Structural assessment and optimization of an offset strip plate - fin heat exchanger for a fuel-cell powered electric aircraft

Abstract

The aviation industry accounts for nearly 3% of all greenhouse gas emissions. Hydrogen fuel cell powered aircraft offer a promising solution to mitigate the effects of climate change, as their emissions are limited to water vapour. This advancement would enable the aviation industry significantly reduce its CO2 emissions by 2050. Among various fuel cell designs, the Proton Exchange Membrane (PEM) fuel cell is the most promising, due to its high technological readiness level and low operating temperature (60oC - 100oC). At the megawatt power scale, typical of aircraft operations, the fuel cell operates at an electrical efficiency of 40% to 50%, resulting in the release of a large quantity of waste heat. The low temperature difference between the fuel cell and the ambient atmosphere introduces a significant challenge for its thermal management. The size and weight limitations associated with conventional heat exchangers is addressed by a compact design, such as an offset strip plate fin heat exchanger. However, other challenges are introduced. Firstly, the flow of air through small, compact channels induces a large pressure penalty. Secondly, repetitive thermal and pressure stresses, particularly during take-off and cruise conditions, would result in structural failure by either fracture or fatigue. Thirdly, both the individual fins and the heat exchanger structure are at a risk of resonance with external dynamic loads, such as flow-induced vibration and the motion of rotating bodies in the vicinity of the thermal management system. These factors highlight the necessity to examine the robustness of offset plate fin geometries for aircraft applications through coupled fluid, structural and modal analyses, and to propose design improvements in the event of failure. In this regard, a critical analysis of standardized offset fin geometry was conducted for a heat exchanger of 1 MW heat duty, using a representative unit cell. The temperature and pressure loads for both takeoff and cruise conditions were derived from validated CFD analyses. As the heat exchanger is situated in a duct, the thermal expansion is constrained. The effects of various constraints on the stress distribution were explored. The structural analysis of the unit cell highlighted the importance of fins on the coolant side, although their presence was deemed unnecessary from an initial thermo-fluid standpoint. The structural influence of three coolant fin configurations was explored, and the best configuration was selected. The optimal design significantly enhanced the gravimetric and volumetric power densities, thermal performance and structural robustness of the heat exchanger. In contrast to the initial assumption, stresses in take-off and cruise conditions were observed to be similar, indicating a greater risk of fatigue failure during stress fluctuations between the operational and idling conditions of the aircraft. Fatigue life was deduced for various constraints. Modal analysis was conducted to evaluate the natural frequencies of the pre-stressed fin geometry and the homogenized heat exchanger. These frequencies were compared against the characteristic frequencies of flow induced vibration and propeller motion, and analyzed for resonance. The results define optimal constraint and fin configurations for heat exchanger integration in hydrogen fuel-cell aircraft. This work presents a framework for enhancing the structural, thermal and fluid performance of compact heat exchangers, supporting the development of reliable, lightweight solutions for zero-emission commercial aviation