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Demonstrator for Aircraft heat exchanger LIfe prediction

Periodic Reporting for period 2 - DALI (Demonstrator for Aircraft heat exchanger LIfe prediction)

Reporting period: 2020-10-01 to 2022-03-31

New generation aircraft engines derived with a strong focus on efficiency, low noise level and low emission are also bigger, leaving less room for surrounding equipment in the engine nacelle. Therefore, there is a need for redesign of several pieces of equipment around those engines which create harsher conditions than onboard current aircrafts.

DALI intends to increase the reliability of the heat exchangers in service, and to improve the qualification process. For that, DALI proposes a fully integrated modelling platform where damage laws will be integrated relying on a multiphysics and multilevel modelling approach and validated by testing to predict the behaviour of compact heat exchangers during service life.

The objectives of Dali, industrial & technical, can be synthetized as follows:

Industrial objectives:
* To reduce the time-to market by reducing the tests duration and the number of manufactured prototypes.
* To reduce the overall cost by 50% due to reduction of time-to market, and cost associated to premature failure or maintenance.
* To improve the quality of heat exchangers that will be more reliable and less prone to failure under harsh environment.
The project has first focused on the definition of the Multi -scale and Multi-Disciplinary Model. The MSMDM is a set of interconnected models necessary to achieve the project goals. It integrates the various physics and the various scales as well as the relevant interdependencies to enable an improved prediction of the mechanical behaviour of a precooler heat exchanger (Hx) in operation. Thus, a proper architecture of models of the MSMDM had be defined and applied in order to derive an efficient, reliable and evolutive MSMDM. This architecture is the definition of all the required necessary models, including scale and physics, as well as the workflows which need to be addressed and therefore the required data transfers between software/tools.

The architecture of models of the MSMDM finally obtained is synthetized in Figure 1. It describes the process which leads to a relevant modelling of the thermomechanical phenomena occurring in an Hx. It shows the matrix nature of this architecture in the physics dimension (Fluid mechanics, Thermal & Mechanical) as well as in the scale dimension (0 to 3).

All the required level and physics required have been defined as well as the interconnection (data transfer)

Besides, in order to perform mechanical simulation of local areas with relevant boundary condition a hybrid modelling methodology has been identified, which relies on the derivation of a macroscale model of the Heat exchanger obtained through an homogenisation process.

Also, the statistical tools to perform the sensitivity analyses have been defined with a specific focus on the uncertainties within the heat exchanger, should they be the uncertainties arising during manufacturing or during usage, and their impact on the structural and/or thermodynamic performance, and the reliability of the system. To model the potential defects, the choice was made to induce geometrically correlated errors, using stochastic processes.
The developments are in progress for now and first tests on basic problems have been initiated, and the results are for now in line with the results presented in the literature.

The mechanical testing on material samples has also been defined, from the nature of required tests to the number required and nature of required specimens. Then, the test apparatus has been tested to ensure that all tests could be performed with a good level of confidence, at room temperature and at high temperature for both tensile tests and fatigue tests.
Some testing has been performed on the thinnest samples (50 microns) with dummy samples so that the feasibility of testing such a narrow sample is ensured. The main goal with these tests was achieved, proving feasibility of testing very thin samples. This is illustrated in Figure 3, where different samples were tested with different gauge lengths but all of them with 50 microns width. The graph bellow shows the strain-stress curves being noticeable that some differences or uncertainties exist from test to test , but these are due to the manufacturing process of the dummy samples.

In the meantime, some non-destructive testing methodologies have been studied and evaluated to identify the best candidate to include crack/failure detection in in-service compact heat exchangers
Advances beyond the state of the art

The novelty is this project is to integrate the defect or local impact of defects in the global model as well as realistic material laws representative of the degradation observed after manufacturing process.
The ability to reproduce the deterioration process (initiation and propagation) and to predict the heat exchanger behaviour will enable the derivation of an appropriate methodology for accelerated design, testing and validation of the heat exchanger life prediction fatigue tests in order to obtain certification.
The DALI project will also provide innovation in the experimental tests area:
Standard specimens for mechanical testing with fins incorporated, sandwich-shape specimens and precooler coupons will be developed and tested as close representation of a precooler.
Ad-hoc instrumented (thermal and mechanical strain) device and methodology with thermal cycling control for thermal fatigue study application for detailed study of thermally caused damage.
Application and validation of NDTs for damage detection and localisation.
New methodology for a representative accelerated test to reproduce fatigue cracks as in fight and evaluate and asses the life duration expectancy based on the known in services loads.

Expected impact

The DALI project will propose a multi-scale and multi-disciplinary model to simulate the aero-thermo-mechanical requirements of the precooler. These simulations will be validated and correlated with laboratory tests to ensure the reliability of the simulation results.

The project also proposes a methodology to perform accelerated test that reproduces the same fatigue cracks as in flight and a measurement set that will enable their detection. This methodology will be based on characterization test results and on the simulations performed with the Multi scale and Multi-disciplinary model.

Highly qualified engineers, physics and chemists face more and more complex challenges to predict the behaviour and conditions of materials, processes and products under real conditions. Advances achieved by means of thermal and mechanical engineering promotes different aspects:
• Prediction of failure of materials and elements
• Optimization of design
• Reliability of design
• Simulation for decision making
Advances in engineering include integration of new knowledge in simulation technologies and ICT tools integrated in simulation programs. Collaborative schemes for simulation experts such as EPSILON and PHIMECA improve the innovation capacity of Europe in advanced engineering leading to outstanding innovative solutions such as the one proposed in DALI.
Figure 3: illustration of feasibility tests performed on 50µm metallic sheets
Figure 1: synthesis map of the MSMDM architecture
Figure 2: Flowchart of the multiphysics and multiscale numerical procedure.