Numerical Investigation of Forced Convection Nonequilibrium Effects on Heat and Mass Transfer in Porous Media

A numerical investigation is performed to analyze the coupled heat and mass transfer in porous media with a strong exothermic reaction. Similar problems have received great attention due to their relevance in a wide variety of engineering applications, such as heat pipes, drying technologies, nuclear reactors, catalytic reactors, environmentally clean utilization of energy, and many others. The momentum transfer in the porous substrate is modeled with the Darcy–Brinkman–Forchheimer law, while the temperature and concentration fields are obtained subsequently from the energy and diffusion equations. The considered configuration consists of a cylindrical duct where a first-order reaction is supposed to occur. The governing equations are solved by using the finite-volume method. The SIMPLER algorithm is applied to solve the momentum and continuity equations. The power-law scheme is used to model the interaction between convection and diffusion terms. The effect of the main governing parameters, such as permeability, aspect ratio, solid-to-fluid conductivity ratio, Reynolds number, Biot number, and the modified Frank-Kamenetskii number, are studied. The comparison with previously published work shows excellent agreement.     

Coupled morphological and thin film instabilities generated by inclusions in crystal growth

The interaction of a foreign particle that is suspended in the melt with a planar solidifying interface may induce the onset of morphological instabilities provided that its distance from the interface falls below a critical value. This distance, which is of the order of the particle’s radius, depends on the governing processing and physical parameters. When the particle is in nearcontact with the solid-liquid interface, the disjoining pressure in the melt film that separates the particle from the interface influences the interaction. We derive an expression for the film thickness at which rupture occurs. The critical film thickness, which depicts the competition between the stabilizing influence of surface tension and thermal gradients and the destabilizing influence of the intermolecular forces, varies as (Sh)^1/4, where Sh is the Scheludko number that is modifed by the imposed thermal gradients. We note the existence of a critical value for the particle’s radius below which the stabilizing effects are primarily due to surface tension and above which they are due to the thermal gradients.     

The mini-conical slump flow test: Analysis and numerical study

This paper provides a general study on cement paste flow. Both mini-slump and Marsh cone tests are used to evaluate the workability of fresh paste mixtures derived from self compacting concretes. A numerical approach is used to reproduce global flow behavior and to check the accuracy of the obtained viscosity as well as the validity of expressions available in the literature giving yield stress from the final diameter of slumped paste. The computational modeling allows access to local information in order to analyze different regions and corresponding flow types, i.e. falling solid and flowing fresh cementing material mixtures.The limitation of some empirical models allowing the prediction of yield stress τ₀ and plastic viscosity μ from mini-slump tests is underlined, conditions of validity are expressed and a new expression is proposed.     

Thermodiffusion in porous media: Multi-domain constitutant separation

A numerical study is carried out on double-diffusive, natural convection within a vertical annular, porous cylinder. The flow is driven by buoyancy force due to externally applied constant temperature difference on the vertical cylinder while the horizontal surfaces are impermeable and adiabatic. The effects of cross phenomena “Soret effect” were considered in the analysis. It is demonstrated that the cylindrical annulus permits a higher thermal gradient. It is established that such system has an optimal separation effect for a given thermal Rayleigh number, Ra_T. To overcome such limitation, two sub-domains (buffer) allowing filtration separation was proposed and investigated. The flow field, temperature and concentration distributions are obtained in terms of the governing parameters. The effect of the sub-domains scales, the Darcy number, Da, and the curvature, R, on flow, temperature and species separation ability is found to be significant.     

Multiple Natural Convection Solution in Porous Media Under Cross Temperature and Concentration Gradients

This article is devoted to the study of air curtains applied to reduce the refrigerated chambers heat gains. The proposed strategy is based on the numerical simulation of air curtains by means of computational fluid dynamics (CFD) using RANS modeling and their corresponding experimental validation. Further work on the reduction of the detailed numerical results into overall energetic parameters is also presented. Unsteady three-dimensional numerical parametric studies are carried out, simulating the process of refrigerated chamber sudden door opening. The numerical solutions are verified and the influence of the turbulence model used is also investigated. The studies are centered on the influence of air curtain location, the air suction combination, and both the air discharge velocity and the discharge angle.     

