A spatially resolved method for measuring heat transfer coefficients of water-sheet sprays at sub-saturation wall temperatures

Spatially resolved heat transfer measurements in spray cooling are critical for the reliable design of high heat flux thermal management systems. This work presents a methodology for quantifying local heat transfer coefficients of water-sheet sprays at sub-saturation wall temperatures. The method reconstructs wetted-surface temperature and convective heat flux fields from backside thermal imaging of a uniformly Joule-heated multilayer stack, using a steady-state inverse conduction model validated against two heater-foil materials (Inconel 600 and Constantan) across multiple heat-flux levels. The method is first validated against round air-jet impingement over R e d = 9 . 35 × 1 0 3 – 6 . 81 × 1 0 4 and heat fluxes of 6.3–35 kW/m 2 , demonstrating invariance of the local Nusselt number across heat-flux levels and foil materials. The technique is then applied to a planar water-sheet spray generated by a microfluidic convergent nozzle (50° convergence angle, 150 μ m × 500 μ m exit), enabling quantification of non-axisymmetric heat-transfer footprints. Experiments at flow rates of 80–120 ml min −1 and heat fluxes of 210–405 kW m −2 show that spray morphology and droplet-size distribution govern the spatial structure and magnitude of the local heat-transfer coefficients. Stable sheets produce broad, uniform cooling regions, whereas increasing flow rate promotes sheet breakup and shifts cooling toward localized impingement zones driven by rim-breakup droplets. The findings establish a robust framework for model validation and for the development of advanced spray-cooling strategies in high-heat-flux thermal management systems.