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Passive cooling solution validation for aircraft application

Final Report Summary - HTCS (Passive cooling solution validation for aircraft application)

Executive Summary:
The proposed project is part of the Cleansky activities for the “More electric” aircrafts. Such aircrafts require more power electronics integration and so “traditional” cooling systems (natural or forced convection, liquid loops,...) may reach their maximal performances because of the too high thermal dissipation density of these components.
A need of heat spreading is clearly identified by the users.

In this way, two-phase fluid capillary pumped heat transfer systems appear as more efficient alternative solutions thanks to their high heat transport capacity and their passive pumping capacity.

Several passive cooling systems can be considered for these applications such as vapour chambers, heat pipes, high temperature loop heat pipes...
Such systems have background on space systems and environments; however, the demonstration of their functioning in avionic application, by modelling and experimental tests, is required before being equipped on an airplane.

The developed technology in the HTCS program corresponds to a thermal system for aeronautical applications based on two-phase heat spreading, meaning that the heat is evacuated thanks to vaporization and condensation of a fluid contained in the thermal device. This device is passive so that no electrical input is necessary to make it function, its activation is only due to the heat dissipation. Effective spreading of the heat offers an elegant and performant way to reduce electronic equipment temperatures or increase their waste heat power

The integration of such a two-phase thermal device in an aircraft allows significantly reducing the maximal temperature at equipment using a light and passive thermal device. This leads to an increase of equipment’s reliability combined with a fuel consumption reduction.

This project has been performed in collaboration with Labinal Power System as topic manager.

Project Context and Objectives:
The EHP proposal will address the development, design, manufacturing and demonstration in a relevant environment - TRL6 (technology Readiness Level) - of a two-phase passive cooling system to be used on aeronautical applications.

The main objectives are :

- Qualify two-phase heat transfer devices in accordance with specific aeronautical requirements such as:
o High acceleration loads
o Long duration vibration stresses

- Reduce global aircraft weight at equipment and/or system level => reduce aircraft consumption and increase operating range

- Lower impact on environment through the use of passive and maintenance free thermal equipment

- Increase reliability of controlled equipment (power electronics) => decrease repair and maintenance aspects

- Pressure containment at high temperature and in flat products (ex: two-phase spreader)

The thermal link must comply with the following main technical constraints :
- The size of equipment to be cooled is close to an A4 format with pass-through holes distributed all over its surface.
- One hundred of watts are dissipated over several localized areas with a heat flux density around 30W/cm².
- The equipment is located in a harsh environment characterized by an ambient temperature of 160°C and a low heat exchange coefficient. Despite these conditions, the equipment maximal temperature should not exceed 200°C.

The EHP activities will consist in the following points:
- Bibliographic review
- Technology selection
- Fluid selection
- Cooling system sizing / modeling
- Prototype construction
- Experimental validation

Project Results:
The aeronautical requirements have been clearly specified by Labinal Power System.

A trade-off of potential couples fluid / material that can be used for the two-phase spreader has been conducted and a technical decision has been made to go further with water / nickel combination.

A risk analysis has been carried out in order to identify the critical failure modes that could occur on the HTCS cooling device and to propose actions and recommendations to reduce their criticality.

Interfaces for the HTCS cooling device have been discussed and agreed between Labinal Power System and EHP.

A development and test plan has been proposed to Labinal Power System to determine which tests to perform on the HTCS cooling device.

The thermal device preliminary performances predictions (hydraulic, thermal and mechanical aspects) have been done leading to the definition of the Bread Board (BB) design.

The BB samples have been submitted to a mechanical, hydraulic and thermal testing campaign.

A thermal model has been issued and adjusted with BB tests results.

The detailed design description and justification has been performed. BB tests results have been integrated to support qualification model (QM) design justification.

The heat sink detailed design has been done.

The QM HTCS cooling device has been manufactured after successful CDR/MRR with Labinal Power System.

The hardware has been submitted to thermal acceptance tests at EHP.

The QM performances test has been conducted and the tests and certification reports delivered to Labinal Power System.

The concept of a large two-phase spreader with pass-through holes being able to spread heat fluxes of 30W/cm² in a wide range of ambient temperature was at TRL2 before this project started, as the concept was known but not developped.
Although the manufacturing of the QM and the tests on smaller-size BB were successful, the full-size spreader needed further minor design modifications and validations to be able to withstand all specifications.
The level of TRL obtained after the HTCS development campaign is estimated at 4.

Main innovations:
- Spreader of larger dimensions
- Spreader with pass-through holes
- Spreader with Nickel/Water assembly
- Spreader working at high temperature (>160°C)

Potential Impact:
By the end of the project, SAFRAN Electrical & Power confirms the increased interest for such two-phase thermal devices, and would be pleased so see these products become « off-the-shelf » solutions.

It has to be noted that EHP has already developed a smaller size version of the HTCS spreader up to TRL5, which would easier and faster become an available thermal solution.

EHP et SAFRAN Electrical & Power are very pleased with the outcome of this project, in terms of technological breakthrough, and are convinced it could bring a lot of opportunities for the future.

Several dissemination activities have been done during the project. These correspond to international dissemination through presentation of EHP developments and products for aircraft applications out of Europe and through attendance to relevant European meeting/conferences.

After HTCS project closure, dissemination of results will be proceeded each time a relevant conference will be identified and by presentations of final results and performances to customers.

The potential impact of integrating such a two-phase thermal device for electronic dissipating equipment is the increase of equipment reliability by a significant reduction of maximal temperature and by the use of a passive thermal device.
Such a thermal device can be integrated and used in aircraft dissipating equipment's but can also be extended to a wider field of applications for heat loads of 200W and environment temperature varying from -55°C to +160°C.

The next step shall be an increasing of the TRL up to a qualification model level. This means the following steps :
- Modification of the inserts welding design in order to improve the mechanical performances ;
- Aeronautical mechanical qualification (acceleration, vibrations, shocks) ;
- DO160 environmental validations (icing, thawing...).
In addition to this, an optimization of the HTCS design shall be performed in order to decrease the weight

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