Theoretical Analysis of the Static Characteristics of the Carbon Segmented Seal

Abstract

The segmented carbon seal is regularly used for sealing bearing chambers of aeronautical turboengines or as part of a buffer seal in space turbopumps. The seal operates with contaminated air or with an inert gas and is made of many identical carbon segments (generally three or six) with reciprocally overlapping ends. The segments are pressed against the rotor by the pressure difference between the upstream and the downstream chambers and by a circumferential (garter) spring. The pressure difference and an axial spring press the segments also against the stator. The inner cylindrical surface of each segment is provided with pads that create an aerodynamic lift proportional to the rotor speed. Following this lift force, the segments of the seal are pushed away from the rotor and the seal opens. The contact between the rotor and the segments is lost, and an axial leakage path is thus created. Although it was developed since long, a model for calculating the characteristics of the segmented seal is completely absent from the scientific literature. The goal of the present work is to fill this gap at least for the static characteristics (leakage and torque). The analysis is carried out for a single segment of the seal by supposing that all the segments have the same characteristics. Each segment has a planar motion (i.e., three degrees-of-freedom (3DOF)), and therefore the film thickness under each pad is not uniform. Given the stationary operating conditions (pressure difference and rotation speed), the present model calculates the equilibrium position of each segment on the bases of the lift and the friction force acting on the pads, the friction forces acting on the nose of the seal, and the radial and axial springs. Once found the static equilibrium position, the leakage and the torque of the seal are calculated. A parametric study enlightens the importance of the pad waviness, the pocket depth, and the spring forces on the characteristics of the segmented seal.