Abstract
Herein, the evolution of initial thermal plumes (TPs) from small heating plates in water was investigated using near-infrared (NIR) imaging. The imaging system comprised a telecentric uniform illumination system and an NIR camera. The system employed the temperature dependence of the absorbance of water at a wavelength of 1150 nm for the measurements. Two-dimensional absorbance images of a 10 mm thick water cell were captured. A 1 mm thick circular or square steel plate was placed at the bottom and heated via induction. The images revealed that the thickness of the thermal boundary layer, followed by the rise in the TP, increased linearly with time, and several patterns of the TP appeared depending on the heat-flux-based Rayleigh number, Ra*. For a small Ra*, a TP with a mushroom-shaped cap was formed. At the critical value of Ra*, the boundary-layer flow on the plate surface was destabilized, leading to a bifurcated TP and an umbrella-shaped cap above it. These results were validated by comparing them with those of numerical simulations. Furthermore, the relationships between the ascent speed of the TP, plate size, and heat flux were assessed quantitatively.
Published Version
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