Spatially resolved heat transfer coefficients in water-sheet spray cooling at sub-saturation wall temperatures using infrared thermography
| Spray cooling is a high-performance thermal-management approach used when conventional single-phase convection becomes limited by available area or mass flow. However, obtaining spatially resolved measurements of local heat transfer coefficients remains challenging. This work presents an experimental method for quantifying spatially resolved local heat transfer coefficients during water-sheet spray cooling at sub-saturation wall temperatures. The approach combines uniform Joule heating of a 25 μm thick metallic foil with infrared thermography in a multilayer glass-adhesive-foil assembly. A conduction-based reconstruction procedure accounting for through-thickness thermal resistance, lateral heat spreading, backside convection, and radiation losses is used to determine the wetted-surface temperature and local convective heat flux, enabling direct mapping of the heat transfer coefficient over the full spray footprint. The measurement technique is first validated using a fully developed single round air jet over a wide range of Reynolds numbers from 9,000 to 60,000, and multiple imposed heat flux levels (0.76 – 2.5 kW/cm2). Measurements obtained with Constantan and Inconel 600 heater foils yield nearly identical Nusselt number distributions and axisymmetric stagnation-region peaks, demonstrating insensitivity to heater material and heat-flux level. The validated method is then applied to cooling produced by a convergent microfluidic nozzle generating a planar water sheet that atomizes prior to impact. The measurements are still independent on the heat flux level (7 – 54 kW/cm2) and reveal strongly non-uniform heat transfer footprints governed by water sheet spreading and breakup dynamics. By restricting operation to sub-saturation wall temperatures, the experiments isolate single-phase convective transport and provide benchmark datasets for spray-cooling models in the absence of boiling. The proposed technique provides a robust framework for high-resolution characterization of spray cooling performance and is broadly applicable to thermal management systems and microfluidic cooling architectures. Charles Touitou has been an MSc student in the Faculty of Aerospace Engineering since 2024 and currently works as an aerodynamics engineer at Elbit Systems.
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