Periodic Reporting for period 2 - SACOC (Aerodynamic upgrade of Surface Air Cooled Oil Cooler (SACOC))
Période du rapport: 2020-04-01 au 2021-09-30
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%.
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%.
A numerical methodology has been implemented to calculate the aerothermal performance of SACOC layouts. The numerical scheme is based on a RANS k-ω SST turbulence model for the fluid dynamics linked to a Conjugate Heat Transfer (CHT) approach to assess the thermodynamics. Both commercial and in-house CFD codes have been used for the numerical study. The commercial CFD software has been used assuming a symmetric study domain, while the in-house made model was designed with a partitioned approach between fluid and solid, using a coupling algorithm. Studies on a periodic condition of a repetitive fin were conducted with this latter approach. Results show that the periodic approach cannot fully predict the heat exchanger behaviour, confirming the need to implement a symmetric model of the domain, which showed better match against the experimental data and highlighted the behaviour of the most external fins, including the appearance of vortices.
An optimization algorithm based on the adjoint method has been designed and implemented. The reliability of such algorithm has been evaluated through the fin geometry optimization of the reference SACOC for reducing the pressure drop induced to the airflow. These results show that with an optimized geometry the pressure loss can be reduced while augmenting the heat transfer against the reference layout.
Two experimental facilities have been designed and assembled to aero-thermally characterize the SACOC prototypes. In the first facility, real size layouts were tested. With this purpose, a bespoke wind tunnel capable of reaching air speeds up to Mach 0.5 has been assembled at UPV. The second facility was designed to perform high-spatial resolution measurements. With this aim, the Linear Experimental Aerothermal Facility (LEAF) at Purdue University was adapted to test scaled-up SACOC prototypes, up to 40% larger than the actual size, so that static pressure profile at convenient zones of the base, the front and between fins could be measured.
Both facilities include settling chambers to control the development of the boundary layer of the air flow and test sections with side-wall optical accesses allowing a clear view of the test model, as well as good transmission wavelengths fully compatible with laser-based optical techniques. Using an in-house semi-analytical methodology, UPV implemented and validated a 3D-printed distortion screen to replicate the total pressure profile of the air upstream the SACOC in a real turbofan engine.
The characterization of different models carried out in the project gave rise to a SACOC geometry which, compared to the current solution, improves the heat exchange capacity by almost 20% and its permeability to air flow by 13%.
The results of SACOC project have been disseminated to the public and the scientific community through two press releases and interviews, one communication at the 2021 AIAA Aviation Forum and Exposition, one article in a scientific journal and two seminars aimed at undergraduate and postgraduate students. In addition, the thematic focus of the project has led to the realisation of two MSc theses defended in 2021 and one PhD thesis to be defended in 2023. Further publication of two additional scientific articles is expected in 2022.
An optimization algorithm based on the adjoint method has been designed and implemented. The reliability of such algorithm has been evaluated through the fin geometry optimization of the reference SACOC for reducing the pressure drop induced to the airflow. These results show that with an optimized geometry the pressure loss can be reduced while augmenting the heat transfer against the reference layout.
Two experimental facilities have been designed and assembled to aero-thermally characterize the SACOC prototypes. In the first facility, real size layouts were tested. With this purpose, a bespoke wind tunnel capable of reaching air speeds up to Mach 0.5 has been assembled at UPV. The second facility was designed to perform high-spatial resolution measurements. With this aim, the Linear Experimental Aerothermal Facility (LEAF) at Purdue University was adapted to test scaled-up SACOC prototypes, up to 40% larger than the actual size, so that static pressure profile at convenient zones of the base, the front and between fins could be measured.
Both facilities include settling chambers to control the development of the boundary layer of the air flow and test sections with side-wall optical accesses allowing a clear view of the test model, as well as good transmission wavelengths fully compatible with laser-based optical techniques. Using an in-house semi-analytical methodology, UPV implemented and validated a 3D-printed distortion screen to replicate the total pressure profile of the air upstream the SACOC in a real turbofan engine.
The characterization of different models carried out in the project gave rise to a SACOC geometry which, compared to the current solution, improves the heat exchange capacity by almost 20% and its permeability to air flow by 13%.
The results of SACOC project have been disseminated to the public and the scientific community through two press releases and interviews, one communication at the 2021 AIAA Aviation Forum and Exposition, one article in a scientific journal and two seminars aimed at undergraduate and postgraduate students. In addition, the thematic focus of the project has led to the realisation of two MSc theses defended in 2021 and one PhD thesis to be defended in 2023. Further publication of two additional scientific articles is expected in 2022.
The numerical models developed in SACOC have provided valuable information of the heat exchanger behaviour, in terms of aerodynamics and thermodynamics, and the experimental data collected during the project has ensured that the models achieved great fidelity when compared to the measurements. Innovative geometries, resulting from the optimization processes aiming to minimise the pressure drop induced by the SACOC in the bypass flow while maximizing the heat transfer have been designed, commissioned, and validated. The experimental validation of the numerical calculations, which predicted the better performance of these heat exchanger geometries, confirmed that the CFD methodology developed in the project will be very useful for future designs, reducing the need for resource-hungry full engine tests and accelerating the development cycle of innovative cooling solutions for next generation engines.
By helping to reduce the time-to-market of these new engine concepts, new numerical and reduced-scale design methodologies such as the ones obtained in SACOC will contribute to the achievement of higher efficiency aircraft engines and consequently, to a reduction in pollutant emissions. Among these, not only chemical pollution such as CO2 and NOx are mitigated; the higher permeability of the obtained SACOC geometries has proved to reduce the fluid-solid interaction and therefore the vibration levels of the heat exchanger fins, prolonging component life and also decreasing noise, whose reduction is also a major challenge for a clean and sustainable aviation.
By helping to reduce the time-to-market of these new engine concepts, new numerical and reduced-scale design methodologies such as the ones obtained in SACOC will contribute to the achievement of higher efficiency aircraft engines and consequently, to a reduction in pollutant emissions. Among these, not only chemical pollution such as CO2 and NOx are mitigated; the higher permeability of the obtained SACOC geometries has proved to reduce the fluid-solid interaction and therefore the vibration levels of the heat exchanger fins, prolonging component life and also decreasing noise, whose reduction is also a major challenge for a clean and sustainable aviation.