Parameters defining flow resistance and the friction factor behavior in liquid annular seals with deliberately roughened surfaces

摘要

Non-contacting annular seals are internal sealing devices used in rotating machinery,such as multistage centrifugal pumps and compressors. Their design affects both efficiencyand rotor stability. Traditional plain and labyrinth seals are being replaced with statorscontaining different roughness patterns to reduce leakage and enhance rotor response.Several roughened seal experiments with liquid and air have produced leakage dataindicating that the friction factor increases as the seal clearance is increased. Simplifiedmodels based on bulk flow theory and Moody??????s approach to characterize wall friction inpipes cannot explain this outcome.This research is an extension of a 2-D numerical analysis of flat plate experiments withwater which found that friction factor of these surfaces is governed by the roughness??????ability to develop high static pressures. An exhaustive 3-D numerical analysis of severalexperiments with liquid annular seals has been performed using a CFD code. Directnumerical simulations (DNS) of turbulent channel flow and smooth seals were replicatedwithin 1% using Reynolds-averaged Navier-Stokes (RANS) equations and turbulencemodeling. Similarly, measured groove seal leakage rates were reproduced within 2%. Onthe other hand, no turbulence model combination predicts the leakage in most 3-D patternroughened seals with the same accuracy. Present results reproduce the friction factor??????plateau?????? behavior predicted with the 2-D analysis and observed in the flat plateexperiments. They also reproduce the friction-factor-to-clearance indifference behavior, themaximum friction factor observed in a specific roughness pattern size is independent of the actual clearance in a certain Reynolds number range, but clarify the role of the roughnesslength-to-clearance ratio and the actual roughness size in defining the friction-factor-toclearanceproportionality.All simulations indicate that roughened surface area and roughness aspect ratios are theparameters defining the friction factor at a given seal clearance. The roughness pattern size,relevant in determining the friction-factor-to-clearance proportionality, plays a moderaterole once the above cited ratios are defined. In any shape and size, shallow patterns arepredicted and observed to provide larger friction factors than deep patterns. Predictions alsoconfirm limited experimental data showing that friction factor is affected by the mean floworientation relative to the roughness pattern.Solving RANS equations is sufficient to model simple seal geometries but might not beenough to replicate turbulent flow in liquid annular seals with roughened surfaces.