Multifunctional Leaf-Vein Micro-Channel Spar Architecture for Cryogenic Hydrogen Thermal Management and Structural Load-Bearing in Long-Endurance UAVs

Abstract

Long-endurance hydrogen PEM fuel-cell UAVs must pre-heat cryogenic hydrogen from −150◦C to 70–85◦C at the fuel cell inlet while simultaneously dissipating kilowatts of motor and stack waste heat. Conventional approaches require separate dedicated subsystems, imposing mass penal ties that reduce endurance.

This paper presents a conceptual design for a leaf-vein micro-channel spar architecture in which the primary wing spars of the MARID UAV simultaneously carry structural bending and torsion loads and serve as cryogenic hydrogen thermal conduits. The architecture uses a paired bridging serpentine channel network: adjacent secondary channels are connected in series by mechanisms of 30 micro-channels each, traversed single-file (no parallel splitting) rather than branching independently to a return line. Two such chains (front and rear of the main spar) run per wing, each a 27.23m single-file path from root to tip and back, substantially reducing dedicated thermal-management hardware while routing cold hydrogen through thermally critical zones to suppress the vehicle’s infrared (IR) signature—a key requirement for covert ISR missions. (This pass covers the main wings only; V-tail chains are deferred to future work.)

A one-dimensional finite-segment thermal model, with an ambient-conductance path (internal convection, Ti wall, CFRP skin, an assumed low-observable coating layer, and external convection in series) and a Darcy-Weisbach pressure-drop calculation, shows that the extended path length yields strong passive pre-heating: with internal waste heat sources (motor, avionics, PEM cooling jacket) included, sea-level ambient heat exchange along the wing raises the outlet from −85.1◦C (internal sources alone) to 51.3◦C, and an electric trim heater of only 86W at sea level (3.3% of total electrical demand) closes the balance to 77.6◦C within the 70–85◦C PEM window. The flow regime is turbulent (Re ≈ 42,900), since only 4 parallel chains share the design flow across a comparatively small number of micro-channels; the resulting pressure drop, including minor losses at the 87 hairpin turns within the zigzag routing (≈145kPa per chain, with turn losses adding ≈39% on top of the friction-only baseline), is non-negligible but still yields a small total pump power (≈2.1W, ≈0.1% of electrical demand) given the low volumetric flow. A wing-exit temperature check—relevant to IR signature, since wall temperature there governs external skin emission—shows the hydrogen (and wing wall) closing most, but not all, of the gap to ambient by the time flow exits the wing structure, at every altitude tested, before any downstream trim heating occurs: a residual of a few degrees remains, attributable to the added thermal resistance of the assumed low-observable coating, itself included specifically because it also suppresses IR emission from the hotter internal components upstream.