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
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.
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.
The main results are as follows.
- Turbine Rim Sealing. Previously undetected flow structures were observed which augment the ingestion of hot main annulus gas into the interior cavities. The presence of these structures was subsequently predicted using unsteady CFD, by modelling the full 360° geometry, rather than a rotationally periodic sector representation – which is common practice. The methods and insight gained in the course of the research were applied to design an optimised rim seal suitable for incorporation into a production engine. The resulting seal design was subsequently tested and achieved a superior sealing efficiency relative to the most effective rim seal geometry tested during the precursor ICAS-GT project. The associated reduction in the internal air system requirements for turbine rim sealing can be exploited as a reduction of SFC.
- Rotating Cavities. The boundaries of the buoyancy- and throughflow-dominated flow regimes have been defined in terms of a non-dimensional group (Buoyancy number) and three component velocity measurements were obtained within a rotating cavity using a novel laser Doppler velocimetry technique. Reynolds-averaged Navier-Stokes CFD methods were exhaustively applied to predict flow and heat transfer behaviour within rotating cavities with limited success. However, large eddy simulation of this class of flow was far more successful, agreeing with heat transfer measurement data to within 6%. The ability to predict the thermal response of compressor disc stacks more accurately will permit reductions in blade tip clearance, thereby improving efficiency/reducing SFC.
Turbine Stator Wells. Various turbine stator well configurations were investigated in the course of the research and a number current production configurations were ranked in terms of the effectiveness with which secondary air was used to cool and seal the stator well cavities. A better understanding of thermo-fluid behaviour in stator wells has been gained and a best practice for stator well design established, ensuring more effective use is made of cooling air with a corresponding reduction in engine SFC. A novel method for cooling critical components in turbine stator wells has been identified, based on insight from both test measurements and detailed CFD modelling. The improved cooling strategy has been protected by a patent and improves cooling effectiveness
The main technical implications are as follows.
- The industrial partners in the consortium have captured the CFD modelling knowledge that has been gained in the course of the ICAS-GT2 project and incorporated these into their respective best practice guidelines.
- The limitations of turbine rim seal ingestion using rotationally periodic sector models has been identified and it is known that this modelling approach under-estimates ingestion.
- In order to resolve rim seal ingestion more accurately, it is necessary to undertake unsteady modelling of the 360° geometry, as steady-state and rotationally periodic sector representations are incapable of resolving key features of this class of flow.
- Flow structures with rotating cavities - which rotate at about 80% of the rotor speed - have been observed experimentally and predicted numerically. The presence of these flow structures – which are broadly similar to those found in Rayleigh-Bernard convection – are thought to have a significant effect on heat transfer.
- The ICAS-GT2 research has clarified the difficulty of resolving rotating flows accurately using RANS-based CFD solvers (using steady, unsteady, sector or 360° representations). In view of this, a change of philosophy is being adopted to design out the problem and impose a more predictable flow structure in rotating cavities.
- The use of large eddy simulation has been found to capture buoyant flow and heat transfer in rotating cavities far better than RANS-based solvers – even on the same calculational mesh. At the current time it is not considered practical to undertake such calculations routinely in an industrial context, however as computing power improves this will provide a fall-back to the approach of designing out the flow structures that are currently difficult to predict.
- Design best practices have been established for pre-swirl systems and turbine stator wells, in order make more effective use of the secondary cooling air. These best practices are now being applied by the industrial partners to current gas turbine designs.
- A range of test facilities have been established and operated by the academic partners in the ICAS-GT2 consortium. In most cases, these test facilities are still in operation and are continuing to produce important new data, e.g. the turbine stator well rig has been further modified under the EU-funded MAGPI project and is being u
The main policy implications are as follows.
- The research undertaken in the course of the ICAS-GT2 project has advanced the design data, technology and tools at the disposal of the European gas turbine manufacturers. As noted above, it would not have been possible for any of the European OEMs in this field to have covered the breadth of the ICAS-GT2 work scope independently. Thus, the work performed has made a significant contribution to the declared policy of maintaining European competitiveness in the field of gas turbine technology.
- A key objective of the ICAS-GT2 research was to improve gas turbine engine performance, by making more effective use of the internal cooling air used to cool critical components. Consistent with the declared objectives of the project, the acquisition of new data, improved design tools and insight have provided a basis for the Industrial Partners to improve their respective products. The achievement of a 1% reduction in engine SFC, via more effective use of secondary air, represents a significant contribution to the declared policy of achieving sustainable growth.
- The perception that the most cost-effective means of improving gas turbine engine performance, with the current state-of-the-art, is to make more effective use of the secondary drawn off from the main annulus has been confirmed. The four year ICAS-GT2 project is deemed to have succeeded in its objective of achieving a 1% reduction in SFC for a total project cost of ~3.5M Euro.
- Over the course of the ICAS-GT2 project, the collaborating partners were able to explore a wide range of different modelling approaches, e.g. pseudo-steady, unsteady, various turbulence models, inlet boundary condition assumptions. It is clear that the combining the resources of the European gas turbine manufacturers permits exhaustive coverage of modelling alternatives and the definition of best practices that would not other wise be possible. It is therefore important for the economic well-being of the organisations in the ICAS_GT2 consortium that further opportunities to collaborate in this way are identified and exploited.
- Based on the ICAS-GT2 research, it is judged that the most fruitful area in which further reductions in gas turbine engine SFC will be achieved via internal air system research is in interactions with the main gas path. To date, little account has been taken of the way in which secondary air is re-admitted into the main gas