Analysis of the 'friction factor jump' phenomenon in hole-pattern seals

Hole-pattern and honeycomb seals are used to replace labyrinth seals in turbomachinery that are experiencing vibration problems, such as high pressure gas compressors. Computer simulations used to investigate the stability of a rotordynamic system require information about the stiffness, damping, and added mass generated by bearings and seals. These codes typically use bulk flow models for the fluid flow inside the bearings and seals which require empirical information about how the friction factor and leakage rate vary with rotor speed and pressure drop across the seal. Historically, experimental facilities were constructed to provide empirical data which were then used in the rotordynamic models. Ha et al (1992) observed a sudden change in the flow rate and resulting friction factor in a honeycomb seal as the pressure differential across the seal increased. This 'friction factor jump' was attributed to the shear flow over a seal cavity changing from a dominant normal mode to a dominant feedback mode. This was confirmed through pressure spectra showing that indeed, the shear layer instability mode changed and the frequencies present compared to predicted values. A similar effect has recently been observed in hole-pattern seals operating at high pressures, 84 bar (1200 psi). However, the pressure fluctuation spectra did not confirm the same mode change observed by Ha. The friction factor changed a by factor of around three in this instance which can drastically change the stability of the rotating system. This high pressure flow has a higher Reynolds number due to the high pressures which may explain the difference. An experimental investigation has confirmed the presence of the "friction factor jump" and that there is a change in the pressure fluctuation spectra. Further experimental investigation coupled with Large Eddy Simulation (LES) of the flow field have confirmed there is a change in the shear layer over the cavity but not the same as observed by Ha. Comparisons between the experimental and computational results are made along with an explanation of the flow phenomena.