In this study, shapes of the molten region and transport processes affected by thermocapillary convection in melting or welding pool irradiated by a low-power-density beam are determined from a scale analysis for the first time. A low-power-density-beam heating implies no deep and narrow cavity or keyhole taking place in the pool. A quantitative determination of the fusion zone shape is crucial due to its close relationship with the strength, microstructure, and mechanical properties of the fusion zone. In this work, the complicated flow pattern in the pool is influenced by an unknown shape of solid-liquid interface, and interactions between the free surface layer, corner regions, and boundary layer with phase transition on the solid-liquid interface. Since Prandtl number is much less than unity while Marangoni and Reynolds number can be more than in melting metals, an appropriate scaling mass, momentum, and energy transport subject to a force balance between viscous stress and surface tension gradient on the free surface account for distinct thermal and viscous boundary layers in these regions of different length, velocity, and temperature scales. The results find that shapes of the fusion zone, free surface velocity and temperature profiles are determined by Marangoni, Prandtl, beam power, Peclet, and Biot numbers, and solid-to-liquid thermal conductivity ratio. The predications agree with numerical computations.

The study presented in this dissertation concerns the evaporation, in normal ambient conditions, of sessile droplets (pinned and freely receding) of various HFE liquids (instead of the widely used water), which are considered so far as environmentally friendly and are often used as heat-transfer fluids in thermal management applications. They are pure perfectly-wetting and volatile liquids with low thermal conductivity and high vapor density. These properties affect in their own way many aspects concerning droplet evaporation such as the evaporation-induced contact angles, evaporation rate of a droplet, contact line pinning and Marangoni flow, all of which are treated in the present dissertation.<p>In general, the thesis starts with a general introduction including but not limited to sessile droplets (Chapter 1). In Chapter 2 we provide a general overview of capillarity-related concepts. Then, in Chapter 3 we present the interferometric setup, along with the liquids and the substrate that is used in the experiments, and also explain the reasons why this particular method is chosen. In Chapter 4 we address, among others, the issue of evaporation-induced contact angles under complete wetting conditions. The behavior of the global evaporation rate is also examined here, whereas in Chapter 5 we discuss the influence of thermocapillary stresses on the shape of strongly evaporating droplets. Finally, before concluding in Chapter 7, we address in Chapter 6 the still open question of the influence of non-equilibrium effects, such as evaporation, on the contact-line pinning at a sharp edge, a phenomenon usually described in the framework of equilibrium thermodynamics. The experimental results obtained are also compared with the predictions of existing theoretical models giving rise to interesting conclusions and promising perspectives for future research.<p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

This thesis focuses on investigating the stability, dynamics and physical mechanisms of thermocapillary flows undergoing phase change by means of direct numerical simulations and experiments. The novelty of the general approach developed in this work lies in the fact that the problems under consideration are addressed with novel fully-coupled transient two-phase flow models in 3D. Traditional simplifications are avoided by accounting for deformable interfaces and by addressing advection-diffusion mechanisms not only in the liquid but also in the gas. This strategy enables a realistic investigation of the interface energy and mass transfer at a local scale for the first time. Thorough validations of the models against theory and experiments are presented. The thesis encompasses three situations in detail: liquid layers in saturated environments, liquid layers in unsaturated environments and evaporation of liquid droplets. Firstly, a model grounded in the volume-of-fluid method is developed to study the stability of laterally-heated liquid layers under saturated environments. In this configuration, the planar layer is naturally vulnerable to the formation of an oscillatory regime characterized by a myriad of thermal wave-like patterns propagating along the gas-liquid interface, i.e. hydrothermal waves. The nonlinear growth of the instabilities is discussed extensively along with the final bulk flow for both the liquid and gas phases. Previously unknown interface deformations, i.e. physical waves, induced by, and enslaved to, the hydrothermal waves are reported. The mechanism of heat transfer across the interface is found to contradict previous single-phase studies since the travelling nature of the hydrothermal waves leads to maximum heat fluxes not at the points of extreme temperatures but somewhere in between. The model for saturated environments is extended in a second stage to assess the effect of phase change in the hydrothermal waves for the first time. New numerical results reveal that evaporation affects the thermocapillary instabilities in two ways: the latent energy required during the process tends to inhibit the hydrothermal waves while the accompanying level reduction enhances the physical waves by minimizing the role of gravity. Interestingly, the hydrothermal-wave-induced convective patterns in the gas decouple the interface vapour concentration with that in the bulk of the gas leading to the formation of high (low) concentrations of vapour at a certain distance above interface cold (hot) spots. At the interface the behavior is the opposite. The phase-change mechanism for stable layers is also discussed. The Marangoni effect plays a major role in the vapour distribution and local evaporation flux and can lead to the inversion of phase-change process, i.e. the thermocapillary flow can result into local condensation in an otherwise evaporating liquid layer. The third problem discussed in this thesis concerns with the analysis of evaporating sessile droplets by means of both experiments and 3D numerical modeling. An experimental apparatus is designed to study the evaporation process of water droplets on superheated substrates in controlled nitrogen environments. The droplets are simultaneously recorded with a CCD camera from the side and with an infrared camera from top. It is found that the contact line initially remains pinned for at least 70% of the time, period after which its behaviour changes to that of the stick-slip mode and the drop dries undergoing contact line jumps. For lower temperatures an intermediate stage has been observed wherein the drop evaporates according to a combined mode. The experimental work is complemented with numerical simulations. A new model implementing the diffuse-interface method has been developed to solve the more complex problems of this configuration, especially those associated with the intricate contact-line dynamics. Further insights into the two-phase flow dynamics have been provided as well as into the initial transient stage, in which the Marangoni effect has been found to play a major role in the droplet heating. For the first time, a fully-coupled two-phase direct numerical simulations of sessile drops with a moving contact line has been performed. The last part of this work has been devoted to the investigation of three-dimensional phenomena on drops with irregular contact area. Non-sphericity leads to complex three-dimensional drop shapes with intricate contract angle distributions along the triple line. The evaporation rate is found to be affected by 3D features as well as the bulk flow, which become completely non-axisymmetric. To the best of our knowledge, this work is the first time that three-dimensional two-phase direct numerical simulations of evaporating sessile drops have been undertaken.

