Keeping ultra-high bypass ratio turbofan engines cool
The aviation industry contributes to 2 to 3 % of total annual global CO2 emissions from human activities and impacts on climate from its non-CO2 emissions. “To reduce the environmental impact of commercial aviation, carbon dioxide, nitrogen oxides and noise must be reduced. To achieve this, a new generation of engines is required, and one of the most promising concepts is the ultra-high bypass ratio (UHBR) turbofan,” highlights Alberto Broatch, project coordinator of the EU-funded SACOC project. While known for generating fewer emissions and being quieter, this engine faces several challenges, one of which is the cooling system. As the front blades of this engine are so large, their optimum rotational speed differs greatly from the turbine that drives the fan. “As a 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,” explains Broatch. With this approach, the gearbox is subjected to a very high thermal load, and that heat needs to be evacuated with a proper refrigeration system.
Finding solutions
The project set out to investigate how this engine can stay cool. “One of our core goals was to build a numerical methodology able to predict the performance of surface air-cooled oil coolers that use air from the bypass duct of the engine to refrigerate the oil from the lubricant system,” explains Jorge García-Tíscar, one of the project’s task managers. As a digital model, it can serve as an optimisation tool for generating new geometries of heat exchangers capable of incrementing the cooling and reducing the impact of the geometries on the air side. The numerical models, however, need to be validated before they can be used to generate innovative designs. “This is where the second core goal of the project appears. Since full engine tests are extremely expensive and time consuming, a reduced-scale experimental test rig able to simulate the actual engine flow conditions around the cooler was required,” outlines Andrés Felgueroso, a member of the project’s consortium. This test rig was designed, commissioned and validated in the project.
A step closer to coolness
The project demonstrated how numerical models can be used not only to simulate this kind of turbofan heat exchanger but also to optimise them. “The numerical optimisation carried out in the project using these models gave rise to a surface air-cooled oil cooler geometry which, compared to the current solution, improves the heat exchange by almost 20 % and its permeability to airflow by 13 %,” confirms García-Tíscar. The project also demonstrated that the evaluation of surface air-cooled oil cooler designs can be carried out in a simplified small-scale wind tunnel, provided that appropriate steps are taken to simulate the actual conditions of the engine airflow. “This will greatly simplify future surface air-cooled oil cooler developments, reducing experimental costs, energy spending, and time to market,” adds García-Tíscar. Discussing the future, Felgueroso concludes: “We would like to further investigate the performance of the current surface air-cooled oil cooler designs and try new ones.” The consortium will continue to work on further understanding the aero-thermal behaviour of heat exchangers.
Keywords
SACOC, surface air-cooled oil cooler, heat exchangers, turbofan, ultra high bypass ratio, UHBR, aviation industry
