Rotordynamic analysis of a pocket damper seal using circular orbits of large amplitude

The solution to the differential equations governing the flow inside a two-blade, four-pocket gas damper seal is determined as a function of any amplitude of vibration (no larger than the seal-rotor clearance) and whirling frequency described by the rotor. The pocket damper seal reaction forces are calculated by integrating the pressure profile around the rotor surface without eliminating nonlinear force terms that are generally neglected by linearized models. A similar methodology to that used to assess the nonlinear effects of squeeze film damper is employed, assuming that the rotor vibration describes circular orbits. In a pocket damper seal, axial and circumferential flows appear and although the seal performance is dominated by the axial flow, the effect of the circumferential flow on the forces generated by the seal is investigated. Past research has shown that circumferential flow induced by the viscous effects at the fluid rotor interface can be neglected suggesting that this flow is mainly caused by the different pressures in the cavities imposed by the rotor as it whirls. Thus the flow model used to determine the seal forces takes into account not only the axial flow but also the pocket to pocket flow due to the pressure gradients between contiguous cavities. The pocket damper seal dynamic force performance is studied by estimating the imbalance response of a rotor-pocket damper seal system as well as by extracting amplitude and frequency dependent force coefficients from the steady state orbits of the system. An investigation of the effect of different parameters such as the orbit radius and static eccentricity on the force coefficients is carried out. The flow model also allows for evaluating the impact of choked flow on the force coefficients.