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Description:
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Non -contacting annular seals are internal sealing devices used in rotating machinery ,
such as multistage centrifugal pumps and compressors . Their design affects both efficiency
and rotor stability . Traditional plain and labyrinth seals are being replaced with stators
containing different roughness patterns to reduce leakage and enhance rotor response .
Several roughened seal experiments with liquid and air have produced leakage data
indicating that the friction factor increases as the seal clearance is increased . Simplified
models based on bulk flow theory and Moodyâ  s approach to characterize wall friction in
pipes cannot explain this outcome .
This research is an extension of a 2 -D numerical analysis of flat plate experiments with
water which found that friction factor of these surfaces is governed by the roughnessâ Â
ability to develop high static pressures . An exhaustive 3 -D numerical analysis of several
experiments with liquid annular seals has been performed using a CFD code . Direct
numerical simulations (DNS ) of turbulent channel flow and smooth seals were replicated
within 1 % using Reynolds -averaged Navier -Stokes (RANS ) equations and turbulence
modeling . Similarly , measured groove seal leakage rates were reproduced within 2 % . On
the other hand , no turbulence model combination predicts the leakage in most 3 -D pattern
roughened seals with the same accuracy . Present results reproduce the friction factor
â  plateauâ  behavior predicted with the 2 -D analysis and observed in the flat plate
experiments . They also reproduce the friction -factor -to -clearance indifference behavior , the
maximum 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 roughness
length -to -clearance ratio and the actual roughness size in defining the friction -factor -toclearance
proportionality .
All simulations indicate that roughened surface area and roughness aspect ratios are the
parameters defining the friction factor at a given seal clearance . The roughness pattern size ,
relevant in determining the friction -factor -to -clearance proportionality , plays a moderate
role once the above cited ratios are defined . In any shape and size , shallow patterns are
predicted and observed to provide larger friction factors than deep patterns . Predictions also
confirm limited experimental data showing that friction factor is affected by the mean flow
orientation relative to the roughness pattern .
Solving RANS equations is sufficient to model simple seal geometries but might not be
enough to replicate turbulent flow in liquid annular seals with roughened surfaces . |