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Turbine Aero-Thermal External Flows 2 (TATEF2)

Final Report Summary - TATEF2 (Turbine Aero-Thermal External Flows 2)

The overall objectives of the TATEF 2 project was to understand the complex aero-thermal phenomena generated in turbines, to build the associated databases and to facilitate the validation of improved computational fluid dynamics (CFD) methods. Indeed, aero-engine manufacturers request to gain understanding of the aero-thermal flow physics, to improve modelling capacity, accuracy and robustness, to try and validate new concepts / designs and methods and improve CFD know-how - leakage flows, heat transfer in particular regions - and validate CFD codes - optimised design process for higher efficiency and specific power, lower emissions, failure risk and global costs.

The work programme has been sub-divided in four complementary work packages (WPs) dealing with the most important turbine external aero-thermal aspects:

WP1 - Inlet temperature distortion and inlet swirl variation effects
This WP aimed to measure the aero-thermal efficiency and losses associated to a shroud-less high pressure turbine, with inlet temperature non-uniformity and inlet swirl variation, and to investigate the transmission of these two phenomena through the turbine stage.

WP2 - Aero-thermal performance of HP turbines and interaction phenomena
The objectives of the WP were:
- to quantify the overall performance of the stage in terms of mass flow, power, etc. in the view of determining its efficiency;
- to investigate the rotor-stator interaction with a strong vane trailing edge shock in order to improve the knowledge on the unsteady aerodynamics and the forcing function;
- to study the aerodynamic and heat transfer of the rotor platform in presence of coolant and with ejection of flow between the stator and rotor hub platforms;
- to determine the aero thermal performance of an innovative second stator combining aerodynamic and structural function.

First of all, high confidence efficiency measurements were obtained for the first time. Additionally, a novel technique to determine the heat flux in cooled airfoils has been implemented. Regarding the flow analysis, interesting features have been observed regarding the strong shocks, rim seal interactions and HP-IP interactions. At high pressure ratio, the analysis of the data indicate that the vane shocks give rise to a vortical structure that is the main cause of the turbine loss. Regarding the rim-seal investigation we observed a rise in efficiency when blowing, contrary to the experience with subsonic turbines. Detailed loss analyses indicate that such increase is associated to a decrease in the shock loss. The multi-splitter IP configuration with equally spaced vanes indicates clear separated regions at off-design conditions.

WP3 - Fundamental film cooling studies
This WP was dedicated to the investigation of the effect of steady and unsteady shock waves on film cooling performance, to the analysis of the flow field inside the film cooling holes for various flow conditions at the hole inlet, to the study of film cooling effectiveness and heat transfer coefficient on a nozzle guide vane (assembled in a linear cascade) and its platforms.

Numerous studies have been performed in the past to obtain a deeper insight into the aero-thermal behaviour of cooling films. In TATEF 1, several basic film cooling issues could be addressed performing generic experiments which allowed separating main influencing effects - such as coolant cross flow, free stream turbulence, wake passing or row interaction.

With the request for powerful optical measuring techniques, a unique design of a generic experimental set-up was developed, allowing for simulating transonic suction-side Mach number distributions at realistic shock strengths, Reynolds numbers, and coolant to free stream density ratios. For the first time, the shock-film cooling interaction zone could be analysed with great thermal as well as local resolution owing to infrared thermography. Facing challenges in applying this measurement technique to the present high temperature gradient flows, new calibration techniques have been developed. The major outcome of the study is that cooling films are quite insensitive to shock waves within the range of Mach numbers and shock strengths usually present in modern transonic turbines. However, the discharge behaviour of cooling holes changes in transonic free streams. Hence, a correlation on discharge coefficient was developed covering a wide range of operating conditions.

A possibility to further increase total cooling efficiency is to combine technologies such as internal convective cooling and film cooling. To promote the internal convective heat flux, obstacles like ribs are positioned at the inner surface. These obstacles cause enhanced turbulent mixing and lead at the same time to an increased heat transferring surface. However, the effect of complex flow structures generated by the internal obstacles on the hole discharge behaviour and film cooling performance cannot be satisfactorily predicted with available design tools. Comprehensive aerodynamic as well as thermal measurements have been performed providing a database allowing for the evaluation of the aero-thermal performance of a cooling film under the influence of internal geometries. Furthermore, the aerodynamic tests performed at the hole entry and exit region as well as pressure measurements inside a film cooling hole help to validate CFD methods and to better understand the flow physics.

The main objective of subtask 3.3 was to measure and analyse the film cooling performance on platforms. A comprehensive database of film cooling performance data obtained on airfoils and platforms of a heavily film cooled nozzle guide vane has been delivered. The experimental and computational results of the test cases allow the industrial partners to for example strengthen film cooling protection schemes without increasing the cooling air consumption. Moreover, the WP has enabled the establishment of expertise, skills and knowledge in the field of turbine cooling technologies. It turned out to be more challenging to measure heat transfer on the platforms with the existing technique than anticipated. An improved thermal measurement procedure has been established and validated. Successful validation and application of pressure sensitive paint for film cooling measurements has been carried out.

WP4 - CFD calculations
The objective of this WP was to perform CFD calculations on the tested configurations and to confront the results to the experimental data aimed to improve the simulation techniques. The CFD tools for heat transfer and unsteady computations were assessed, as their usability for the most suited ones. Moreover, the comparisons between the results from the different partners (industrial and also research) allow these partners to assess their code competence.

The assessment of CFD tools for heat transfer of turbine stage components has been achieved through the improvements of physical model on the basis of CFD simulation of selected cases focused on the basic of film cooling. The physical model improvement for HT calculations have been completed and a transition model for the boundary layer has been successfully added to the two equation turbulence models already present in the HybFlow code, a research code developed in the past at University of Florence. Grid generation for the fan-shaped holes test case has been concluded to share the grid with other partners.

A second activity was focused on the steady and unsteady stage aerodynamics and heat transfer simulation of the selected cases. A lot of efforts have been dedicated to the improvement of the computing efficiency in terms of accuracy of unsteady treatment, computational time diminishing and transition modelling. The computational time has been diminished by a factor of 33 % by changing the CFD code algorithm.

All the collected data from calculations and experiments have been compared and some guidelines and suggestions on the best practise for improved accuracy of the CFD tools were prepared together with a report on the improvements achieved on the numerical and physical modelling of the computational tools. Validation of numerical methods and assessment has been achieved with improvements in modelling capacity. Deep investigation of the complex time-averaged and time resolved flow field has resulted in a better understanding of the aerodynamics and the heat transfer in basic film cooling configuration, as well as in stage behaviour.

The socio-economic impacts of this work can be summarised in the improvements , through the high number of PhD student and young researchers who have been involved in the calculations, research, investigation and analyses of the results, of the competitiveness the fluid dynamics and engineering European community, in an enlargement of the employment opportunity and in increase of the feeling towards the environmental items, as the achievement of better and longer performance of the aero-engines will result in a reduced environmental impact of the aeronautical transportation.

From the point of view of the universities the improved knowledge of the problems and tools involved in the design of aero-engines will results in a better teaching capability closer and closer to the need of an advance society based on the 'knowledge', and to the European feeling of the crucial needs for the future of our new generations: environment, competitiveness and skills increase as engines for the harmonic development of European and world economy.