10,803 research outputs found
Numerical simulation of density-driven flow and heat transport processes in porous media using the network method
Density-driven flow and heat transport processes in 2-D porous media scenarios are governed by coupled, non-linear, partial differential equations that normally have to be solved numerically. In the present work, a model based on the network method simulation is designed and applied to simulate these processes, providing steady state patterns that demonstrate its computational power and reliability. The design is relatively simple and needs very few rules. Two applications in which heat is transported by natural convection in confined and saturated media are studied: slender boxes heated from below (a kind of Bénard problem) and partially heated horizontal plates in rectangular domains (the Elder problem). The streamfunction and temperature patterns show that the results are coherent with those of other authors: steady state patterns and heat transfer depend both on the Rayleigh number and on the characteristic Darcy velocity derived from the values of the hydrological, thermal and geometrical parameters of the problems.The first author acknowledges the support of the Universidad Politécnica de Cartagena through a pre-doctoral scholarship and the economic support of the Universidad Católica del Norte to cover the costs to publish in open access
Non-equilibrium thermodynamic analysis of double diffusive, nanofluid forced convection in microreactors with radiation effects
This paper presents a theoretical investigation of the second law performance of double diffusive forced convection in microreactors with the inclusion of nanofluid and radiation effects. The investigated microreactors consist of a single microchannel, fully filled by a porous medium. The transport of heat and mass are analysed by including the thick walls and a first order, catalytic chemical reaction on the internal surfaces of the microchannel. Two sets of thermal boundary conditions are considered on the external surfaces of the microchannel; (1) constant temperature and (2) constant heat flux boundary condition on the lower wall and convective boundary condition on the upper wall. The local thermal non-equilibrium approach is taken to thermally analyse the porous section of the system. The mass dispersion equation is coupled with the transport of heat in the nanofluid flow through consideration of Soret effect. The problem is analytically solved and illustrations of the temperature fields, Nusselt number, total entropy generation rate and performance evaluation criterion (PEC) are provided. It is shown that the radiation effect tends to modify the thermal behaviour within the porous section of the system. The radiation parameter also reduces the overall temperature of the system. It is further demonstrated that, expectedly, the nanoparticles reduce the temperature of the system and increase the Nusselt number. The total entropy generation rate and consequently PEC shows a strong relation with radiation parameter and volumetric concentration of nanoparticles
Finite element modeling of free surface flow in variable porosity media
The aim of the present work is to present an overview of some numerical procedures for the simulation of free surface flows within a porous structure. A particular algorithm developed by the authors for solving this type of problems is presented. A modified form of the classical Navier–Stokes equations is proposed, with the principal aim of simulating in a unified way the seepage flow inside rockfill-like porous material and the free surface flow in the clear fluid region. The problem is solved using a semi-explicit stabilized fractional step algorithm where velocity is calculated using a 4th order Runge–Kutta scheme. The numerical formulation is developed in an Eulerian framework using a level set technique to track the evolution of the free surface. An edge-based data structure is employed to allow an easy OpenMP parallelization of the resulting finite element code. The numerical model is validated against laboratory experiments on small scale rockfill dams and is compared with other existing methods for solving similar problems.Peer ReviewedPostprint (author’s final draft
Overlimiting Current and Shock Electrodialysis in Porous Media
Most electrochemical processes, such as electrodialysis, are limited by
diffusion, but in porous media, surface conduction and electro-osmotic flow
also contribute to ionic fluxes. In this paper, we report experimental evidence
for surface-driven over-limiting current (faster than diffusion) and
deionization shocks (propagating salt removal) in a porous medium. The
apparatus consists of a silica glass frit (1 mm thick with 500 nm mean pore
size) in an aqueous electrolyte (CuSO or AgNO) passing ionic current
from a reservoir to a cation-selective membrane (Nafion). The current-voltage
relation of the whole system is consistent with a proposed theory based on the
electro-osmotic flow mechanism over a broad range of reservoir salt
concentrations (0.1 mM - 1.0 M), after accounting for (Cu) electrode
polarization and pH-regulated silica charge. Above the limiting current,
deionized water ( ) can be continuously extracted from the
frit, which implies the existence of a stable shock propagating against the
flow, bordering a depleted region that extends more than 0.5mm across the
outlet. The results suggest the feasibility of "shock electrodialysis" as a new
approach to water desalination and other electrochemical separations.Comment: 39 pages, 9 fig
UNSTEADY MIXED CONVECTION WITH SORET AND DUFOUR EFFECTS PAST A POROUS PLATE MOVING THROUGH A BINARY MIXTURE OF CHEMICALLY REACTING FLUID
This study investigates the unsteady mixed convection flow past a vertical porous
flat plate moving through a binary mixture in the presence of radiative heat transfer
and nth-order Arrhenius type of irreversible chemical reaction by taking into account
the diffusion-thermal (Dufour) and thermo-diffusion (Soret) effects. Assuming an
optically thin radiating fluid and using a local similarity variable, the governing
nonlinear partial differential equations have been transformed into a set of coupled
nonlinear ordinary differential equations, which are solved numerically by applying
shooting iteration technique together with fourth-order Runge-Kutta integration
scheme. Graphical results for the dimensionless velocity, temperature, and concentration
distributions are shown for various values of the thermophysical parameters
controlling the flow regime. Finally, numerical values of physical quantities, such as
the local skin-friction coefficient, the local Nusselt number, and the local Sherwood
number are presented in tabular form
A hierarchy of models for simulating experimental results from a 3D heterogeneous porous medium
In this work we examine the dispersion of conservative tracers (bromide and
fluorescein) in an experimentally-constructed three-dimensional dual-porosity
porous medium. The medium is highly heterogeneous (), and
consists of spherical, low-hydraulic-conductivity inclusions embedded in a
high-hydraulic-conductivity matrix. The bi-modal medium was saturated with
tracers, and then flushed with tracer-free fluid while the effluent
breakthrough curves were measured. The focus for this work is to examine a
hierarchy of four models (in the absence of adjustable parameters) with
decreasing complexity to assess their ability to accurately represent the
measured breakthrough curves. The most information-rich model was (1) a direct
numerical simulation of the system in which the geometry, boundary and initial
conditions, and medium properties were fully independently characterized
experimentally with high fidelity. The reduced models included; (2) a
simplified numerical model identical to the fully-resolved direct numerical
simulation (DNS) model, but using a domain that was one-tenth the size; (3) an
upscaled mobile-immobile model that allowed for a time-dependent mass-transfer
coefficient; and, (4) an upscaled mobile-immobile model that assumed a
space-time constant mass-transfer coefficient. The results illustrated that all
four models provided accurate representations of the experimental breakthrough
curves as measured by global RMS error. The primary component of error induced
in the upscaled models appeared to arise from the neglect of convection within
the inclusions. Interestingly, these results suggested that the conventional
convection-dispersion equation, when applied in a way that resolves the
heterogeneities, yields models with high fidelity without requiring the
imposition of a more complex non-Fickian model.Comment: 27 pages, 9 Figure
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