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Acoustic and thermal instrumentation, tests and modelling of engine surface coolers in representative aerodynamic conditions

Final Report Summary - ACOC-TH (Acoustic and thermal instrumentation, tests and modelling of engine surface coolers in representative aerodynamic conditions)

The recent technological developments in the aeronautical domain and the continuous search for more efficient engine architectures demands a parallel investigation on advanced oil cooling strategies. The usual cold sources like the inlet air stream and the fuel circuit are approaching their limits as new engine designs are exploited. The higher level of complexity on the mechanical systems requires an adequate thermal management of the systems. The heat removal by the aircraft structure will be limited by the use of composite materials with lower operational temperature and thermal conductivity properties. Furthermore, the limitation on the maximum fuel temperature decreases the viability of the fuel tank as a cold source.
The present work is included in the research frame of novel engine cooling strategy. It has the objective to quantify the thermal performance of an Air Cooled Oil Cooler (ACOC) heat exchanger assembled on the inner wall of the secondary duct of a turbofan. The goal of such design is to use the available surface as a heat exchanger between the air and the oil. In order to increase the thermal performance, the wet area is increased by adding longitudinal fins, reaching the required heat dissipation power. Such a design implies a strong compromise between the aerodynamic penalties, (introduced by the increased drag) and the thermal performance of the heat exchanger.
The developed research presents an innovative aero-thermal study by testing the new heat exchanger concept in a 3D shaped transonic wind tunnel capable of reproducing the flow condition within the bypass of an engine. Innovative data processing approaches, based on inverse heat conduction methods (IHCM), were developed and employed during the course of this work.
Inverse heat conduction methods provide the possibility to reconstruct the convective wall heat flux responsible for the surface temperature evolution during an experimental test. This approach, when coupled with spatial temperature transducers such as Infrared Thermography (IR), contributed to an important increase of flexibility. This fact allowed the analysis of models with non-negligible three dimensional heat conduction effects. The implemented IHCM was based on an iterative Alifanov procedure. Conjugate gradient methods with the adjoint problem were used as a minimization technique. The coupling of the inverse heat conduction solver with a commercial finite element program (COMSOL MULTIPHYSICS) provided the possibility to solve a generalized transient 3D IHCP.
For validation purpose, numerical and experimental methodologies were developed to test the solver for a variety of situations. This process revealed to be of paramount importance to understand the sensitivity of the method to different parameters that typically govern the heat transfer process. The validation was obtained by imposing a known heat flux on a surface of the numerical domain. Afterwards, the resulting temperatures were used as an input to the IHCM.

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