Analysis of a novel turbomachinery shaft seal design

Advanced sealing systems are needed to control parasitic leakage flows to achieve high turbine engine efficiency and low emissions. Typical extreme turbomachinery engine operating conditions when combined with rotor excursions do not lend simple sealing solutions. This work presents an in-depth analysis of a novel robust yet simple sealing system that is capable of maintaining long life under high speed and high temperature operating conditions. The proposed seal design is actually a gas bearing that is carefully tailored, analyzed, and designed to function as a differential pressure seal. The design involves a simple rigid/semi-flexible seal ring that is attached to a stationary support plate via flexible metal cloth structure. The seal body is capable of moving under the effect of aerodynamic lift force. Therefore, above a certain clearance limit, which ensures that asperity contact is avoided, the seal follows shaft excursions to avoid damaging hard rubs. Applying established working principles for gas lubricated journal bearing, governing Reynolds equation for compressible gas flow is solved using both finite difference and finite element methods. The results of these analyses are validated with other published work in literature. The calculated seal aerodynamic pressure profile is integrated to obtain the lift force. This force is compared to the frictional drag and other resistive forces that need to be overcome to move the seal with rotor during transients. Four different seal designs with varying geometries are studied, and resulting lifting forces are evaluated. It is verified that necessary lifting force for these designs can be obtained without exceeding lower limits of seal clearance. Furthermore, to ensure dynamic stability of the sealing system, natural frequency analyses are conducted. Results from the analysis under typical operating conditions of a gas turbine indicated that seal natural frequencies are well above rotor operating speeds. Finally, leakage analysis of new seal system is performed. Comparison with brush seal leakage performance rates under typical operating conditions reveals that the analyzed seal designs show satisfactory leakage performances that are similar to those of brush seals.