Mechanisms of hot gas ingestion in turbine disk cavities

Abstract

In a gas turbine engine, hot gas ingestion occurs when gas from the mainstream flow path migrates into the cavities between stator and rotor disks. In order to mitigate the damage caused by these high-temperature gases, the cavities are purged with colder air from the compressor, and rim seals are used at the entrance from the mainstream. Various mechanisms of ingestion have been proposed, and studied in great detail, but other potential mechanisms have been largely neglected. This thesis aims to test the sensitivity of ingestion to features of flow and geometry about which little is known, and to investigate how their effects might be predicted by engine designers. The thesis uses experimental testing, together with two- and three-dimensional modelling methods, to demonstrate that long wavelength, small amplitude variations in mainstream pressure are the dominant cause of hot gas ingestion for an engine-representative test case. This mechanism creates a flow pattern in the rim seal that drives hot mainstream gas into the cavity in discrete locations, increasing the risk of localised thermal damage. CFD simulations incorporating these variations predict average cavity seal effectiveness to within 2% of measured values, and capture local variations in seal effectiveness. A two-dimensional model of ingestion is developed and tested, based on the linearised Euler equations. The model uses a parametric representation of the mainstream pressure variation to model the flow in the seal, and tracks the transport of mainstream gas into the disk cavity. Results from the model show good agreement with experimental data for different mainstream pressure distortions and purge flow rates. The model is sensitive to several key geometric and flow parameters, and it is used to estimate the effects on ingestion of changes to mainstream pressure, rim seal geometry, and engine operating conditions. The final chapter of the thesis reports an experimental study of the effects of rotor leakage flows on hot gas ingestion. It is shown that the presence of leakage flows increases the purge flow required to maintain desired cavity temperatures, and amplifies local variations in surface temperature. A simple, semi-empirical model for the flow in the cavity is presented that estimates the effect of leakage on ingestion, based on data with no leakage present. The model shows good agreement with experimental measurements, and provides valuable insight into the structure of the flow in the disk cavity.