It is a well-known phenomenon that during Marangoni condensation of binary mixtures, a small concentration of more volatile constituent with smaller surface tension gives significant heat transfer enhancements. This is due to surface tension gradients causing instability in condensate film, resulting in a pseudo-dropwise mode of condensation which resembles closely to dropwise condensation of pure fluid on the hydrophobic surface, consequently, the film gets thinner with lower thermal resistance across the condensate film and thus higher heat transfer coefficient is achieved. Marangoni condensation of steam-ethanol mixtures has been widely investigated in the past. However, Marangoni condensation of self-rewetting fluids e.g. steam-butanol is yet to be investigated where the constituent in a small concentration is a less volatile component. Marangoni condensation of steam-ethanol, steam-butanol and steam-propanol mixtures has been investigated on a horizontal smooth tube at an atmospheric pressure. For all experiments, concentrations by mass in the boiler feed when cold prior to start of the experiment were 0.001%, 0.005%, 0.01%, 0.025%, 0.05%, 0.1%, 0.5% and 1.0%. The coolant temperature rise was measured accurately with a ten-junction thermopile. Tube wall temperature was measured using four thermocouples embedded in the test tube wall. Effects of pressure and vapour velocity over a wide range of vapour-to-surface temperature difference have been investigated. Care was taken to avoid error due to the presence of air in the vapour. Marangoni condensation of steam-butanol and steam propanol mixtures show significant heat transfer enhancements compared with that of steam-ethanol mixtures. Higher Heat flux and heat-transfer coefficients were observed. For the steam-ethanol mixtures, enhancement ratio (heat flux or heat-transfer coefficient divided by the corresponding value for pure steam condensation on a horizontal smooth tube for the same vapour-to-surface temperature difference and vapour velocity) of 5.5 was found at an ethanol concentration of 0.01%. For steam-butanol mixtures, the maximum enhancement ratio was found to be 11 at a concentration of 0.005% and 0.01%. For steam-propanol mixtures, the maximum enhancement ratio of 8.5 was found at the same mass concentrations as steam-butanol mixtures. Enhancement ratio was generally higher at lower ethanol concentrations, increases at first with increasing vapour-to-surface temperature difference and subsequently decreases at high vapour-to-surface temperature difference. Finally, a semi-empirical model was proposed to predict the Marangoni condensation of steam-ethanol mixtures based on the vapour phase diffusion theory of Sparrow and Marchall (1969) and pure steam dropwise theory of Rose (2002).

Numerical simulations of combined buoyant and surface tension driven flow, also known as Rayleigh-Bénard-Marangoni (RBM) convection are conducted for heated fluid layers of small aspect ratio (defined as the ratio of the horizontal extent of the domain divided by the depth of the fluid) in square cross-section containers. A particular non-dimensionalization of the governing equations is developed in which the aspect ratio of the domain appears as a continuous parameter. The simulations extend and enhance existing experimental studies of the RBM convection phenomenon by mapping continuous solution branches in aspect ratio and Marangoni number parameter space. Key implementation aspects of the development of the adaptive mesh refinement (AMR) library libMesh are discussed, and a series of simulations of the RBM problem with a stick-slip boundary condition demonstrate the suitability of AMR for computing these flows. / text

A half-zone experimental set-up was used to study the effects of various liquid bridge and helium flow parameters on the onset of thermocapillary convection in silicone oil liquid bridges. Experiments confirmed that helium flow has a stabilizing effect, with the effect increasing with helium velocity. Furthermore, helium flow in the same direction as surface flow due to Marangoni convection had a more stabilizing effect than countercurrent flow. It was established that increasing helium temperature has a mixed effect, producing a less stable bridge at low helium flow rates, but a more stable flow pattern at higher helium flow rates. Finally, it was confirmed that decreasing the cold disk temperature results in a decrease in critical temperature difference.

