Behaviors of a Hole-Pattern Seal With Wet-Gas

Hole-pattern (HP) seals are widely used in centrifugal compressors to control leakage. This paper investigates the behaviors of an HP with wet-gas mixtures. The mixture consists of oil and air with inlet liquid volume fraction (LVF) up to 8%. Injecting oil into the air stream increases the leakage mass flowrate. Direct stiffness K is frequency-dependent and increases with increasing excitation frequency Ω. Injecting oil into the airflow makes this stiffening effect more pronounced. At low frequencies, increasing inlet LVF shows no appreciable impact on K; however, as Ω increases, the effects of changing LVF become more pronounced; i.e., at high frequencies, increasing LVF increases K. The effective damping C_eff value at half of the running speed is indicative of the system stability because many compressor rotors frequently show instabilities at ∼50% of the running speed. At 50% of the running speed, C_eff is positive, and it increases with increasing inlet LVF. Predictions based on San Andrés's (2011) homogenous-mixture bulk-flow model show a good agreement with test results for leakage mass flowrate, K, and the C_eff value near 50% of the running speed. When Ω = 0.5ω, the predicted value of C_eff is smaller than the measured value by ∼12.5%, giving a safe margin for the compressor design.