Morphology-Controlled Sheet Spray Cooling from Convergent Micronozzles for Hypersonic Leading Edges

Thermal management for hypersonic leading edges must remove extreme heat fluxes within ultra-thin, aerodynamically constrained structures, where passive approaches are limited by available volume and surface-temperature margins. This study experimentally investigates convergent micro-nozzle sheet sprays as an active cooling concept for leading-edge integration. The goal is to combine minimal coolant thickness and consumption with strong near-wall renewal and mixing.
Liquid sheets are generated using lithographically fabricated micronozzles and characterized via high-speed shadowgraph imaging to quantify breakup and atomization pathways. The measurements show that increasing the flow rate and outlet confinement expand the sheet size and extend the stable-sheet region. In contrast, the convergent angle systematically reshapes the sheet by redistributing momentum at the nozzle exit. A scaling analysis predicts stable-sheet dimensions and is used to construct a Weber-number-convergent angle-regime map that classifies the observed sheet morphologies.
Downstream atomization is quantified using laser diffraction, which reveals single-peak and bimodal droplet-size distributions associated with specific regimes and highlights the strong influence of the outlet aspect ratio on mean droplet diameters. Steady-state infrared thermography provides spatially resolved heat-transfer coefficient maps, directly linking spray structure to cooling performance. For the conditions tested, more coherent, planar sprays provide higher area-averaged cooling than fully atomized sheets. However, targeted regime selection and local morphological manipulation can enhance peak heat transfer at the leading edge, where it is most needed. These results provide experimentally grounded design rules for micro-nozzle geometry and flow rate to tailor cooling footprints for hypersonic thermal protection.