An Experimental and Theoretical Investigation of Evaporating Meniscus Dynamics and Instabilities

Abstract

High power density systems utilising phase change heat transfer devices such as heat pipes can be susceptible to evaporation driven meniscus dynamics and instability.To better understand this, an study of evaporating meniscus dynamics and instability was needed. A mathematical model describing evaporating meniscus dynamics was developed in which meniscus height was correlated with superheat. Subsequent validation experiments confirmed the model was consistent with the general trends including the superheat to meniscus height relation.The study of evaporating meniscus instability was investigated using a one-sided model and a linear stability analysis. The analysis considered the effects of long range molecular forces, surface tension, vapour recoil, evaporation, thermocapillarity and viscous forces. The potential for instability was studied for three film geometries, for which the potential for instability was found to be spatially dependent for the curved cases, with perturbation growth rates increasing with superheat.An experimental study of channel based evaporating meniscus instability was performed for eight channel widths and three fluids: n-pentane, iso-octane and acetone.The meniscus height to superheat correlation was used to infer the superheat at which the meniscus destabilised. The experiments revealed two kinds of instability. The first was localised to a narrow range of superheats and unique to the alkanes, the second common to all three fluids and sustained for higher superheats. The second kind of instability was found to require larger superheats for decreasing channel widths.

Experimental Study on Two-Phase (Solid-Liquid) Flows of Ground Wheat Straw in Inclined Pipes

“…Many studies are available on the hydraulic transport of conventional solids, i.e., sand, glass beads, etc., through inclined pipelines (Al-Mutahar, 2006;de Vreede, 2018;Matousek et al, 2018;Rai, 1972;Shook et al, 1974;Vlasak et al, 2018;Vlasak et al, 2019b). The flow of solids-water mixtures through inclined pipes is more complex than in horizontal and vertical pipes because in inclined pipes the phases can separate and slip at the same time (Polanský, 2014). Pipe inclination affects the flow characteristics (i.e., pressure drop, concentration distribution across the pipe cross-section, velocity profile, the deposition limit velocity, etc.)…”