Thermosolutal Convection During Melting in a Porous Medium Saturated with Aqueous Solution

The thermosolutal convection in a porous medium saturated with an aqueous solution near the temperature of the density maximum is studied. The fixed temperatures applied to vertical walls include the density maximum. The formulation of the problem is based on the Darcy-Brinkman model and the density variation is governed by a nonlinear approximation. The equations are solved by a finite-volume method. The numerical model is validated through experimental results. We show that the nonlinear variation of the density influences strongly the flow structure and the heat transfer. The structures of this flow show that the density maximum generates a complex flow structure of two contrarotating cells of unequal importance.     

THE EFFECT OF HEAT FLUX DISTRIBUTION ON THERMOCAPILLARY CONVECTION IN A SIDE-HEATED LIQUID BRIDGE

A numerical analysis is performed on thermocapillary convection in a liquid bridge. The effect of various imposed heat flux distributions on the free surface for axisymmetric liquid bridge flows, in zero gravity, is presented. Constant, polynomial, and different Gaussian profiles are considered, and the resulting velocity and temperature fields are compared. It is shown that a more concentrated heat flux distribution has a significant effect on the hydrodynamics of the liquid bridge. The transition from steady to unsteady flows also depends on the distribution of applied heat flux. The frequency of the oscillation increases with Ma increase.     

Natural convection in a shallow cavity containing two superposed layers of immiscible binary liquids

This paper reports an analytical study of the natural convection in a shallow rectangular cavity containing two superposed layers of binary immiscible fluids. A uniform heat flux is applied to the horizontal walls of the enclosure while the vertical walls are impermeable and adiabatic. The solutal buoyancy forces are assumed to be induced either by the imposition of constant fluxes of mass on the horizontal walls (double-diffusive convection) or by a temperature gradient (Soret effects). An analytical solution of the steady form of the governing equations is derived using a parallel flow approximation. The critical Rayleigh numbers for the onset of critical, ${R_{\rm TC}^{\rm sup}}$ , or subcritical, ${R_{\rm TC}^{\rm sub}}$ , convection are predicted by the present theory. For finite amplitude convection the present model yields explicitly the flow patterns, Nusselt and Sherwood numbers in terms of the governing parameters of the problem. In the limit of a single fluid layer the present theory is found to be in agreement with the results reported in the literature.     

Heterogeneous nanofluids: natural convection heat transfer enhancement

Convective heat transfer using different nanofluid types is investigated. The domain is differentially heated and nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account the thermal conductivity, viscosity versus nanofluids type and concentration and the spatial heterogeneous concentration induced by the Soret effect is presented. The obtained results, by solving numerically the full governing equations, are found to be in good agreement with the developed solution based on the scale analysis approach. The resulting convective flows are found to be dependent on the local particle concentration φ and the corresponding solutal to thermal buoyancy ratio N. The induced nanofluid heterogeneity showed a significant heat transfer modification. The heat transfer in natural convection increases with nanoparticle concentration but remains less than the enhancement previously underlined in forced convection case.     

Double dispersion,natural convection in an open end cavity simulation via Lattice Boltzmann Method

Double dispersion in an open end cavities are simulated using Lattice Boltzmann Method (LBM). The flow is driven by the buoyancy effect due to the heated vertical wall and species concentration at the heated wall of the cavity (closed end). The paper is intended to address the physics of flow, heat and mass transfers in open ended cavities and close end slots. Prandtl number (Pr) is fixed to 0.71 (air) for the thermal Rayleigh number (Ra_T) of 10⁴, 10⁵ and 10⁶. The results are presented for moderate Lewis number of 2, 4 and 8 and for a range of buoyancy ratio, N, (species to thermal). The species concentration induced buoyancy force either aids or opposes the thermally driven flow, which is determined by the value of buoyancy ratio (positive or negative, respectively). Interesting flow patterns were predicted for opposing buoyancy forces.