The Role of Diffusion in an Integrated Fan-Heat Exchanger Configuration

Hydrogen-powered fuel cells are a promising pathway toward cleaner regional aviation. However, this solution comes with several concerns, such as heat dissipation. Integrating a ducted heat exchanger in the flowpath of a fan can turn this challenge into an opportunity, leveraging the “Meredith effect” to generate additional thrust. A critical aspect of this concept is the diffusion needed in the transition duct between the fan and the heat exchanger, as they operate at different Mach number ranges. The efficiency of the flow deceleration process has a direct impact on the overall cycle performance, posing key questions about the overall feasibility of the concept. To evaluate the cycle-level performance, a 1D cycle sizing model is deployed. Promising results are obtained with an inclined heat exchanger arrangement, where part of the diffusion is achieved using flow-turning guide vanes, upstream of the heat exchanger, to redirect the flow in the meridional plane. This setup mitigates heat exchanger diameter limitations, enhances diffusion potential, and enables lower Mach at the heat exchanger inlet. Based on the cycle-level results, a fan design is developed using a 2D throughflow analysis. The sensitivity of the fan stage and installed performance to different fan-face Mach is explored. Furthermore, typical fan design parameters are investigated showing the potentiality of redistributing the diffusion from the down stream duct to the fan stage itself. Lower fan-face Mach and increased diffusion introduce operability challenges. Therefore, a full performance map is generated and off-design conditions are assessed.