In all-electrical aircrafts, the corresponding electrical ram air fan should be optimised in order to improve its current design in two aspects:
- generate pressure drop at low flows without surge issues;
- cooling of the fan electrical motor at high inlet air temperatures.
The ECS control logics produces two main typical operating points, one with high flow when the ram-air fan creates enough flow to cool both the ECS main heat exchanger and the ECS motor stator, and the second when it only needs to provide a low airflow for the latter necessity. As the pressure drop is similar in both situations but the flow very different, this creates surge problems that should be eliminated for adequate operation.
Meanwhile, the fact that the ram-air fan is located downstream the ECS main heat exchanger implies that the air inlet arrives to the fan at high temperatures, what difficulties its own correct electrical motor cooling, both from a mechanical and electrical points of view.
Considering these objectives, a new fan concept was selected from available bibliographic and consortium expertise, and designed using the consortium skills in deep fundamental fluid mechanics and heat transfer knowledge (Technical University of Catalonia - UPC ), advanced CFD tools and aerodynamic know-how (Thermo Fluids - TF) and engineering capacities of a fan manufacturer (LMB SAS). The proposed design related to surge problem was implemented in a prototype and tested accordingly. In a similar manner, a solution for electrical motor cooling was found. The final solution also considered its impact on the whole ECS pack, in order to maintain its global performance objectives.
On an electrical ECS pack, outside air is used to remove heat from the cooled system. The outside air flow is generated on ground thanks to a vacuum device. The present subject dealt with electrical fan solution used as vacuum device for ram air flow generation. The challenge of optimising such a component for this application was double.
First, from a aerodynamic point of view, the fan was capable of generating a pressure drop whatever the flow without surge issues. Indeed, the ram air fan was used to suck main air flow through ECS pack main heat exchanger for cabin cooling and in the meantime to suck small amount of flow through ECS pack electrical motor stator for cooling. Due to ECS control logics, the fan was also capable to ensure ECS motor stator cooling without any flow from main heat exchanger. In this case it provided the same pressure rise with a very limited flow.
Then, the fan was installed downstream of the ECS main heat exchanger. Therefore, high temperature air can enter the fan and specific concept was implemented for fan integration to enable fan mechanical and electrical subpart cooling.
Finally, technology and solution selected to comply with these last constraints minimised impact on performance efficiency and component reliability and availability so as to achieve global ECS objectives.
Considering these objectives, a new fan concept had been selected from available bibliographic and consortium expertise, and designed using the consortium skills in deep fundamental fluid mechanics and heat transfer knowledge (Universitat Politècnica de Catalunya- UPC ), advanced CFD tools and aerodynamic know-how (Termo Fluids - TF) and engineering capacities of a fan manufacturer (LMB SAS).
After a careful selection of anti-surge technologies based on state of the art carried out within the first project period, some predesigns had been numerically tested and aerodynamically analysed (WP2). A final modeling of the ram-air fan and the consequent prototype was designed, constructed and successfully tested at both flow range conditions (WP3). The numerical results and the experimental data assured that final proposed solution avoid surge problem under both flow conditions initially expected.
In a similar manner, a thermal analysis of the selected electric motor coupled with the fan designed had been thermally analysed. The thermal design had been numerically modeled and experimentally tested under high temperature conditions in a climatic chamber. The fan thermal validation allowed to numerically check the temperature level at the actual conditions indicated by the Topic Manager in the initial project requirements.
In conclusion, an adequate efficient fan design had been obtained to cover at the same time, both high and low flow rate working conditions and high inlet air temperatures. Thus, LMB has now this new efficient fan design, TF had improved its in-house CFD numerical tool capable to predict the surge effect under numerical simulation of fans, while UPC developed a detailed thermal model capable to analyse thermal effects in electric motor driven fan configurations. Finally, LTS acquired the know-how and the technological readiness to drive the ram-air flow with a wide flow spectrum fan, thus covering the initial objective of cooling the ECS main heat exchanger or alternatively the ECS electrical motor stator.