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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.

Periodic Reporting for period 2 - 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: 2020-10-01 to 2021-09-30

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.
In the ACCENTO (Active Clearance Control dEsigN and characTerizatiOn) project a series of advanced investigations on different LPT Active Clearance Control (LPTACC) system pipes and target plates have been carried out. Outcomes of the project are the development of design and verification procedures to reliably predict the aero-thermal behavior of such impingement cooling system. This goal has been reached by means of dedicated experimental tests and numerical simulations. A dedicated modular test rig has been designed, manufactured and then operated at engine representative conditions providing reliable data for impingement cooling heat transfer characterization in a wide range of operating points. The radiative heat transfer impact on the system performance has been also accounted for, implementing a dedicated test rig upgrade capable of high target surface temperature levels. The rig provided high quality data under highly controlled operating conditions generating boundary conditions to validate CFD tools for the heat transfer and, more in general, for the ACC system characterization.
The project is successfully completed reaching the planned goals that can be summarized as follows:
• the design, commissioning and testing of a modular rig for LPTACC impingement heat transfer coefficient measurements
• the validation of suitable CFD methodologies
• development of reliable tools and methodologies, experimentally validated, for a dependable and really optimized ACC configurations design
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 were 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 one is dedicated to the study of radiation effects (high temperature rig).
The experimental data obtained from the Cd rig have been compared with literature correlations confirming measurements consistency and allowing to characterize from the pressure loss point of view several innovative pipe drilling patterns.
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 with a customized finite difference procedure, which solves the inverse heat conduction problem within the target plate.
The high temperature rig consists in a modification of target surface of the heat transfer rig which is capable of reaching higher temperature values, relevant to investigate radiative effects too.
As impingement tests are considered, more than 1000 heat transfer experiments were performed considering more than 200 different impingement configurations.
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. At the end of the project, more than 600 simulations were performed. The simulations included relatively simple models of a single impingement hole as well very computationally expensive cases of the entire ACC pipe, reaching about 100 million mesh elements. Additionally, almost 50 high-fidelity LES simulations were carried out.
The findings of ACCENTO were published in three research papers published at the ASME Turbo Expo conferences across the 2020-2022. Paper GT2020-15540 and paper GT2021-59158 were really appreciated during the reviewing process and received the transaction on the ASME Journal of Gas Turbine and Power. Paper GT2022-81791 is currently under preparation.
The achievements of ACCENTO have been disseminated continuously through a dedicated LinkedIn page ( with more than 400 followers.
Improvements beyond the state of the art 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 adopting Scale Resolving CFD simulations such as Hybrid-LES has been done. 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 were coupled in a Conjugate Heat Transfer strategy.
Regarding the experimental activities, the ACCENTO project allowed to improve the measurement techniques knowledge based on IR-thermography and also allowed to better understand the effects of cooling pipes geometrical details on the detailed heat transfer coefficient distribution, also taking into account the geometrical features of the target surface. Particular efforts were paid to investigate the effect of the surface/coolant temperature ratio and the influence of thermal radiation on the heat transfer performance.
The 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 carried out in ACCENTO, and in particular the detailed experimental database and the validated numerical setup, provided 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 resulting 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 can be of benefit for different industrial areas thus improving the general societal impact of the CS2 actions.
Target surface HTC map
Manufactured ACC pipes
Impingement 3D unsteady simulation
Target surface temperature map