Heat Transfer in a Porous Electrode of Fuel Cells

2005 ◽  
Vol 128 (5) ◽  
pp. 434-443 ◽  
Author(s):  
J. J. Hwang
Keyword(s):  
Heat Transfer ◽  
Diffusion Layer ◽  
Wall Temperature ◽  
Solid Phase ◽  
Porous Electrode ◽  
Catalyst Layer ◽  
Fluid Phase ◽  
Stanton Number ◽  
Thermal Nonequilibrium ◽  
Local Thermal Nonequilibrium

The thermal-fluid behaviors in a porous electrode of a proton exchange membrane fuel cell (PEMFC) in contact with an interdigitated gas distributor are investigated numerically. The porous electrode consists of a catalyst layer and a diffusion layer. The heat transfer in the catalyst layer is coupled with species transports via a macroscopic electrochemical model. In the diffusion layer, the energy equations based on the local thermal nonequilibrium (LTNE) are derived to resolve the temperature difference between the solid phase and the fluid phase. Parametric studies include the Reynolds number and the Stanton number (St). Results show that the wall temperature decreases with increasing Stanton number. The maximum wall temperatures occur at the downstream end of the module, while the locations of local minimum wall temperature depend on the Stanton numbers. Moreover, the solid phase and the fluid phase in the diffusion layer are thermally insulated as St⪡1. The diffusion layer becomes local thermal nonequilibrium as the Stanton number around unity. The porous electrode is local thermal equilibrium for St⪢1. Finally, the species concentrations inside the catalyst and diffusion layers are also provided.

Convective Instability of the Darcy Flow in a Horizontal Layer With Symmetric Wall Heat Fluxes and Local Thermal Nonequilibrium

2013 ◽  
Vol 136 (1) ◽  
Author(s):  
A. Barletta ◽  
M. Celli ◽  
A. V. Kuznetsov
Keyword(s):  
Rayleigh Number ◽  
Linear Stability ◽  
Solid Phase ◽  
Fluid Phase ◽  
Heat Fluxes ◽  
Local Thermal Equilibrium ◽  
Neutral Stability ◽  
Thermal Nonequilibrium ◽  
Local Thermal Nonequilibrium ◽  
Darcy Rayleigh Number

The linear stability of the parallel Darcy throughflow in a horizontal plane porous layer with impermeable boundaries subject to a symmetric net heating or cooling is investigated. The onset conditions for the secondary thermoconvective flow are expressed through a neutral stability bound for the Darcy–Rayleigh number associated with the uniform heat flux supplied or removed from the walls. The study is performed by taking into account a condition of local thermal nonequilibrium between the solid phase and the fluid phase. The linear stability analysis is carried out according to the normal modes' decomposition of the perturbations to the basic state. The governing equations for the disturbances are solved numerically as an eigenvalue problem leading to the neutral stability condition. If compared with the asymptotic condition of local thermal equilibrium, the regime of local nonequilibrium manifests an enhanced instability. This behavior is displayed by lower critical values of the Darcy–Rayleigh number, eventually tending to zero when the thermal conductivity of the solid phase is much larger than the conductivity of the fluid phase. In this special limit, which can be invoked as an approximate model of a gas-saturated metallic foam, the basic throughflow is always unstable to external disturbances of arbitrarily small amplitude.

A Local Thermal Nonequilibrium Analysis of Silicon Carbide Ceramic Foam as a Solar Volumetric Receiver

2012 ◽  
Vol 134 (2) ◽  
Author(s):  
Y. Sano ◽  
S. Iwase ◽  
A. Nakayama
Keyword(s):  
Heat Transfer ◽  
Silicon Carbide ◽  
Pore Diameter ◽  
Solid Phase ◽  
Flow Direction ◽  
Ceramic Foam ◽  
Thermal Nonequilibrium ◽  
Silicon Carbide Ceramic ◽  
Local Thermal Nonequilibrium ◽  
Solar Receiver

A volumetric solar receiver receives the concentrated radiation generated by a large number of heliostats. Heat transfer takes place from the receiver solid phase to the air as it passes through the porous receiver. Such combined heat transfer within the receiver, associated radiation, convection and conduction, are investigated using a local thermal nonequilibrium model. The Rosseland approximation is applied to account for the radiative heat transfer through the solar receiver, while the low Mach approximation is exploited to investigate the compressible flow through the receiver. Analytic solutions are obtained for the developments of air and ceramic temperatures as well as the pressure along the flow direction. The results show that the pore diameter must be larger than its critical value to achieve high receiver efficiency. Subsequently, there exists an optimal pore diameter for achieving the maximum receiver efficiency under the equal pumping power. The solutions serve as a useful tool for designing a novel volumetric solar receiver of silicon carbide ceramic foam.

