P. Lavieille

About Stability of a Vapour Jet Condensing in a Micro Channel

An unsteady model of condensation flow in capillary regime inside a cylindrical tube was developed, based on a two-fluid approach. The model takes into account the coupling between the liquid film zone (where the quality is low) and the nearly hemispherical meniscus which is present at the end of the condensation region. Numerically, a major difficulty is that this type of problem has a free boundary condition. Indeed the end location of the two-phase zone is the result of all the heat exchanges occurring upstream of this area. To overcome this difficulty a mathematical representation was specifically developed. The unsteady model presented is based on five dimensionless numbers characterizing this type of flow. A comparison between the results of this model and those of a previously developed stationary model is made. An excellent agreement is obtained. The presence of self-sustaining oscillations due to the intrinsic mechanisms of condensation are also obtained numerically. The variation of the Nusselt number with the boiling number is then determined and presented. These results complement the previous study by determining the frontier between the stable and unstable situations. The atypical behaviour of the Nusselt number preceding the onset of instability is also analyzed.Copyright

Dropwise Condensation Enhancement Using a Wettability Gradient

ABSTRACT This paper deals with the modeling of dropwise condensation process on a wettability gradient surface. The proposed heat transfer model explicitly takes into account the mechanical nonequilibrium on the periphery of the droplet due to the surface-energy gradient and the contact-angle hysteresis. The model aims to predict the dynamic behavior of a droplet placed on a wettability gradient surface regarding the temperature difference between the wall and the saturated vapor. A comprehensive analysis of all the contributing thermal resistances is proposed. The influences of contact angle, temperature difference, and other representative parameters on a single droplet on a horizontal surface are also discussed. The results indicate that a wettability gradient can cause a reduction of the mean size of the droplets on the condensing surface and thus enhance significantly the heat transfer rate.