Experiments to understand microlayer and dry patch dynamics under subcooled nucleate flow boiling in a vertically oriented rectangular channel

Abstract

Nucleate flow boiling offers high heat transfer rates and is considered an effective mode of heat transfer in many systems involving high heat loads. The phenomenon is characterized by the inception of vapor bubble(s) and its growth, followed by its departure in a periodic manner. The evolution of the nucleating bubble's footprint—microlayer and dry patch dynamics—is important in understanding the heat transfer capability and limiting heat flux values. However, efforts toward developing a fundamental understanding of this phenomenon during the nucleate flow boiling regime under subcooled bulk conditions are scarce in the open literature. Toward bridging this gap, we report flow boiling experiments on a hydrophilic surface for investigating the plausible influence of subcooling and minimize the influence of the hydrodynamic movement of contact lines on the dry patch dynamics. Experiments have been conducted in a vertically oriented rectangular channel with water as the working fluid for a Reynolds number of Re = 2400. Real-time microlayer dynamics have been mapped using thin-film interferometry, while the bubble evolution has been captured using one of the gradients-based imaging approaches employed from the side view. Experiments revealed a noticeable influence of subcooling on dry patch and microlayer dynamics. The size of the dry patch and the radial spread of the microlayer showed a decreasing trend with increasing subcooling level. Experimental conclusions are also supported with theoretical considerations.