Design Assessment and Test of Compact Gyroid TPMS Heat Exchanger with Embedded Coolant Channels for LTPEM Fuel Cell Powered Regional Aircraft

Abstract

All-electric aircraft with fuel cell-powered propulsion are considered a key technology for enabling low-emission flights. However, the feasibility of fuel cell systems for regional flights is challenging because their current mass-specific power densities are lower than required. Heat exchangers (HEXs) are critical component which are necessary for the cooling of fuel cell stacks, power electronics and pre-conditioning of fuel. HEXs must be designed lightweight to improve the power density of the propulsion system and compact for easy integration and drag reduction. Among several existing types of HEXs, designs based on Triply Periodic Minimal Surface (TPMS) geometries are used for compact HEXs. This paper introduces a compact gyroid-pipe HEX, designed using TPMS gyroid surfaces and with integrated coolant channels. Three design variants of gyroid-pipe HEX with thickness of 20 mm, 30 mm and 40 mm are generated using TPMS unit cell aspect ratio (AR) of 1, 1.5 and 2 respectively. Thermal-hydraulic performance is calculated using Computational Fluid Dynamics (CFD) simulations on small periodic cells, denoted simply as AR1, AR1.5 and AR2 with their respective above-mentioned thicknesses. Results show that with increasing ARs, pressure losses reduce, improving the hydraulic performance but the thermal performance decreases. Prototypes with a frontal cross-section of 200 mm x 200 mm are developed using additive manufacturing with ARs of 1, 1.5 and 2. Experiments are conducted to measure the pressure drop across the air side of the HEX segments, denoted as AR1, AR1.5 and AR2. Eight additional combinations of AR1, AR1.5 and AR2 HEX segments are assembled to test the effect of compactness variation (increasing or decreasing compactness along the air side) on pressure drop measurements. The best result is achieved with a combination of AR2 and AR1.5 HEX segments (denoted AR2-AR1.5), where the AR2 segment with lower compactness is attached to the AR1.5 segment with higher compactness and facing the air inflow. This HEX configuration delivers a 3% reduction in pressure drop and pumping power while increasing the heat transfer surface area by a factor of 3.12 relative to the AR1 configuration, without altering the frontal area or mass flow rate. Hence, it is a potential candidate for future studies and thermal tests. Structural calculations are performed to estimate the stiffness and thermal conductivity matrices for the AR1, AR1.5 and AR2 segments. Using these characteristics, a stress assessment is performed on all the ARs and their eight additional combinations at different angles of inclination inside the air duct. The results reveal that, while AR1 and AR1.5 are reliable options individually, especially at higher inclinations, Combinations of all three ARs offer the most favorable structural performance across all angles of inclination. This makes them the strongest candidates for integration and structural testing in future studies. The methodologies used in this paper for the thermal, hydraulic and structural assessment of the TPMS gyroid pipe HEX will be validated using further experiments.