Research On Leakage Characteristics And Improvements Of Labyrinth Seals

Labyrinth seals are widely used in the field of turbomachines with a great influence on the efficiency and economy issues. This thesis studies leakage characteristics, seal mechanism and proposes a seal structure with higher performance by flow modeling, experimental investigation and numerical simulation.Based on scaling analysis for leakage flow, the leakage mass flow rate is greatly influenced by three parameters: pressure ratio, geometry structure and rotational speed.Characteristics of leakage flow is similar to that of pipe flow, so analogy is made between these two flow phenomena. To illustrate dissipation contributions made by every point in seal structure, viscous entropy generation is applied. From dissipation contour, the dissipation generated by every point varies obviously with a higher value at orifice and stagnation point and a much lower value at the cavity flow. Then design criteria for higher performance is proposed: new design should increase dissipation of orifice and stagnation point in seal structure and also it should do improvements directly to the primary flow of the leakage.As for pressure ratio, a flow model is derived to obtain relation between leakage mass flow rate and pressure ratio, which is proved by a comparison between model predictions and experimental results. As for geometry structure, the relative position between stator and rotor is studied and a conclusion is summarized that leakage is determined by the length of rotor step in front of seal fin which determines the size of vortex in front of the seal fin. Furthermore, a minimal leakage can be obtained by varying the length. And this conclusion is applied and validated by the shaft-shift cases which is a normal phenomenon in real working conditions of a turbomachine. As for rotational speed of the rotor, a modified flow model, based on the pressure ratio model, is developed with both axial and circumferential direction equations. This modified model is validated by experimental results of different seal structures in different working conditions, which convinces the understanding of rotational effects that the rotor provides the tangential component of main flow velocity which leads to a higher dissipation. Then numerical simulation is implemented to validate the model in flow details aspect.Based on the understanding of seal mechanism and new design criteria, a new design is proposed with a leakage reduction of about 25%, convincing the proposed design criteria. Due to geometry limitation of shaft-shift working conditions, this new structure is modified to be applicable for real turbomachines. It turns out that this modified new structure is better than the original design at most relative positions by a leakage reduction of about 20%.