Bubble nucleation quantification for a single droplet impact onto a heated substrate

Spray cooling has numerous high-temperature industrial applications, including metallurgical processes and wall cooling during fuel injection. In terms of cooling effectiveness, spray systems is among the best solutions to obtain high heat transfer coefficients. However, the underlying heat transfer mechanisms remain poorly understood, and current predictive models offer limited reliability. The complexity of spray cooling arises from the interplay of multiple variables, such as fluid properties, surface characteristics, ambient conditions, and surface temperature. Although single droplet impact studies simplify the dynamics of spray behavior, they serve as a fundamental basis for understanding key phenomena such as wall rewetting and bubble nucleation. This work proposes the investigation of single droplet impacts on heated substrates using Total Internal Reflection (TIR), an optical technique that enables visualization of the solid-liquid interface. TIR facilitates the detection of interfacial singularities such as dry-out zones and nucleation sites where heat flux variations are most pronounced. To quantify the effective solid-liquid contact area during droplet impact in the nucleate boiling regime, a bubble detection algorithm is under development. A major challenge in this context is the presence of light reflections and interference patterns, which can mimic the appearance of bubbles due to their similar shapes and pixel intensities. To mitigate this issue, the algorithm incorporates segmentation criteria such as bubble size and circularity, which significantly improve detection accuracy, particularly during the early spreading stage, when a large number of small bubbles rapidly form at the interface. Further improvements, including the integration of additional segmentation parameters and a comprehensive sensitivity analysis, are being considered to enhance the algorithm’s robustness and reliability.

Work In Progress