Development of a Coupled CHT-CFD Solver for the Modelling of Secondary Air Flows in Gas Turbines

Abstract

With one of the major contributors to climate change being air travel, future gas turbine designs are striving towards higher efficiencies by achieving greater turbine entry temperatures. This is accomplished via higher overall pressure ratios in the compressor stage of the engine, with newer designs featuring smaller core dimensions. The combination of increased dimensional sensitivity and higher temperatures experience by the rotating components can lead to thermal expansion, resulting in the risk of contact between the compressor blades and the stationary casing. For engine designers, tools to help predict this radial growth are vital for optimisation, however modelling this is not trivial. The complexity of this problem lies within the conjugate nature of the compressor design: the radial temperature gradient that forms in the discs drives the formation of large-scale buoyant structures in the core of the enclosed fluid, which in turn governs the heat transfer and temperature in the discs.

This thesis describes the development and implementation of a novel coupling strategy for Computational Fluid Dynamics (CFD) with Conjugate Heat Transfer (CHT) for use in highly buoyant flows. Computations were conducted using the open-source package OpenFOAM, with initial validation undertaken to determine the suitability of the available software. The existing conjugate solver was found to be unsuitable for resolving the heat transfer in buoyant flows with highly rotational boundaries, where inconsistencies in the formulation of the Navier-Stokes equations resulted in an under-prediction of the heat transfer. A new solver was developed to resolve the conjugate flow in the rotating frame of reference. To overcome the large difference in thermal inertia between the fluid and solid domains, an unsteady-fluid/steady-solid coupling was used. To achieve an axisymmetric solution in the solid, a new boundary condition was implemented to provide a mixing plane on the coupled surfaces, preventing the unsteady features of the fluid from disturbing the convergence of the solid domain. Validation was performed against experimental and numerical data.

Unsteady Reynolds-Averaged Navier-Stokes computations were conducted on an engine representative geometry for a closed compressor cavity. The computations provided further insight beyond the experimental results, while validating assumptions made by experimentally-derived models. The presence of laminar Ekman-layers near the radial surfaces of the discs was captured by the computations, and the variation of the radial velocity within this region was linked to the change in tangential velocity in the core. The predictive nature of the tool was demonstrated through a theoretical case where the flow in the core approached stratification.

Date of Award	26 Jun 2024
Original language	English
Awarding Institution	University of Bath
Supervisor	James Scobie (Supervisor), Mauro Carnevale (Supervisor) & Hui Tang (Supervisor)