Development of Fluid-Curtain Sealing Technology to Improve the Efficiency and Operational Flexibility of Large Power Generation Turbines

Abstract

Fluidic curtain sealing has recently been shown to offer significantly reduced leakage in rotating turbomachinery seals. The seal type uses an additional flow injected into the leakage path to reduce some existing leakage flow. Shrouded steam turbine tip seals were the focus of research in this thesis, but the seal has potential applications in blade tip seals, stator root seals, shaft seals, and end gland seals in steam turbines as well as in gas turbines. The implementation of such a seal may be simplified in the case of gas turbines since secondary flows of air are already built into the machine to provide cooling flows to high temperature components. The fluidic curtain seal is especially effective when a combination of fluidic curtain and a conventional labyrinth seal is used, and the research presented will generally feature a fluidic curtain placed upstream of a labyrinth fin type restriction. The new addition to knowledge on fluidic curtain sealing described in this work is in characterising seal performance in terms of its design parameters. Better characterisation of the seal allows the development of a set of realistic design rules to specify how fluidic curtains may be applied to the design of new, high performance turbomachinery seals.
Two main advances in characterising fluidic curtain seals resulted from the research. The first advance was to numerically and experimentally test basic geometric parameters and their influence on performance to identify design rules which maximize the performance gain of incorporating a fluidic curtain. A series of fundamental dimensionless geometric ratios were proposed and the design space created by these parameters was explored and validated experimentally using a simple annular test rig. CFD was then used to demonstrate that it is possible to incorporate a high performance design into a labyrinth seal independent of the existing labyrinth seal geometry. The second advance is to explore the effect of swirl velocity at the leakage channel inlet on overall seal performance. This was first achieved using CFD to model the selected realistic tip seal design with different levels of inlet swirl. This CFD study was then validated by building the design in a rotating annular test rig where the inlet swirl velocity was controlled. The research findings resulted in a proposed design process for new fluidic curtain seals (Section 8.2) which considers; the geometry of an existing seal, fluid conditions in the leakage path and elsewhere in the turbine stage, rotational speed, and minimum allowable physical clearances.