This study investigates the transient thermo-hydrodynamic behaviour of millimetric water droplets impacting heated solid substrates under subcooled conditions. The effects of wall temperature, wall material, and impact velocity on droplet spreading, heat transfer, and cooling performance are examined using high-resolution CFD simulations, validated against in-house experimental measurements of transient temperature evolution. The results show that droplet spreading is highly affected by impact inertia, with higher velocities producing faster radial expansion and larger maximum spreading. In contrast, the thermal response is strongly influenced by substrate properties. Steel exhibits steeper temperature gradients and stronger localized cooling within the substrate, while aluminium, owing to its higher thermal diffusivity and effusivity, sustains higher total heat-transfer rates at the wall–liquid interface. Increasing wall temperature significantly enhances the absolute heat-transfer rate due to the larger thermal driving potential, although normalized temperature profiles indicate reduced relative cooling. The analysis highlights the distinct roles of hydrodynamic and thermal mechanisms: impact velocity governs the lateral distribution of cooling, whereas substrate properties control the depth-wise thermal response. These findings provide a comprehensive understanding of droplet-induced cooling from a substrate perspective and offer insights for optimizing material selection and operating conditions in spray cooling, surface quenching, and high-heat-flux thermal management applications.
Shankar et al. (2026) studied this question.