In order to reduce the fuel consumption and CO2 emissions of future aircrafts new engines are being developed, such as UltraFan (Rolls Royce), UHBR (Airbus) or Open Rotor (Safran). Those engines aim to improve performance in two ways that complexify thermal management.
First, the dilution rate is increased. This requires using a bigger fan, which results in a lower downstream dynamic pressure in the region where air is extracted to cool down the engine core zone.
Second, the engine efficiency is improved, increasing the operating temperatures inside the combustion chamber. The heat generated by the combustion process is only partially used and the rest heats the engine components. The cooling of the engine core zone is necessary to avoid an excessive temperature rise of the engine which would cause structural damage to the engine carter and/or accessories located in this zone, impacting the safety of the aircraft.
Besides, new engines require in a repositioning of the equipment in the core zone. This repositioning, combined with additional accessories in this area (e.g. air system, engine monitoring sensor, etc.) increases the need for cooling the core zone. In addition, it is also necessary to ensure the integrity of the equipment located in the engine area by not exceeding the acceptable temperature limits of the materials. All these elements lead to a rethinking of the thermal management architecture, which should be as little intrusive as possible in this important area of the plane.
Currently, although approaches might differ per engine and manufacturer, the core zone is cooled either by using bleed air from the compressor, or by continuously extracting air from the secondary flow. This is done by using permanently open inlets or through actuated valves of the engines' ventilation system.
An innovative solution has been patented by Airbus which describes the design of a passive solution that enables the cooling to be adapted to the real needs of the engine core zone. The solution, based on heat pipes and on the use of shape memory alloys, will actuate the opening of an inlet (or flap) depending on the temperature of the core zone. It is expected that several inlets are radially positioned around the axis of the engine and nacelle. As it will probably be necessary to keep permanent openings to ensure minimum ventilation for safety reasons, a study will be carried out to determine the number of passive openings and the number of permanent openings needed.
The objective is to use the temperature of a hot structure in the engine core compartment to trigger the opening of the flap. For this purpose, a heat pipe, considered as a passive dissipation technology, is used to transport the heat from the structure's hot spot to a colder area where this heat is transmitted to a shape memory alloy. The latter, under the effect of the temperature gradient, will activate the opening mechanism and allow the cooler air to enter the area.
PALOMA aims to improve and optimise the patented concept design and to validate this new solution through appropriate tests and suitable modelling activities. This optimised solution will be passive, not requiring any active actuation either by using electric or pneumatic actuators.
