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COOLING OF SEAT ELECTRONIC BOX AND CABIN EQUIPMENT

Final Report Summary - COSEE (Cooling of seat electronic box and cabin equipment)

The main aim of the COSEE project was to develop a new cooling enhanced thermal link dedicated to the cabin in flight entertainment (IFE) equipment based on heat pipe technique and having the following characteristics:
- transfer capacity up to 100 W
- thermal conductivity equivalent or greater than 800 W/m/degrees K (twice that of copper)
- heat transportation distance 500 mm (max)
- resistance to aircraft cabin environment (vibrations, acceleration, shocks, airbus specifications)
- minimum volume and weight
- ease maintainability
- affordability: cost target less or equal than the cost of a fan system

The project was structured into individual work packages (WPs) as follows:

WP 1000 : System specifications, comparison of existing cooling, system mock up definition, test file definition
The fill charge ratio has a significant effect if the void fraction in the evaporator core varies, leading to a radial heat leak variation. The radial heat leak, as well as the ratio of radial to axial heat leak, is affected by the wick characteristics, and the evaporator and compensation chamber designs. The pore size is an important parameter, which should be as low as possible to increase the capillary and boiling limits. The fluid selection mainly depends on its saturation pressure, which should be sufficiently high at the considered operating temperature. Thus, ammonia and propylene are used for low temperature applications; water, alcohols, acetone and R134a may be used for higher temperature applications. In addition, the compatibility of the fluid with the loop materials should be carefully considered. The gravity effect is important for terrestrial applications: an adverse elevation or tilt decreases the LHP performance, especially at low heat loads. Likewise, a fluid pressure drop increase tends to decrease the performance. The temperature difference between the ambient and the heat sink affects the transition heat load between variable and fixed conductance modes of the LHP operation.

WP 2000: Loop heat pipes studies
The scope of WE2000 was to design a LHP adapted to the specifications defined in WP1000. For the simulation of LHP behaviour, the model should include following parameters:
- capillary structure parameter (porosity, permeability, etc )
- fluid type and characteristics
- thermal operating conditions (heat flux level, cooling fluid temperature, elevation).
When using a low conductivity capillary structure, the LHP performance is sensible to the latent heat of vaporisation, the liquid specific heat and the evaporator thermal resistance RE (which includes container / wick mechanical contact and fluid / wick wettability), particularly when the LHP operates at variable conductance mode. When operating at fixed conductance mode, the LHP performance mainly depends on the heat transfer resistance between the working fluid and the heat sink. The LHP is more sensitive to elevation or acceleration forces when using a high conductivity capillary structure rather than a plastic mesh. A composite wick enables to decrease the operating temperature at low heat inputs and reduces the sensitivity of the LHP performance to the elevation. The simulations performed with the geometry of the LHP manufactured and delivered by ITP to IKE lead to the following conclusions:
- the increase of the liquid line length results in an increase of the LHP operating temperature. This increase is about 6 K when the liquid line length is increased by 50 %.
- FC-72 or R-245fa are probably not good candidates as working fluids (as compared to water), in that sense that they lead to a low or very low capillary limit (about 90 W and 20 W, respectively) with respect to the specifications, especially under vertical orientation or when subjected to acceleration forces.
A model was developed to simulate the loop heat pipe designed and manufactured by EHP. Simulations were performed to compare the performance of the ITP and EHP loop heat pipes. The ITP performance seems to be better due to the enhanced heat exchange from the reservoir to the ambient. However, this result has to be confirmed after identification of the thermal resistances RE and Rwall of the EHP loop heat pipe, using experimental data.

The principal possibility of the LHPs making in conformity with the specification was demonstrated. It is necessary to proceed efforts aimed on the further device optimisation and search of the methods that will allow solving the problem of the water freezing in the LHPs.

WP 3000 : System integration designs
WP 3000 objective is to design and optimise the interfaces of the equipment and connecting structure (seat and aircraft) in order to minimise the thermal resistance and take full benefit of the new cooling system. This simulation tools based on existing software: Flowtherm from Flomerics and PCB thermal from Pacific Numerix will be used to design improved conductive packaging options. The conclusions of this preliminary simulation were:
- In order to be efficient, the loop heat pipe must be associated with one or more internal standard heat pipe for the cooling of the SEB.
- The LHP evaporator can be reported on the SEB cover: this solution is very efficient for the cooling of discrete components but cannot be applied if there is more than two cards in the SEB or if there are too many components to cool.
- The best solution is to report the LHP evaporator on the side of the SEB near the slides. This solution allows using the thermal conductivity of the PCB for the cooling of all the components located on the card. Local solutions with heat pipes link to the slide side permit to have an efficient cooling even for large dissipating components.

WP 4000 : System mock up development
Two types of prototypes have been realised and tested. The manufacturing of these first versions LHP's allows tuning some manufacturing processes mainly relative to the evaporator primary parts and sub-assy. Based on these tests results, the solution with R245fa was kept as nominal configuration for COSEE mini-LHP's. The solution with water needs still some improvements and is considered as back-up.

The objective of the second version LHP was to modify the SS-R245fa LHP condenser design so that it can be mounted on the seat interfaces. The condenser new design was based on the geometry of the seat I/F. The seat model that has been selected for mounting the EHP LHP's is the RECARO one. Two SS-R245fa LHP's have been manufactured; one LHP presents the condenser plate on the right side and the other presents the condenser plate on the left side. Only evaporator in vertical position has been tested to simulate the mounting configuration in the SEB (mounted on the lateral walls). The LHP's have been tested with a tilt of 500 mm between evaporator and condenser.

The thermal conductivity of the two seats was too weak for the application targeted: for Avio seats it comes from the small cross section of the beam. It cannot be increased as a matter of weight. For Recaro seat the difficulty lies in the thermal conductivity of the carbon beam (1 W/mK in the vertical direction) and too small in the horizontal plane (50 W/mK). A simple solution for Recaro and Avio seats would be the use of a copper heat pipe fixed on the main structure aluminium beam or carbon beam. Additional heat pipes would reduced the LHP condenser temperature the main problem is to integrate the copper heat pipe in the beams. Painting on the hottest areas of the Avio seats allows some additional cooling.

WP 5000 : Performance evaluation
The seats equipped with representative electronic equipments have been submitted to the test programme that has been previously defined in the project. Each LHP condenser was fixed to a seat beam face. The SEB was placed on the back of the seat and each LHP evaporator was mounted on the SEB lateral face. The tests have been done considering the evaporator above the condenser with a tilt of 500 mm. The ambient temperature was of 20 degrees Celsius. Only radiation and convection have been considered. The improvement of the SEB cooling efficiency was limited by the beam thermal conductivity. However, the LHP efficiency was good: 5 degrees Celsius of thermal gradient whereas the tilt between evaporator and condenser was of 500 mm.

All the objectives of the COSEE project have been realised. The final testing has shown that the two developed technologies are able to divide by a factor of two the temperature elevation on the critical components or to multiply by two the power dissipated. The heat transportation distance between the box and the structure initially 500 mm has been increased to 700 mm with a good flexibility of the tubes. Software and simulation tools associated with experimental measurement techniques have provided good design optimizations. The quality and excellent cooperation within the consortium has permitted to successfully terminate the project.

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