Shrestha, Hari Krishna (2018-03). 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. Master's Thesis.
Thesis
Wet-gas compression has become an important technology to focus on, requiring a concentrated effort to predict the behavior of compressors when exposed to wet gas conditions. The compressor's stability is highly influenced by the rotordynamic behavior of seals. Labyrinth seals are widely used in compressors to reduce leakage. There have been many studies of labyrinth seals operating under mainly-air conditions. However, there are very few studies on labyrinth seals operating under wet gas conditions. A 2 phase annular-seal stand (2PASS) at Turbomachinery Laboratory of Texas A&M University is utilized to experimentally investigate a labyrinth seal operating under 2- phase flow conditions (a mixture of silicone oil and air). A long labyrinth seal (L/D = 0.75) is tested at supply pressure of 70 bar-a with gas volume fraction (GVF) ranging from 90% -100%. Tests were conducted at pressure ratios of 0.3, 0.4 and 0.5, three rotating speeds of 5, 10 and 15 krpm, and three preswirl ratios at a radial clearance of 2mm (8 mils). The results show that the direct stiffness and cross coupled stiffness of the labyrinth seal are frequency dependent for all three preswirls. Also, direct stiffness is negative. As inlet GVF decreases (more liquid), direct stiffness becomes more negative for zero preswirl, causing the seal's centering force and direct stiffness to decrease. The effect of GVF on direct stiffness for medium and high preswirl is not as defined as for zero preswirl. Similarly, for zero preswirl, as inlet GVF decreases cross-coupled stiffness increases. Hence, decreasing GVF promotes instability by developing a transverse (to the eccentricity vector) reaction force in the direction of rotor precession. On the other hand, for high preswirl, cross-coupled stiffness decreases as GVF decreases. Hence, decreasing GVF enhances the stability of the seals. The increase of liquid in the flow stream increases the direct damping of the system. When the GVF increases, cross-coupled damping increases; however it remains small. Also, for zero preswirl, decreasing GVF from 100% to 90% makes effective damping negative at low frequencies; however effective damping converges to almost the same positive value for frequencies higher than ?c (the frequency at which Ceff changes from negative to positive), indicating a stable system at higher frequencies. However, for medium and high preswirl, as GVF decreases, Ceff becomes less negative and eventually becomes positive for frequencies higher than ?c. This result indicates that the presence of some liquid in air flow can make the labyrinth seals more stable at higher frequencies. For a compressor running at 15 krpm and PR 0.5 with the first critical speed of 7500 rpm (125 Hz), decrease in GVF increases the stability for medium and high preswirl. Similarly, leakage rate m increases with decreases in GVF for all three preswirls.
