Unsteady Flow in a Labyrinth Seal

Abstract

Unsteady flow phenomena in turbomachinery are a source of aerodynamic losses and can adversely affect the durability of an engine. Examples for such flows are the flow through the cavities of the turbine hub or shroud labyrinth and rim seal. The re-entering flow causes mixing losses with the main flow and increases incidence on the following stator. In order to increase the efficiency level of modern gas and steam turbines, an improved understanding of the interaction between the secondary and primary air system is required. Turbulence-resolving numerical methods can provide such insights if the computational domain captures all dominant scales of the flow in time and space. For turbine rim seals, various experimental and numerical studies in the literature report the presence of large scale cavity modes with a periodicity of up to 120◦ in the circumferential direction. For turbine shroud cavities such modes have not yet been observed. In this paper the time-resolved flow in the cavities of a stepped labyrinth seal is investigated. The aim is to identify the largest scales of the flow which determines the required size of the computational domain for scale-resolving simulations. A small CFD domain of 1◦ circumferential extent leads to an incorrect prediction of the frequencies associated to leakage pulsation. This domain cannot resolve the largest scales present in full annulus CFD models. The largest scales, referred to as lobes, have a periodicity of 12◦ in the circumferential direction. As experimentally shown for turbine rim seals, these lobes are moving in the direction of rotation with 0.76ΩRotor. The frequency of leakage pulsation is higher then the frequency of the lobes and its dependence on the periodic boundary conditions is weaker. Furthermore, the presence and the frequency of the lobes depends on the seal pressure ratio and the pre-swirl of the flow.