On the Performance Potential of Ducted Fans With Integrated Heat Exchangers

Fuel-cell-powered aircraft propulsion is a key hydrogen-based technology that could enable zero in-flight carbon emissions. However, managing the dissipated heat of such systems poses significant design and operational challenges, but at the same time provides potential for additional thrust generation from the waste heat. Integrating ducted heat exchangers within the flowpath of an electrically-driven propulsor could present a promising solution, which this study refers to as the “Heat Propulsor.” A virtual propulsion system framework is deployed to develop a dedicated cycle design and 2D sizing model for the Heat Propulsor. The fan is sized at cruise while the heat exchanger at take-off, to accommodate the high heat load demands during this phase of operation. An analytical heat exchanger model is integrated within the Heat Propulsor simulation framework. A linear diffuser model is used to decelerate the flow before the heat exchanger inlet, with its length calculated to minimize the risk of flow separation. The analysis reveals a trade-off on the choice of Mach number at the heat exchanger inlet. While heat transfer is more efficient at lower Mach numbers, this leads to configurations with excessively long linear diffusers and increased duct losses. It is shown that designing the fan for lower fan-face Mach numbers and shifting part of the required diffusion to the intake duct can revitalize the integration of heat exchangers within the fan flowpath. The study also demonstrates that fuel-cell propulsion systems with high efficiency — and consequently low heat dissipation — render integrated ducted heat exchangers with linear diffusers an unsuitable solution.