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ACCENTO (Active Clearance Control dEsigN and characTerizatiOn). Advanced investigations on different Low Pressure Turbine Active Clearance Control (LPTACC) system by means of CFD and experiments.

Periodic Reporting for period 1 - ACCENTO (ACCENTO (Active Clearence Contol dEsigN and characTerizatiOn). Advanced investigations on different Low Pressure Turbine Active Clearance Control (LPTACC) system by means of CFD and experiments.)

Reporting period: 2019-04-01 to 2020-09-30

The activities carried out in ACCENTO project contributes to the WP2.4.2 of the ENGINE GAM of the CleanSky2 JU. The WP2 - Ultra High Propulsive Efficiency (UHPE) demonstrator, addressing Short/Medium Range aircraft market, represents one of the pillar demonstrator and activities of the Engines ITD of the CleanSky2 JU. In particular, the development of the LPT (WP2.4) is one of the most critical steps for the achievement of the high level objectives of the demonstrator in terms of aerodynamic efficiency. The LPT efficiency optimization requires to minimize airflow leakages between the static parts and the rotating parts; such goal can be achieved implementing an active clearance control system.
The objective of ACCENTO (Active Clearance Control dEsigN and characTerizatiOn) project is then to carry out a series of advanced investigations on different LPT Active Clearance Control (LPTACC) system pipes and target plates. The aim is to develop design and verification procedures to reliably predict the aero-thermal behavior of such impingement cooling system. This goal is pursued by means of dedicated experimental tests and numerical simulations. The great ambition of ACCENTO is to design a modular test rig which will be operated at engine representative conditions and that will be able to provide reliable data for impingement cooling heat transfer characterization in a wide range of operating points. The radiative heat transfer impact on the system performance will be also accounted for. The rig is now providing high quality data in conjunction with controlled operating conditions to validate CFD tools for the heat transfer and, more in general, for the ACC system characterization.
The expected outcomes of the project can be summarized as follows:
• design, commissioning and testing of a modular rig for LPTACC impingement heat transfer coefficient measurements
• validation of suitable CFD methodologies
• development of design correlations
Moreover, thanks to the modular nature and flexibility of the developed rig, future and successive tests and analyses will be possible.
Exploiting the applicants’ experience the test rigs have been designed and manufactured. Three different benches are designed and operated. The first rig is devoted to the impingement holes discharge coefficient (Cd) characterization, the second is related to the impingement heat transfer performance (low temperature rig) while the third is dedicated to the study of radiation effects (high temperature rig).
The rig for the Cd evaluation has been already designed, manufactured and operated. Leakage tests have been performed and the experimental data obtained in this rig commissioning phase have been compared with literature correlations confirming the rig measurements consistency.
The low temperature heat transfer rig was designed in order to retrieve detailed heat transfer data on the impingement jets’ target surface. To achieve this goal, the air jets generated from the ACC pipe are directed towards a heated surface and the resulting temperature distribution is recorded by means of an IR camera. Heat transfer coefficient distributions are obtained by postprocessing the recorded temperature data.
A specific procedure has been developed to retrieve temperature and heat flux on the inner (impingement) side of the target starting from temperature measurement on the outer side. This goal is achieved by a custom finite difference procedure, which solves the inverse heat conduction problem within the target plate.
The high temperature rig consists in a modification of the heat transfer rig described above. More in details, differences between the rigs are limited to the target plate.
As impingement tests are considered, currently around 490 experiments were performed considering more than 100 different impingement configurations. Experimental results, together with relevant numerical studies, were then exploited to define new impingement configuration improving both the knowledge associated to the impingement related phenomena and the muscle of the LTPACC system. As computational analyses are concerned, different CFD approaches were tested to identify the related accuracy and the trade-off between models’ reliability and computational costs. It was proved that scales resolved approaches are actually able to improve the computations accurateness providing also significant insights on the jets mixing and on the turbulence development.
The work done up to now has put the light on several aspect that affect the system performance but are not covered by the ACCENTO test plan. New ideas to push forward the knowledge associated to LPTACC system behavior arose, putting the basis for future research studies.
A technical publication is now in press at the ASME Journal of Engineering for Gas Turbines and Power, and a new paper focused on the ACCENTO obtained results will be presented in the next ASME TurboExpo 2021 conference.
Improvements beyond the state of the art expected through the activities carried out in ACCENTO are both in the numerical modelling and in the experimental testing fields.
Besides standard simulation approaches, an in deep investigation will be also done adopting Scale Resolving CFD simulations such as Hybrid-LES. Moreover, in the second phase of the project, an experimentally validated numerical methodology aimed at combining the radiative heat transfer calculations and the impingement jets simulation in a single CFD calculation will be coupled in a Conjugate Heat Transfer strategy.
Regarding the experimental activities, the ACCENTO project will allow to improve the measurement techniques knowledge based on IR-thermography and will also allow to better understand the effects of cooling pipes geometrical details on the detailed heat transfer coefficient distribution, also taking into account the micro and macro geometrical features of the target surface. Particular efforts will be also paid to investigate the effect of the surface/coolant temperature ratio and the influence of thermal radiation on the heat transfer performance.
The expected impact arising from an optimized LPTACC system can be summarized as follows:
• Reduction of flow leakages between stator and rotor parts to maximize turbine aerodynamics efficiency
• Minimizing the airflow by-passed to the ACC system will limit the reduction of LPT performance related to mass flow curtail and to reinjection process
• The optimization of the impingement process as well as the proper prediction of mass flow distribution across the ACC pipes and holes can help to properly size the system thus limiting the compressor bled mass flow rate
The fundamental studies planned in ACCENTO, and in particular the detailed experimental database and the validated numerical setup, will provide the basic knowledge and reliable design tools to support the Topic Leader and the CleanSky2 JU in the design of a really optimized and innovative ACC system.
An additional impact on innovation that will result from the outcomes of ACCENTO, is related to the innovative experimental methodology proposed to evaluate the heat transfer process due to impinging jets. Taking into account the widespread application of impingement cooling in several engineering processes (from engines up to electronic devices), the mentioned goal of ACCENTO will be of benefit for different industrial areas thus improving the general societal impact of the CS2 actions.