To reduce the environmental impact of commercial aviation, three main pollutants need to be reduced: carbon dioxide, nitrogen oxides, and noise. To fulfil these objectives, a new generation of engines is needed, and one of the most promising concepts is the ultra-high bypass ratio (UHBR) turbofan. This type of engine has much larger blades in the front fan than any other turbofan nowadays.
Nevertheless, they face several challenges. Among them is the fact that, since their front blades are so large, their optimum rotational speed differs too much from that of the turbine that drives the fan. As result, UHBR engines need a gearbox between the fan and the turbine, so the latter rotates slower than the former, and the tips of the blades avoid reaching noisy sonic conditions. One of the main issues with this approach however is that the gearbox is subjected to a very high thermal load. That heat needs to be evacuated with a proper refrigeration system.
The SACOC project, framed within the Clean Sky 2 programme, addresses this challenge by proposing innovative designs for a surface air-cooled oil-cooler (SACOC) capable of extracting the heat from the lubrication system by taking advantage of the airstream that goes through the turbofan bypass. The main goal is to be able to extract as much heat from the oil as necessary, while the aerodynamic interference is kept at minimum, in order to maintain the engine propulsive efficiency and thus, its low fuel consumption.
To this end, a numerical methodology based on high-fidelity computer fluid-dynamics (CFD) simulations to predict the thermo-aerodynamic characteristics of the heat exchanger was developed. By using this methodology, optimization methods have been used to improve the heat exchanger geometry maximizing the heat transfer and reducing the pressure drop as much as possible. Different facilities were then setup to experimentally study the performance of the SACOC through state-of-the-art techniques and to provide crucial validation data for the CFD simulations.
In conclusion, reliable numerical and experimental methodologies to characterize the aero-thermal features of surface air-cooled oil-coolers for UHBR engines were implemented and validated. The application of these methodologies has led to the design of an optimized heat exchanger which, compared to the current solution, improves the oil cooling by almost 20% and reduces the impact on the airflow by 13%.
