| The annular seal is an important component of turbomachines to reduce the fluid leakage.Compared with the labyrinth seal,the pocket seal can provide greater damping for the rotor system,and alleviate the instability problem caused by the fluid-induced force.Therefore,the research on the stability mechanism and structure optimization of the pocket seal will be beneficial to further improve the operation efficiency of the unit and the stability of the rotor system.The research has certain theoretical significance and engineering value.The Helmholtz effect of the pocket seal was analyzed.The influence of pre-swirl direction on the stability of fully partitioned pocket seal was studied.The results show that when the inlet pre-rotation is same with the whirling direction,the cross-coupled stiffness increases,and the effective damping decreases and the system stability decreases.As the inlet pre-rotation and the whirling direction are opposite,the cross-coupled stiffness decreases,the effective damping increases and the system stability improves.Compared with other cavities,the viscous shear force on the rotor surface of the first cavity shows most violent fluctuation,and the reverse pre-swirl can effectively reduce the viscous shear force on the rotor surface.Based on the Helmholtz effect,the traditional fully partitioned pocket seal was modified by varying the baffle thickness along the leakage direction.Then the cavity volume was changed divergently and convergently in the axial direction.A computational fluid dynamics(CFD)model of the divergent,convergent,traditional pocket seal was established.The effect of the divergent/convergent angle,inlet and outlet pressure,rotational speed on rotordynamic characteristics was analyzed.Results show that the effective damping coefficients of the novel divergent/convergent pocket damper seal are larger than those of the traditional fully partitioned pocket damper seals.The divergent pocket damper seals possess the best stability,and the three kinds of seals show little changes in their crossover frequencies.The design of the gradually divergent and convergent cavity increases the type of resonance frequency produced by the pocket damper seal,and reduces the instability of the rotor sealing system in a wide frequency range.The large divergent/convergent angle tends to enhance the effective damping.With the increasing of pressure difference,the effective damping coefficient increases gradually at high frequencies.The increase of rotational speeds will reduce the effective damping and increase the crossover frequency.The increasing rotational speed deteriorates the system stability.A new type of the pocket seal with self-regulated injection was proposed.Opposed jets were arranged on the first tooth of the conventional pocket seal to generate a gas flow opposite to the circumferential flow direction in the cavity to suppress the circumferential flow.The CFD model of the pocket seal structure with different rotation angles and different inlet/outlet area nozzles was established.The results show that reducing the circumfluence of the first cavity is beneficial to reducing the crossover frequency and enhancing the stability of the pocket seal,while the opposed jet can effectively restrain the circumfluence of the cavity.For the opposed jet,the larger the inlet area,the better the stability of the pocket seal under the same outlet area;and the smaller the jet outlet area under the same inlet area,the worse the sealing stability.Increasing the rotation angle of the opposed jet can effectively increase the effective damping of the pocket seal,reduce the crossing frequency,and enhance the system stability. |
