Adel M. Benselama

Thermocapillary effects on steadily evaporating contact line: A perturbative local analysis

The evaporation process taking place close to the three-phase contact line is considered and studied theoretically using a linear stability analysis approach. A domain perturbation method, taking into consideration thermocapillary effects and surface forces, is used to develop the higher-order solution in terms of series expansion about lubrication condition. A closed-form solution is found for the film thickness, the pressure jump across the liquid-vapor interface and the evaporative flux in the vicinity of the contact line. The key novelty in this work is considering thermocapillary instability for very thin films (∼10 nm) accounting for surface forces. For (quasi-) flat-very-thin films, the analysis shows no instability, which is consistent with general knowledge in this field. However, for films extending from a meniscus, as encountered in wetting configurations, it is found that the competition between London–van der Waals, capillary, and thermocapillary forces leads to contact line instability and b...

A perturbation method for solving the micro-region heat transfer problem

A perturbation method is proposed and used to model the two-dimensional equations governing evaporation in the micro-region of a meniscus on a heated substrate. The novelty of the method lies in the choice of the physical quantities which are used to describe the hydrodynamic and heat transfer phenomena. The chosen quantities are the pressure jump function across the liquid-vapor interface and a modified-shape function. The problem is thus transformed into a set of decoupled initial-value sub-problems that can be solved recursively from lower to higher orders. This approach represents many advantages compared with existing theories. The model is then applied, accounting for the effect of gravity, to describe the micro-region shape and heat transfer. The results obtained following this approach are then validated against those given in literature. The comparison demonstrated the validity of the developed model as well as its wider range of applicability. The influence of the interaction between liquid, vap...

Expanding Taylor bubble under constant heat flux

Modelization of non-isothermal bubbles expanding in a capillary, as a contribution to the understanding of the physical phenomena taking place in Pulsating Heat Pipes (PHPs), is the scope of this paper. The liquid film problem is simplified and solved, while the thermal problem takes into account a constant heat flux density applied at the capillary tube wall, exchanging with the liquid film surrounding the bubble and also with the capillary tube outside medium. The liquid slug dynamics is solved using the Lucas-Washburn equation. Mass and energy balance on the vapor phase allow governing equations of bubble expansion to be written. The liquid and vapor phases are coupled only through the saturation temperature associated with the vapor pressure, assumed to be uniform throughout the bubble. Results show an over-heating of the vapor phase, although the particular thermal boundary condition used here always ensures an evaporative mass flux at the liquid-vapor interface. Global heat exchange is also investig...

Evaporating bubble in a heated capillary: effect of passage on the temperature field of the external wall

The nonisothermal Taylor liquid-slug-vapor-bubble problem, occurring inside a capillary of circular cross-section, is investigated numerically. The underlying hydrodynamic and mass transfer phenomena are considered the major heat transfer means in pulsating heat pipes. The temperature signature at the outer side of the capillary, inside which the bubble travels, is particulary examined. It is shown that for typical flow conditions, i.e. for liquid flow velocity and applied heat flux about 0.1 m s−1 and 105 W m−2, respectively, wall thickness effects on capillary wall temperature are negligible in terms of diffusion and lag. In addition, the larger the liquid flow velocity, the more likely the bubble grows (due to evaporation) axially. This investigation opens new avenue to inverse methods where the bubble position is identified only through the temperature profile at the outer side of PHPs channels wall.