A synthesis of fluid and thermal transport models for metal foam heat exchangers

An investigation of variants within the porous media transport models is presented in this work. Four major categories in modeling the transport processes through porous media, namely constant porosity, variable porosity, thermal dispersion, and local thermal non-equilibrium, are analyzed in detail. The main objective of the current study is to compare these variances in models for each of the four categories and establish conditions leading to convergence or divergence among different models. To analyze the effects of variants within these transport models, a systematic reduction and sensitivity investigation for each of these four aspects is presented. The effects of the Darcy number, inertia parameter, Reynolds number, porosity, particle diameter, and the fluid-to-solid conductivity ratio on the variances within each of the four areas are analyzed. For some cases the variances within different models have a negligible effect on the results while for some cases the variations can become significant. In general, the variances have a more pronounced effect on the velocity field and a substantially smaller effect on the temperature field and Nusselt number distribution

Analysis of Variants Within the Porous Media Transport Models

The wetting of solid surfaces using liquid droplets has been studied since the early 1800s. Thomas Young and Pierre-Simon Laplace investigated the wetting properties, as well as the role of the contact angle and the coupling of a liquid and solid, on the contact angle formation. The geometry of a sessile droplet is relatively simple. However, it is sufficiently complex to be applied for solving and prediction of real-life situations (for example, metallic inks for inkjet printing, the spreading of pesticides on leaves, the dropping of whole blood, the spreading of blood serum, and drying for medical applications). Moreover, when taking into account both wetting and evaporation, a simple droplet becomes a very complex problem, and has been investigated by a number of researchers worldwide. The complexity is mainly due to the physics involved, the full coupling with the substrate upon which the drop is deposited, the atmosphere surrounding the droplet, and the nature of the fluid (pure fluid, bi- or multi-phase mixtures, or even fluids containing colloids and/or nano-particles). This review presents the physics involved during droplet wetting and evaporation by focusing on the evaporation dynamics, the flow motion, the vapour behaviour, the surface tension, and the wetting properties.

Recent advances in droplet wetting and evaporation

An experimental and numerical study on heat transfer enhancement for gas heat exchangers fitted with porous media

Evaporation of a Droplet: From physics to applications

Analytical solution of non-Darcian forced convection in an annular duct partially filled with a porous medium

A convection-diffusion model is developed to analyze the effect of buoyant convection in the surrounding air on the heat and mass transfer phenomena during the evaporation of a pinned water drop deposited on a horizontal substrate of large dimensions. The substrate is maintained at constant temperature which can be equal or higher than the temperature of the ambient air. The mathematical model accounts for the motion of the gas phase surrounding the drop due to thermal and solutal buoyancy effects, while only thermal diffusion is considered in the liquid phase. A quasisteady state regime is adopted because of the slow motion of the liquid-gas interface as well as the induced heat and mass transfer phenomena in both phases. The numerical results obtained with the diffusion model or the convection-diffusion model show that heat and mass transfer rates are important toward the contact line. The heat required for evaporation process is taken from the environment, both the liquid and the gas phase, and results in a small cold zone on both sides of the interface. The influence of the buoyancy in air is of greater importance in the lower part of the interface and beyond a distance of a contact radius above the droplet. A weak variation of the evaporation rate is observed on a wider range of contact angle for high wall temperatures. The diffusion model underestimates the overall evaporation rate by 8.5% for a wall temperature equal to an ambient temperature of 25 ° C and by 27.3% for a wall temperature of 70 ° C . Numerical calculations show that the length of the heated wall has very little effect on the evaporation process when it exceeds 25 times the contact radius.

Numerical investigation of heat and mass transfer of an evaporating sessile drop on a horizontal surface

Forced convection cooling enhancement by use of porous materials

A numerical investigation of electronic cooling enhancement is carried out in this study in order to determine how the operating temperature can be maintained under the allowable level. A new technique based on use of porous or foam material inserted between the components on a horizontal board is studied. One energy equation model has been adopted to analyse the thermal field. The control volume method based on finite differences with appropriate averaging for diffusion coefficients is used to solve the coupling between solid, fluid and porous regions. The effect of parameters such as Reynolds number, Darcy number and thermal conductivity ratio are considered in order to look for the most appropriate properties of the foam or porous substrate that allow optimal cooling. The results show that for high thermal conductivity of the porous substrate, substantial enhancement is obtained compared to the fluid case even if the permeability is low. In the mixed convection case, the insertion of the foam between the blocks leads to a temperature reduction of 50%.

Salah Chikh

Papers

Analytical solution of non-Darcian forced convection in an annular duct partially filled with a porous medium

Numerical investigation of heat and mass transfer of an evaporating sessile drop on a horizontal surface

Forced convection cooling enhancement by use of porous materials

Enhancement of electronic cooling by insertion of foam materials

Evaporation of a sessile drop with pinned or receding contact line on a substrate with different thermophysical properties