Film condensation in a porous medium has been receiving increasing attention due to its wide range of heat transfer applications. Some examples of these practical applications are distillation, drying technology, geothermal energy, cooling towers, heat exchangers, and air conditioning. One of the characteristic features of film condensation in porous media is the formation of a two-phase zone separating the liquid film and the vapour zone due to capillary pressure. In this paper, a physico-mathematical model of liquid film condensation on a surface embedded in a porous medium with a two-phase region effect is developed and presented. The model is based on momentum and continuity equations as applied to the liquid film and the two-phase flow region supplemented with the Darcy flow assumption and assumptions on the Leverette J-function and the saturation behaviour near the edge of the liquid film. The developed model allows a simple analytical solution to the problem in distinction to semi-analytical and numerical solutions published by different authors. From the model developed, it shows that the presence of the two-phase region decreases the liquid film thickness. By taking the capillary effects into consideration results in higher heat transfer and condensation rates due to the decrease in the liquid film thickness. The presented model yields good agreement when compared to the theoretical results and experimental data by other authors. The developed model addresses the fundamental concepts of phase transition in porous media which can effectively find applications in many areas.
Evaporation and boiling from porous media has drawn a broad attention in many applications ranging from industrial, environmental to engineering and technologies. However, prediction of evaporation rates from porous media remains a challenge due to complex interactions between ambient conditions and the porous media properties such as complex geometry, material of the matrices composing porous medium, many length scales, surface phenomena, and effective transport coefficients. In this study, the evaporation rates of distilled water from porous media of different matrices were studied experimentally. Porous media were made up from spherical brass balls of different diameters of 2 mm, 3 mm, 4 mm and 5 mm. The temperature profiles across porous media, air temperature, humidity, and amount of water evaporated were measured in experiments. The effects of the porosity, the thickness of a porous layer, and particle size on the evaporation from porous media were investigated. Different materials of the porous matrices have allowed bringing to light the influence of the material properties. The experimental data show that the rate of evaporation of distilled water with brass balls is higher than that of only distilled water without a porous medium embedded into the water layer and does not change significantly after a steady state condition is achieved.
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