Experimental determination of dynamic force coefficients of a pocket damper seal at higher frequencies

Abstract

Experimental results are presented for a two-bladed pocket damper seal (PDS) with eight partition walls on a high-speed test rig. The objective of these experiments is to measure damping coefficients, stiffness coefficients and leakage rates at higher frequencies than tested previously. The seal coefficients are determined by measuring the logarithmic decrement and damped natural frequency in non-rotating free vibration tests and by computer simulations of these results using a computer program called XLTRC. Rotating tests, which consist of coastdown tests from 14000 rpm to 0 rpm were also conducted to corroborate the effectiveness of the PDS in suppressing the vibration amplitudes at the critical speeds. The initial rotating tests on the originally designed PDS were discouraging, and highlight the importance of the optimum design of the inlet plenum chamber of the PDS in producing damping. Suitable modifications are carried out in the PDS inlet plenum design and the non-rotating tests were repeated on the modified PDS. These tests show positive damping and negative stiffness in both the horizontal and vertical directions. The tests also show that the magnitude of both the damping and the stiffness coefficients increase with the inlet pressure. The rotating tests corroborate the effectiveness of the PDS in suppressing the vibration at critical speeds around 13,000 cpm, which is about twice the frequencies tested previously. Subsequently a comparison is made with the predictions from previous pocket damper seal codes.