Experimental Study of the Static and Dynamic Characteristics of a Long (L/D = 0.75) Labyrinth Annular Seal Operating Under Two-Phase (Liquid/Gas) Conditions

A two-phase annular-seal stand at the Turbomachinery Laboratory of Texas A&M University is utilized to experimentally investigate a labyrinth seal operating under two-phase flow conditions (a mixture of silicone oil and air). A long labyrinth seal (length-to-diameter ratio L/D = 0.75, diameter D = 114.729 mm, and radial clearance C_r = 0.213 mm) is tested at a supply pressure of 62 bars-g with inlet gas volume fraction GVF_i ranging from 90 to 100%. Tests were conducted at three pressure ratios PR (0.3, 0.4, 0.5), three rotating speeds (5, 10, 15 krpm), six GVF_i (90%, 92%, 94%, 96%, 98%, and 100%), and three inlet-preswirl inserts, namely, zero, medium, and high. Specifically, the ratio between the fluid's circumferential velocity and the shaft surface's velocity are in ranges of 0.0–0.2, 0.5–1.6, and 0.5–2.7 for the zero, medium, and high preswirls respectively. The direct dynamic stiffness K_Ω is negative. As GVF_i decreases (more liquid), K_Ω becomes more negative for the zero preswirl. The effect of changing GVF_i on K_Ω for the medium and high preswirls is not as clear as for the zero preswirl. For the zero preswirl, as GVF_i decreases, the cross-coupled dynamic stiffness k_Ω and direct damping C damping increase. However, the effective damping C_eff values converge to almost the same positive value for higher frequencies_. Hence, there is no significant effect of change in GVF_i for the zero preswirl. For the high preswirl, as GVF_i decreases, k_Ω decreases and C increases. As GVF_i decreases, C_eff becomes less negative and eventually becomes positive for frequencies higher than Ω_c. This result indicates that at certain frequencies, the presence of liquid can make the labyrinth seals with high preswirl more stable. For the seal tested, a compressor running at 15 krpm and PR (ratio of seal exit pressure and seal inlet pressure) = 0.5 with the first critical speed of 7500 rpm (125 Hz) would experience an increase in stability with presence of liquid in the flow stream for the medium and high preswirls. However, for the range of GVF_i considered here, if swirl brakes are used in a compressor application to reduce the preswirl, there would be no impact of liquid presence on the stability of the compressor. Concerning static measurements, leakage rate $m˙$ increases with decreases in GVF_i but remains unchanged with increasing preswirl.