Is CHF triggered by the vapor recoil effect?

This paper deals with the triggering mechanism of the boiling crisis, a transition from nucleate to film boiling. We observe the boiling crisis in pool saturated boiling experimentally at nearly critical pressure to take advantage of the slowness of the bubble growth and of the smallness of the Critical Heat Flux (CHF) that defines the transition point. Such experiments require the reduced gravity conditions. Close to the CHF, the slow growth of the individual dry spots and their subsequent fusion on the transparent heater are observed through the latter. As discussed in the paper, these observations are consistent with numerical results obtained with the vapor recoil model of the boiling crisis

Quasi-static relaxation of arbitrarily shaped sessile drops

International audienceWe study a spontaneous relaxation dynamics of arbitrarily shaped liquid drops on solid surfaces in the partial wetting regime. It is assumed that the energy dissipated near the contact line is much larger than that in the bulk of the fluid. We have shown rigorously in the case of quasi-static relaxation using the standard mechanical description of dissipative system dynamics that the introduction of a dissipation term proportional to the contact line length leads to the well known local relation between the contact line velocity and the dynamic contact angle at every point of an arbitrary contact line shape. A numerical code is developed for 3D drops to study the dependence of the relaxation dynamics on the initial drop shape. The available asymptotic solutions are tested against the obtained numerical data. We show how the relaxation at a given point of the contact line is influenced by the dynamics of the whole drop which is a manifestation of the non-loca

Modelling of the moving deformed triple contact line: influence of the fluid inertia

For partial wetting, motion of the triple liquid-gas-solid contact line is influenced by heterogeneities of the solid surface. This influence can be strong in the case of inertial (e.g. oscillation) flows where the line can be pinned or move intermittently. A model that takes into account both surface defects and fluid inertia is proposed. The viscous dissipation in the bulk of the fluid is assumed to be negligible as compared to the dissipation in the vicinity of the contact line. The equations of motion and the boundary condition at the contact line are derived from Hamilton's principle. The rapid capillary rise along a vertical inhomogeneous wall is treated as an example.Comment: 19 pages and 3 figure

Dynamics of the triple contact line on a non-isothermal heater at partial wetting

The dynamics of the triple gas-liquid-solid contact line is analysed for the case where the gas is the saturated vapour corresponding to the liquid, like in the vapour bubble in boiling. It is shown that even small superheating (with respect to the saturation temperature) causes evaporation of the adsorption liquid film and the true triple contact is established. It is shown that the hydrodynamic contact line singularity cannot be relaxed with the Navier slip condition under such circumstances. Augmented with the second derivative slip condition is proposed to be applied. For the partial wetting conditions, a non-stationary contact line problem where the contact line motion is caused by evaporation or condensation is treated in the lubrication approximation in the vicinity of the contact line. High heat fluxes in this region require the transient heat conduction inside the heater to be accounted for. Two 2D problems, those of drop retraction with no phase change and of drop evaporation are solved and analysed as illustrations of the proposed approach

Evaporation at microscopic scale and at high heat flux

This thesis theoretically investigates the transport processes in the vicinity of the triple gas-liquid-solid contact line and its impact on macroscopic evaporation. In the first part of the thesis, the hydrodynamics close to the contact line at partial wetting is studied. Specifically, evaporation into the atmosphere of pure vapor driven by heating of the substrate is considered. The question of singularity relaxation is addressed. The main finding of the thesis is that the Kelvin effect (dependence of saturation temperature on pressure) is sufficient by itself to relax the hydrodynamic contact line singularity. The proposed microregion (the contact line vicinity) model for small interface slopes is solved numerically. Asymptotic solutions are found for some specific cases. The governing length scales of the problem are identified and the multiscale nature of the phenomenon is addressed. Parametric studies revealing the role of the thermal resistance of vapor-liquid interface, slip length, thermocapillary term, the vapor recoil and surface forces are also performed. An extension of the lubrication approximation for high slopes of the gas-liquid interface at evaporation is discussed.In the second part of the thesis, the previously established microregion model is coupled to a simplified single vapor bubble growth numerical simulation. The bubble departure from the heater at boiling is also studied. It was proposed in the thesis, that under high heat loads, the increase of the apparent contact angle causes the vapor bubble to spread over the heated substrate. Such a behavior may cause the heater dry-out that occurs during the boiling crisisCette thèse étudie théoriquement les processus de transport au voisinage de la ligne triple de contact liquide-gaz-solide et leur impact sur l'évaporation macroscopique. Dans la première partie de la thèse, l'hydrodynamique au voisinage de la ligne de contact est étudiée sous les conditions de mouillage partiel. L'évaporation induite par le chauffage du substrat dans l'atmosphère de vapeur du même fluide est considérée. La relaxation de la singularité hydrodynamique de la ligne triple est considérée. La principale conclusion de la thèse est que l'effet Kelvin (dépendance de la température de saturation de la pression) est suffisant en soi, pour faire disparaitre la singularité des variables hydrodynamiques. La microrégion (le voisinage de la ligne de contact) est résolue numériquement et analytiquement pour de faibles pentes de l'interface liquide-vapeur. Les échelles de longueur caractéristiques du problème sont identifiées et la nature multi-échelle du phénomène est prise en compte. Les études paramétriques effectuées révèlent le rôle de la résistance thermique de l'interface vapeur-liquide, de la longueur de glissement, du terme thermocapillaire, du recul de vapeur et ainsi que des forces de surface. Une extension de l'approximation de lubrification pour de pentes élevées de l'interface gaz-liquide à l'évaporation est discutée. Dans la seconde partie de la thèse, le modèle précédemment établi pour la microrégion est couplé à des simulations numériques de la croissance d'une bulle de vapeur. Le départ de la bulle de vapeur de la paroi chauffante pendant l'ébullition a également été étudiée. Il a été proposé dans la thèse, que sous des charges thermiques élevées, l'augmentation de l'angle de contact apparent provoque l'étalement de la bulle de vapeur sur la paroi chauffante. Ce phénomène peut conduire, au séchage de la paroi observé pendant la crise d'ébullitionPARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF

Pulsating Heat Pipes:Basics of Functioning and Modeling

International audienceFor cooling of electronic or electric equipment, there is a growing industrial demand of high-performance thermal links. One such thermal device is the recently invented pulsating (called also oscillating) heat pipe (PHP). It consists of a closed capillary tube folded into meander and partially filled with a liquid. One side of the meander is in thermal contact with a hot spot, the other with a cold spot. The oscillation of the liquid plugs and vapor bubbles spontaneously occurs after the start of heating by the action of evaporation/condensation at the menisci. The plugs move between hot and cold areas by creating an efficient convective heat exchange. This advantage and also the simplicity of PHP make it highly competitive with respect to other kinds of heat pipes. However, the PHP functioning is non-stationary and depends on a large number of physical and material parameters. As a result, application of empirical correlations is quite unsuccessful, and more sophisticated theoretical and basic experimental studies are necessary. In this chapter, we present the current level of understanding and existing approaches to the PHP modeling and design. We start by describing the basic experiments with the simplest, single-branch PHP that contains only one bubble–plug couple. We show how the results of these experiments help to understand the PHP functioning and introduce the reader to the theoretical and numerical approaches to the PHP modeling by describing the relevant physical phenomena. Finally, we review the state-of-the-art of modeling of the multi-branch PHP. This chapter is complementary to the review of the experimental work on multi-branch PHPs presented in Chapter 1