Buoyancy-Induced Flow and Heat Transfer inside Compressor Rotors

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.

The impact of this research will benefit the economy and society. Rolls-Royce plc. are a global supplier of gas turbines for aerospace applications, and their success is critical to UK manufacturing industry. Over 30,000 Rolls-Royce engines are currently in service with 650 airlines, freight operators and lessors and 4,000 corporate operators. In the UK Rolls-Royce employs over 21,000 personnel to design, manufacture and maintain gas turbine engines for worldwide distribution, with 85% of sales coming from abroad. In this very completive market, small service and performance changes can have significant impact directly on sales as these strongly influence the life-cycle cost of the products which is a major performance indicator to customers.

This proposal addresses an area of critical importance to the production of ever more efficient and cleaner aero-engines. The direct impact of this research will be to improve the design of aero-engine compressors through informing the design practices employed at the company. By enabling tighter blade tip clearance control, an increase in compressor pressure ratio from 50 to 60 would give a 1.7% increase in thermodynamic efficiency. This improvement can be readily converted in savings in specific fuel consumption (SFC): for a Trent 900 powered Airbus A380 at cruise conditions this equates to a reduction of over 20,000 tonnes of fuel over the lifetime of the aircraft, equivalent to a saving of £5M at today's oil prices (£62,000 per engine per year).

More broadly, the impact of the research will also contribute to the company's current level of technology and competitiveness in the aerospace industry. The principal route to impact will be through the Thermo-Fluids Systems University Technology Centre (TFSUTC) at The University of Surrey. This proposal will build on this association by enabling a cross-institutional syndicate of established experts to work together in this collaborative venture. The proposed combination of experimental, theoretical and computational modelling is vital to ensuring that the fundamental investigations are developed through to tangible industrial impact.

At Surrey there is an established track record of such impact. The improved methods for predicting aero-thermo-mechanical behaviour of machines (developed by Surrey) benefit industry (and hence the economy) through contributions to improved design, reduced development costs and timescales, and less reliance on engine testing. Surrey's CFD methods have been used by Rolls-Royce plc, since 2006. Uptake by the company of the aero-thermo-mechanical modelling techniques has expanded rapidly since 2010. As an example of impact, in 2011 the coupled CFD/FE capability enabled Rolls-Royce to cancel at least one telemetry test (estimated at between one and five million pounds), and allowed declaration of a viable HP turbine disc life avoiding penalties that would have significantly affected the business position.

Software development, validation studies, and working practices developed at Surrey's TFSUTC underpin such applications. In 2012 Rolls-Royce internally agreed that requirements for engine tests to validate thermal models would be reduced by 20% over the following 5 years. A conservative estimate of the cost savings over this period is £6 million. Of direct relevance to this proposal is the increased Time Between Overhaul (TBO) that will result from this research. It is expected that the more accurate predictions of disc temperatures, made possible by the vast improvements in modelling accuracy, will dramatically increase the compressor disc life. Accurate predictions of disc life are of critical importance to gas turbine designers, and directly impact Rolls-Royce's market competiveness.