Design and optimization of microchannel ceramic heat exchanger for electric and hybrid aircraft using multiscale approach

Kurzfassung

One of the key challenges for the future of aviation is the development of electric and hybrid aircraft that result in low to zero CO2 and NOX emissions without compromising payload capacity and range. Conventional commercial aviation relies on refined carbon-based fossil fuels to power turbofan engines, which are responsible for 13.9 % of the greenhouse gases produced by the transportation sector [17]. The use of electric propulsion systems powered by zero-carbon or sustainable aviation fuels can help achieve the goal of carbon-neutral commercial aviation by 2050. Current solutions of high-power batteries and fuel cell systems are too heavy and their system-level power densities are not high enough for a regional to long-range aircraft application. Among different types of fuel cells, Solid Oxide Fuel Cell (SOFC) offers high efficiency and high system-level power density if its waste heat is recovered within its system boundary [39][82]. This requires lightweight and compact heat exchangers with minimal pressure drop and high heat transfer rate. The thesis presents the design and heat transfer evaluation of a compact heat exchanger made of ceramic microchannels for waste heat recovery of a SOFC system. Multiple numerical simulations are performed using a multi-scale approach for different channel sizes and counterflow gas-to-gas configurations. In addition, a porous media approach is introduced and applied to calculate the heat transfer on a full-scale heat exchanger. Several calculations are performed to validate the computational fluid dynamics (CFD) methodology, mesh independence, and boundary condition sensitivity. The square duct delivered the lowest pressure drop and a heat transfer value of 12 MW. The material selection of silicon carbide, which has a low density and high thermal conductivity compared to other alternatives, is implemented in a porous media approach and uses a full-scale heat exchanger. This resulted in a high volumetric power density of 22.45 MW/m³ and a gravimetric power density of 46.74 kW/kg. This demonstrates a more than 4.5× increase in gravimetric power density compared to an offset strip fin-plate heat exchanger. This multi-scale approach enables analysis on large-scale models with low computational cost compared to conventional CFD simulations, enabling high-fidelity CFD studies for microchannel heat exchangers.