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

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 surface of the IR transparent heater, that influences the local temperature distribution at the heater wall. The temperature distribution on the backside of the Chromium Nitride layer at a distance of less than 1 micrometer away from the heater/fluid-interface is measured with a high speed IR camera at a framerate of 1000 Hz and a resolution of 29.27 micrometer/pixel. The high speed IR camera is synchronized to a high speed black-and-white camera, that allows detection of the liquid/vapour-interface with a resolution of 4 micrometer/pixel. From the temperature fields the local heat flux distribution is calculated numerically with the same spatial and temporal resolution. Experiments were performed at reduced pressures in the range of p_R = 0.05 to p_R = 0.7 and with liquid/vapour-interface velocities of up to v_int = +-40 mm/s. Comparisons to earlier experiments on three-phase contact line heat transfer show, that the results obtained using thin foil heaters are transferable to heaters with substantial larger thermal inertia, like the IR transparent heaters used within this work. Merely the extremely high temperature differences, that are present in proximity of the contact line, are significantly smaller on walls with higher thermal inertia. The conducted experiments clearly show a local heat flux peak in proximity of the contact line, which is accompanied by a local temperature minimum. At increased system pressure and equal wall superheat and contact line velocity, the local heat flux peak at the contact line decreases with increasing pressure. This effect can be attributed to the reduction of latent heat of evaporation with increasing pressure. Considering the influence of the contact line velocity, one must distinguish between advancing and receding contact lines. At a receding contact line and equal reduced pressure and wall superheat, no influence of the contact line velocity onto the local heat flux distribution in proximity of the contact line is discernible. At an advancing contact line on the other hand, an increase of the heat flux maximum at the wall and thereby the heat transfer in proximity of the contact line with increasing contact line velocity is observed. Both the increase of the heat flux maximum with increasing contact line velocity at an advancing contact line and the independence of the maximum heat flux on the contact line velocity at a receding contact line is observed at low and high levels of the reduced pressure. In some experiments with negative meniscus velocity (and therefore receding contact line) it was observed, that a thin, evaporating liquid film can be deposited on the heater surface by the moving liquid/vapour-interface. If and to which extend the thin film is deposited, depends on the wall superheat, the velocity and acceleration of the liquid/vapour-interface, the latent heat of evaporation of the fluid and the wetting characteristics of the heated wall. Furthermore it was observed, that film rupture due to instabilities can occur. If the thin film was present, the contribution of thin film evaporation to the overall heat transfer was dominating compared to the contribution of contact line heat transfer. This makes it necessary to describe the thin film with a model, if it appears during an evaporation process in order to capture the underlying physics correctly. Based on an estimation of the thickness of a liquid film staying behind on a wall drawn out of a quiescent liquid by Landau and Levich, a model for stationary thin film evaporation was developed. It is assumed, that thin film evaporation has no influence onto the initial film thickness, so it can be calculated according to Landau and Levich. Starting from the mass and energy balance at an infinitesimal thin segment of the film, an equation for the film thickness gradient in direction parallel to the wall is derived. By making the equation dimensionless, non-dimensional quantities governing thin film evolution are identified. Comparison of the length of the thin film region calculated using the model to lengths of the thin film region observed in the experiment shows good agreement up to a certain velocity, at which an increasing deviation between theoretical value and experimental data is observed. This deviation is probably caused by the limited length of the IR transparent heater, which does not allow for reaching steady state thin film evaporation