Experimental Investigation of Heat Transfer during Evaporation in the Vicinity of Moving Three-Phase Contact Lines

by Sebastian Fischer

Institution: Technische Universit├Ąt Darmstadt
Department: Fachbereich MaschinenbauTechnische Thermodynamik
Degree: PhD
Year: 2015
Record ID: 1103809
Full text PDF: http://tuprints.ulb.tu-darmstadt.de/4396/


The subject of the present work is heat transfer close to moving three-phase contact lines. The term three-phase contact line designates the area in which the liquid/vapour- or liquid/gas-interface approaches a solid wall. Due to the small thermal resistance of the extremely thin liquid film high heat fluxes are reached within this area. These can have significant influence onto the overall heat transfer process within two-phase systems. Examples of such systems are pool and flow boiling, droplet evaporation during spray cooling applications or heat pipes used for high power electronics cooling. The more the interface approaches to the heated wall, the stronger the influence of intermolecular interactions between wall molecules and those at the liquid/vapour-interface onto the local phase equilibrium becomes. This results in a shift of the equilibrium to higher temperatures, so that local evaporation is entirely inhibited through intermolecular forces at a certain minimum liquid film thickness. A few molecule layers thin liquid film that cannot be evaporated remains on the apparently dry surface. Apart from the wall superheat, the direction of contact line movement and its velocity, as well as the system pressure influence the local heat transfer in the contact line area decisively. While there is some experimental work on the influence of contact line velocity and its movement direction, the influence of system pressure has remained uninvestigated up to date. Aim of this work is therefore a target oriented experimental investigation of the pressure and velocity influence on heat transfer in the proximity of moving three-phase contact lines. Core of the dedicated experiment setup is an infrared transparent heater element, which provides the possibility to measure the temperature fields at the heater/fluid interface with high spatial and temporal resolution using infrared thermography. The heater developed for this purpose consists of an infrared transparent substrate, onto which a two-layer composition of black Chromium Nitride and pure Chromium with an overall thickness of approximately 800 nm is applied through Physical Vapour Deposition. While the black Chromium Nitride layer enhances the surface emissivity and thereby increases the signal-to-noise-ratio of the IR thermography drastically, the pure Chromium is employed as resistance heater to achieve the wall superheat necessary for evaporation. As experiment fluid degassed FC-72 (n-perfluorohexane) is used. Within the experiment setup a single capillary slot with a width of 1.4 mm is created between the infrared transparent heater and a polished copper wall. Liquid rises between the walls of the slot due to capillary forces and forms a single extended meniscus. The system pressure is adjusted through the saturation state of the fluid by variation of the system temperature, while the movement of the meniscus is realized through a volume displacement within the system. Movement of the meniscus results in an advancing or a receding contact line situation at the…