Rotordynamic force coefficients of pocket damper seals
Ertas, Bugra Han
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The present work describes experiments conducted on several pocket damper seal (PDS) designs using a high pressure annular gas seal test rig. Both rotating and non-rotating tests were conducted for a 12, 8, and 6 bladed PDS. The objective of the tests was to determine the rotordynamic force coefficients and leakage for the different PDS while varying parameters such as: (1) clearance ratio, (2) rotor surface speed, (3) PDS pressure differential, and (4) excitation frequency. Two different methods were used to determine frequency dependent force coefficients: (1) the impedance method, which involved using a baseline subtraction and (2) the dynamic pressure response method, which comprised of measuring seal cavity dynamic pressure and phase relationship to vibration. Both methods were used to determine coefficients, but the dynamic pressure response method revealed insights to the dynamics of the PDS that were the first of its kind and allowed the comparison to the damper seal theory at the most fundamental of levels. The results indicated that the conventional PDS possessed high positive damping, negative and positive stiffness, and same sign cross-coupled coefficients. Another objective of the work is to investigate a new fully partitioned PDS design and accompany experimental results with the development of a modified damper seal theory. The new fully partitioned PDS design was shown to give twice as much damping as the conventional design and revealed the ability to modify direct stiffness without degradation in direct damping. Finally, both the conventional theory and the newly proposed theory predictions are compared to experimentally determined force coefficients. The last objective was to evaluate the leakage characteristics of the different designs and to investigate the effect of blade profile on seal leakage. Results showed that beveled tooth blade profiles yield higher mass flow leakage compared to rectangular blade profiles.