Development and validation of an analytical model for the notched pocket damper seal

Abstract

Experiments conducted at the Texas A&M Turbomachinery Laboratory and field applications have shown that pocket damper seals (PDS) can be used to suppress vibrations in compressors. A mathematical model is presented for the notched PDS. The notch is a prominent feature in all the PDS manufactured in recent times. The notch is provided at the exit blades of the PDS to act as a diverging clearance, which is one of the conditions for the damper seal to perform satisfactorily The model to be presented has been adapted from a theory previously developed to predict the direct stiffness and damping coefficients. The flow equations are numerically solved and a computer program is developed correspondingly. The predictions from this notched model are compared with the existing model to highlight the effect of the notch in the analysis. These predictions correlate well with the experimental results from the notched PDS. Also experimental results from testing of a two bladed PDS are compared to the code predictions thus validating the notched model. The notched model performs satisfactorily to predict the direct damping coefficients. Coastdown tests are conducted on a four bladed eight pocket PDS with a partial arc notch of large radius across the exit blades. The PDS offers positive direct damping which increases with an increase in seal inlet pressure. The low stiffness of the test rig combined with the negative stiffness of the seal made it impracticable to conduct testing above inlet pressures of 64.7 psia (4.461 bar). The existing theoretical models are compared with the experimental data collected up to 64.7 psia (4.461 bar).