ICAS-GT2 - Fluid flow and heat transfer within the rotating internal cooling air systems of gas turbines (2)
Overview
Background & policy context:
The socio-economic requirements to advance European gas turbine technology are well recognised and with the current state-of-the-art, it is judged that the most cost-effective route to access performance gains is through reductions in internal losses associated with the internal air system.
The optimisation of the internal air systems of gas turbines has previously received relatively little attention or funding and therefore was identified as a technology area in which significant improvements could be made. The objective of advancing internal air systems technology, through the acquisition of new experimental data at engine-representative conditions and the development and validation of predictive design methods, was consistent with the aims of the New Perspective in Aeronautics Key Action of the Competitive and Sustainable Growth Programme.
The research programme addressed fluid flow and heat transfer in the following five distinct – but related – areas of gas turbine internal air systems design:
- Turbine rim sealing
- Rotating cavities
- Turbine stator wells
- Pre-swirl systems
- Engine parts testing and windage losses
The experimental strand of the research included new build/radical modification of six engine-representative test rigs, which were subsequently operated by the consortium’s academic partners. The industrial partners in the consortium were responsible for analysis, ranging from the development of design models based on new experimental data through to validated heat transfer modelling using both Computational fluid dynamics (CFD) and Large Eddy Simulation (LES) techniques.
The industrial partners in the ICAS-GT2 consortium planned to exploit the technology advances derived from ICAS-GT2 in order to:
- Reduce time to market by 3-months
- Improve aircraft efficiency (1% reduction in specific fuel consumption)
- Improve environmental friendliness by reducing CO2 emissions
- Reduce maintenance costs
Objectives:
The technical and scientific objectives of the ICAS-GT2 programme were:
- to perform a range of new experiments on five distinct, but related, aspects of gas turbine engine internal air systems, using high rotational Reynolds number experimental facilities;
- to improve predictive capability by evaluating and validating existing numerical modelling methods (CFD & LES) against the experimental database, and by developing correlations from experimental and numerical results;
- from the experiments and the numerical modelling, to improve physical understanding of two key areas identified by the precursor ICAS-GT programme as critical to the overall optimisation of air system designs, specifically the prediction of sealing/main annulus flow interactions, and rotating cavity heat transfer in buoyancy-driven flow regimes;
- to apply the derived design methods and enhanced predictive capability in the design of geometrical features for optimising internal air system performance;
- to demonstrate the effectiveness of these new designs by experiment.
The main deliverables from the programme were:
- a database of experimental information, including flow and velocity distributions, heat transfer rates and unsteady pressure measurements at engine-representative non-dimensional conditions in all five rotating flow systems;
- validated CFD and LES numerical modelling methods, and validated design methods in the form of correlations which are applicable at engine-representative conditions, and which can be applied directly by gas turbine engine air system designers;
- up to 10% reduction in new engine product development costs and a 3-month reduction in development timescales as a result of exploiting the validated predictive capability;
- a 1% reduction in engine SFC with associated reductions in CO2 emissions.
Methodology:
The methodology adopted in the ICAS-GT2 project was a close coupled combination of experimentation, on test rigs capable of operating at engine-representative non-dimensional conditions, and numerical analysis. CFD modelling was used from the outset to support experimental design and to inform instrument placement and measurement range. The measured data obtained from the test rigs were then used both as the basis for new design correlations (heat transfer, rim sealing flows, windage losses) and validation of CFD and LES models.
It was recognised that acquisition of data from engine-representative test rigs alone could provide new and useful data, but experience has shown that in many cases this yields insufficient information to gain physical insight into complex flow and heat transfer phenomena – such as the unsteady behaviour observed in rotating cavities. On the other hand CFD provides whole field insight into complex flows, however in the absence of a uniqueness theorem for the Navier-Stokes equations means the approach cannot be relied upon to unerringly yield the solutions found in Nature.
Therefore it was judged that only by adopting a strategy of combined and coupled experimentation and numerical modelling (CFD, LES and other approaches) would it be possible to acquire the data and insight needed to achieve the project objectives within the budget and timescale available.
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