow for a centered seal and the perturbed flow for small amplitude rotor motions, respectively. The one-control volume model considers the circumferential flow within the seal cavity (or pockets), the flow across the radial baffles and the mass flow rates through the blade tips. Flow turbulence is accounted for with turbulent shear stress parameters and Moody's friction factors in the circumferential bulk-flow momentum equation. The effects of the radial baffles' thickness and clearance on decelerating the swirl flow are of importance for a proper analysis of pocket damper seals. The zeroth- and first-order flow equations are solved numerically using a robust CFD method. A parameter investigation evidences the effects of the seal geometry and operating conditions on the dynamic force coefficients of pocket gas damper seals. Comparisons between predictions and experimental results show that the current bulk-flow model predicts reasonably well the leakage and dynamic force coefficients of multiple-blade, multiple-pocket gas damper seals. The pressure in the baffle vicinity and the differential pressure across the baffle are measured in an existing test facility and compared to numerical predictions. The test results show that the shear flow introduces a minute pressure drop across the radial baffle separating two pockets. Computed predictions agree well with the measurements, although the experimental measurements show an unusual pressure differential drop at high rotor speeds, and perhaps due to transition flow regime conditions and local recirculating flows in the baffle vicinity.