Experimental evaluation of force coefficients of metal mesh hybrid pocket damper seal

Abstract

The topic of this thesis is metal mesh hybrid pocket damper seals (MHS). This seal design is a combination of the pocket damper seal and a metal mesh element. The metal mesh element not only reduces the leakage but it also provides damping. Early in its development, the tested metal mesh hybrid pocket damper seal induced a gripping force on the shaft that increased with upstream pressure. The resulting friction torque was beyond acceptable limits. The target after that was to develop an experimentally verified design of a metal mesh hybrid seal that has low leakage and positive damping, and to determine rotordynamic effects and leakage. The hybrid seal design was revised to eliminate the gripping effect. The inner diameter of the retaining ring was increased and holes were provided at the periphery of the ring. With this revised design, tests were carried out on the non-rotating rig. Results showed a considerable decrease in stiffness of the seal. This indicated that the stiffness of the seal is dependent on the press fit interference of the metal mesh doughnut. Tests were carried out on the rotating rig using a grooved journal, which allowed a low press fit interference. Metal mesh was placed at the exit blade in these experiments. The results showed negative damping and positive stiffness. Results also showed appearance of instability at 58 Psig, which increased with increase of pressure. Next, the metal mesh was moved to the inlet blade. This modification required design and fabrication of a complete new seal. The experiments were repeated on the rotating rig. This time positive damping and positive stiffness was achieved with no sign of instability. The pocket depth of the hybrid seal was decreased to achieve more damping as predicted by the pocket damper seal code. However experimental results did not show any improvement in damping from this modification. Finally, the Bode plots from rotating tests were curve fitted using XLTRC to obtain damping, stiffness and force coefficients.