Experimental and theoretical rotordynamic coefficients and leakage of straight smooth annular gas seals

摘要

Results are presented for experimental and theoretical rotordynamic coefficientsand leakage of straight smooth annular gas seals. Experimental rotordynamiccoefficients were measured and trends in changes of rotordynamic coefficients withoperating variables such as rotor speed, back-pressure, fluid preswirl, and seal clearanceare analyzed.Experimental results show that cross-coupled stiffness coefficients are highlyinfluenced by fluid preswirl and only moderately influenced by other operatingparameters, whereas direct damping is nearly unaffected by changes in operatingparameters. Effective damping, a good indicator of stability, is highly affected by fluidpreswirl. Although rotordynamic coefficients of straight smooth annular gas seals areassumed to be frequency independent, experimental results suggest a frequencydependent nature at high back-pressures and high excitation frequencies.Experimental results for rotordynamic coefficients and leakage are comparedwith theoretical predictions of ISOTSEAL, an isothermal-flow, two-control-volume,bulk-flow rotordynamic analysis program. All rotordynamic coefficients are underpredicted.Direct stiffness is poorly predicted while cross-coupled stiffness and directdamping are predicted reasonably well. Leakage is also consistently under-predicted.Theory predicts a slight frequency dependent nature for a limited number of testconfigurations.