A half-zone experimental set-up was used to study the effects of various liquid bridge and helium flow parameters on the onset of thermocapillary convection in silicone oil liquid bridges. Experiments confirmed that helium flow has a stabilizing effect, with the effect increasing with helium velocity. Furthermore, helium flow in the same direction as surface flow due to Marangoni convection had a more stabilizing effect than countercurrent flow. It was established that increasing helium temperature has a mixed effect, producing a less stable bridge at low helium flow rates, but a more stable flow pattern at higher helium flow rates. Finally, it was confirmed that decreasing the cold disk temperature results in a decrease in critical temperature difference.

The stability of evaporating liquids is examined. The geometries investigated are semi-infinite liquid sheets, bounded liquid sheets, sessile droplets, and funnels. Stability parameters are generated to characterize the stability of evaporating semi-infinite liquid sheets, and bounded liquid sheets. The derivation is made possible by introducing evaporation as the specific heat transfer mechanism at the interface, and using the statistical rate theory expression for evaporation flux so there are no fitting parameters. It is demonstrated that a single parameter can be used to predict the onset criterion instead of two parameters. A linear stability analysis is performed for spherical sessile droplets evaporating on substrates constructed of either insulating or conducting materials. A stability parameter is generated to characterize the stability of sessile droplets evaporating on insulating substrates and conducting substrates. The results indicate that spherical sessile droplets evaporating on insulating substrates are predicted to transition to Marangoni convection. Since there are currently no experimental results to compare the theory with, another analysis is performed for liquids evaporating from funnels, which can be compared with existing experimental observations. A linear stability analysis predicts stable evaporation for funnels constructed of insulating materials, in contrast to the sessile droplet case, and generates a new stability parameter for funnels constructed of conducting materials. The stability parameter is free of fitting variables since the statistical rate theory expression for the evaporation flux is used. The theoretical predictions are found to be consistent with experimental observations for water evaporating from a funnel constructed of poly(methyl methacrylate) (PMMA) and for water and heavy water evaporating from a funnel constructed of stainless steel. A parametric analysis is performed on the new stability parameter for liquids evaporating from funnels constructed of conducting materials, indicating that smaller interfacial temperature discontinuities, higher evaporation rates, and smaller radii correspond to less stable systems. It is also illustrated that calculations using statistical rate theory predict an instability, which is consistent with experimental observations, whereas using the Hertz-Knudsen theory does not predict any instability.

The stability of evaporating liquids is examined. The geometries investigated are semi-infinite liquid sheets, bounded liquid sheets, sessile droplets, and funnels. Stability parameters are generated to characterize the stability of evaporating semi-infinite liquid sheets, and bounded liquid sheets. The derivation is made possible by introducing evaporation as the specific heat transfer mechanism at the interface, and using the statistical rate theory expression for evaporation flux so there are no fitting parameters. It is demonstrated that a single parameter can be used to predict the onset criterion instead of two parameters. A linear stability analysis is performed for spherical sessile droplets evaporating on substrates constructed of either insulating or conducting materials. A stability parameter is generated to characterize the stability of sessile droplets evaporating on insulating substrates and conducting substrates. The results indicate that spherical sessile droplets evaporating on insulating substrates are predicted to transition to Marangoni convection. Since there are currently no experimental results to compare the theory with, another analysis is performed for liquids evaporating from funnels, which can be compared with existing experimental observations. A linear stability analysis predicts stable evaporation for funnels constructed of insulating materials, in contrast to the sessile droplet case, and generates a new stability parameter for funnels constructed of conducting materials. The stability parameter is free of fitting variables since the statistical rate theory expression for the evaporation flux is used. The theoretical predictions are found to be consistent with experimental observations for water evaporating from a funnel constructed of poly(methyl methacrylate) (PMMA) and for water and heavy water evaporating from a funnel constructed of stainless steel. A parametric analysis is performed on the new stability parameter for liquids evaporating from funnels constructed of conducting materials, indicating that smaller interfacial temperature discontinuities, higher evaporation rates, and smaller radii correspond to less stable systems. It is also illustrated that calculations using statistical rate theory predict an instability, which is consistent with experimental observations, whereas using the Hertz-Knudsen theory does not predict any instability.

Thesis (M. Sc.)--University of Alberta, 2010. / Title from pdf file main screen (viewed on July 14, 2010). A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Master of Science, Department of Mechanical Engineering, University of Alberta. Includes bibliographical references.