A NUMERICAL AND EXPERIMENTAL STUDY OF WINDBACK SEALS

摘要

Windback seals work similarly to labyrinth seals except for the effect of helicalgroove. These seals are essentially a tooth on stator or tooth on rotor labyrinth sealwhere the grooves are a continuous helical cut like a thread. Windback seals are used incentrifugal gas compressor to keep oil out of the gas face seal area. These face sealscannot be contaminated by oil. A purge gas is applied to the seal to help force the oilback into the bearing area.The windback seal should be designed to prevent any oil contamination into thesupply plenum and to reduce purge gas leakage. The CFD simulations have beenperformed with the effect of clearance, tooth width, cavity shape, shaft rotation,eccentricity, and tooth location on the seal leakage performance and the flow field insidethe seal. The leakage flow rate increases with increasing the pressure differential, rotorspeed, radial clearance, cavity size, and shaft diameter and with decreasing the toothwidth. The eccentricity has a minimal effect for the windback seal. From oil simulations,the windback seal with 25% rotor eccentricity has some of the journal bearing action anddrives back flow into the gas plenum. However the windback seal can be used to force the oil back into the bearing side before starting the compressor by applying a purge gasflow since the positive axial velocity inside the cavity is larger than the negative axialvelocity. m A Rw cav & / ? is constant for varying shaft rotation since the leakage flow ratefor the windback seal increases linearly as the the rotor speed increases. The leakageflow rate for the windback seal increases as the groove size increases due to the pumpingaction of the windback seal. A windback seal design based upon the numericalsimulations that minimize gas leakage and help prevent gas face seal oil contaminationwas optimized.The windback seal has two leakage flow paths. Since the leakage flow rate underteeth of windback seals is the same as for a similar geometry labyrinth seal, the flowunder the teeth can be predicted by two-dimensional labyrinth seal analysis. Anempirical model for the leakage rate through the cavity has been developed which fitsthe data with a standard deviation of 0.12.