Buoyancy-Induced Flow and Heat Transfer Inside Compressor Rotors

Lead Research Organisation: University of Surrey
Department Name: Mechanical Engineering Sciences

Abstract

The gas turbine represents one of the most highly advanced examples of modern day engineering. With over 100,000 scheduled flights per day globally, there is an ever increasing reliance on jet engines to meet our transportation demands. Increasing fuel costs and demanding environmental legislation have driven jet engine manufacturers to produce increasingly efficient power-plants in order to remain competitive. The Advisory Council for Aerospace Research in Europe (ACARE) have set out the target of a 20% reduction in engine fuel consumption and carbon dioxide (CO2) emissions by 2020, relative to 2000 levels.

To increase the power output and efficiency - and consequently to reduce the fuel consumption and CO2 production - of gas turbines, it is necessary to increase the pressure ratio of the compressors. This presents a challenge for designers of aeroengines: the higher the pressure ratio, the smaller the compressor blades become, and the size of the clearance between the blades and casing has an increasing effect on the compressor performance and stability. To calculate (and control) these small clearances for transient and steady conditions, it is necessary to determine the radial growth of the compressor discs. This in turn requires the calculation of the transient temperatures of the discs, which involves the calculation of the buoyancy-induced rotating flow and heat transfer inside the compressor rotors. These flows - which are three-dimensional, unsteady and unstable - are extremely difficult and expensive to compute, even by the biggest computers now available. This presents a challenging problem for engine designers, and the research involves an integrated theoretical, computational and experimental programme to address this problem.

This project aims to combine experiment, computation and theory to generate a fundamental understanding of buoyancy-induced rotating flow and to develop CFD codes and a theoretical model for use in the compressor-clearance-control technology of gas turbines. The proposal represents an exciting new collaboration between two of the UK's leading research institutes in this area, both with a proven track record of delivering impact to industry. The complementary experience and expertise of the research teams at Bath and Surrey are perfectly suited for such a collaborative enterprise, and the advice and support from Rolls Royce is vital for its success. Not only would this research seek to understand these complex rotating flows, it would also lead to the development of CFD codes and theoretical models that would be used by the designers of the next generation of aeroengines.

Publications

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Description Early computational fluid dynamics studies clarified the development of flow instabilities and flow characteristics in a sealed rotating annulus heated on the outer shroud and cooled on the inner cylinder. Further analysis using an incompressible solver with the Boussinesq approximation gave insight into the surface heat transfer within the sealed cavity. The phenomenon of "Ekman layer scrubbing" was identified showing the disc boundary layers to be highly unsteady Ekman layers. The effect of imposing a central axial throughflow in the cavity was also investigated. Although this has important effects, the flow and heat transfer in the outer part of the cavity remains similar to that in the sealed case.

A second phase of the research used a fully compressible flow solver and compared fully-resolved large eddy simulations with those from the incompressible solver and experimental results. A surprising finding was that the Ekman layer scrubbing gave rise to significant frictional heating in the disc boundary layers. With this and other factors now understood, many discrepancies and questions raised by previous published work in the area can be explained. A result of practical interest to gas turbine designers is that the shroud heat transfer agrees with correlations obtained from free convection under gravity.

In the final phase of the research wall modelled large eddy simulation (WMLES) was introduced into the compressible flow solver, comparisons were made with meaTsurements obtained at the University of Bath, and the WMLES was used to study low Rossby number conditions and flow interactions in a dual cavity configuration. The model proved viabl at the most extreme experimental conditions with potential for application to engine conditions. Comparisons with measurements from the University of Bath gave very encouraging levels of agreement. Difficulties were encountered and overcome in modelling low cooling flow rates where an issue with reversed flow in the central axial flow was identified (and is also believed to occur in the experiment). Some experiments at Bath were delayed due to COVID restrictions, and completion of the CFD studies was achieved in 2021, using a funding extension to mitigate the effects of COVID-19.
Exploitation Route During the project regular reviews with the academic and industrial partners took place. The papers published are also attracting considerable interest. The latest paper (to be presented at the 2021 ASME Turbo Expo 2021 and a corresponding journal paper published in 2022) is of particular interest to industry as it gives a clear account of the flow and heat transfer mechanisms (correcting some assumption made in other workers' publications) at research rig conditions with indications of how to extrapolate results to engine conditions.

Exploitation of the research findings has progressed through application of WMLES to a Rolls-Royce Trent engine geometry with funding from an Impact Acceleration Account. The understanding of the flow mechanisms developed and the WMLES technique has been used in consideration of elemental models that can be applied in industry,

Further collaboration with Rolls-Royce and the University of Bath is under discussion.
Sectors Aerospace, Defence and Marine,Energy

 
Description The results of the research project and the associated Impact Acceleration Account study are being used at Rolls-Royce in defining correlations and assumptions in engine thermal modelling, and are informing interpretation of data and further experimental work at the Universities of Surrey and Bath.
First Year Of Impact 2021
Sector Aerospace, Defence and Marine,Energy
Impact Types Economic

 
Description EPSRC Impact Acceleration Account - Surrey
Amount £31,557 (GBP)
Organisation University of Surrey 
Sector Academic/University
Country United Kingdom
Start 06/2020 
End 03/2021
 
Description UKRI COVID-19 Grant Extension Allocation (CoA)
Amount £85,259 (GBP)
Funding ID UKRI CoA RN0450: UKRI COVID-19 Grant Extension Allocation (CoA) 
Organisation University of Surrey 
Sector Academic/University
Country United Kingdom
Start 01/2021 
End 09/2021