New models for heat flux splitting at the boundary of a porous medium: three energy equations for nanofluid flow under local thermal nonequilibrium conditions

2014 ◽  
Vol 92 (11) ◽  
pp. 1312-1319 ◽  
Author(s):  
M. Nazari ◽  
M.J. Maghrebi ◽  
T. Armaghani ◽  
Ali J. Chamkha
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Porous Medium ◽  
Solid Phase ◽  
Solid Matrix ◽  
Fluid Temperature ◽  
Transfer Coefficients ◽  
Thermal Nonequilibrium ◽  
Local Thermal Nonequilibrium ◽  
Energy Equations

One of the challenging points in the simulation of a nanofluid flowing through a porous medium is modeling the surface heat flux in the presence of nanoparticles and internal solid matrix. The question is how much energy is absorbed by the solid phase, fluid phase, and particles at the surface of imposing heat flux? To reach a suitable answer, a local thermal nonequilibrium approach (including three energy equations) is presented in this paper and three heat flux models are proposed for the first time. The proposed models are compared and analyzed. The effects of interstitial heat transfer coefficients on the heat transfer in a porous channel are completely studied. The fluid temperature distributions and heat transfer rate obtained by homogenous and nonhomogenous approaches (for the proposed models) are completely studied and compared. The results show that the nonhomogeneous approach experiences larger Nusselt number than the homogeneous one for all the recommended heat flux models.

Heat Transfer in Fixed Bed Elliptic Cylindrical Reactor via Two-Phase Model

2020 ◽  
Vol 400 ◽  
pp. 45-50
Author(s):  
Antonildo Santos Pereira ◽  
Rodrigo Moura da Silva ◽  
Maria Conceição Nóbrega Machado ◽  
Luan Pedro Melo Azerêdo ◽  
Anderson Ferreira Vilela ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Solid Phase ◽  
Homogeneous Model ◽  
Fixed Bed ◽  
Fluid Phase ◽  
Packed Bed ◽  
Two Phase ◽  
Heterogeneous Model ◽  
Fully Implicit ◽  
Cylindrical Reactor

The study of heat transfer in fixed bed tubular reactors of heated or cooled walls has presented great interest by the academy and industry. The adequate and safe design of such equipment requires the use of reliable and realistic mathematical. Unfortunately several studies are restrict to homogeneous model applied to circular and elliptic cylindrical reactors. Then, the objective of this work was to predict heat transfer in packed-bed elliptic cylindrical reactor, by using a proposed heterogeneous model. The mathematical model is composed for one solid phase and another fluid phase, in which the balance equation for each constituent is applied separately. The finite volume method was utilized to solve the partial differential equations using the WUDS scheme for interpolation of the convective and diffusive terms, and the fully implicit formulation. Results of the temperature distribution of the fluid and solid phases along the reactor are presented and analyzed. It was verified that the highest temperature gradients of the phases are located close to the wall and inlet of the reactor.

scholarly journals Effect of Local Thermal Nonequilibrium on the Stability of Natural Convection in an Oldroyd-B Fluid Saturated Vertical Porous Layer

2017 ◽  
Vol 139 (4) ◽  
Author(s):  
B. M. Shankar ◽  
I. S. Shivakumara
Keyword(s):  
Natural Convection ◽  
Solid Phase ◽  
Viscoelastic Fluids ◽  
Maxwell Fluid ◽  
Base Flow ◽  
Temperature Fields ◽  
Thermal Nonequilibrium ◽  
Interphase Heat Transfer ◽  
Local Thermal Nonequilibrium ◽  
The Stability

The effect of local thermal nonequilibrium (LTNE) on the stability of natural convection in a vertical porous slab saturated by an Oldroyd-B fluid is investigated. The vertical walls of the slab are impermeable and maintained at constant but different temperatures. A two-field model that represents the fluid and solid phase temperature fields separately is used for heat transport equation. The resulting stability eigenvalue problem is solved numerically using Chebyshev collocation method as the energy stability analysis becomes ineffective in deciding the stability of the system. Despite the basic state remains the same for Newtonian and viscoelastic fluids, it is observed that the base flow is unstable for viscoelastic fluids and this result is qualitatively different from Newtonian fluids. The results for Maxwell fluid are delineated as a particular case from the present study. It is found that the viscoelasticity has both stabilizing and destabilizing influence on the flow. Increase in the value of interphase heat transfer coefficient Ht, strain retardation parameter Λ2 and diffusivity ratio α portray stabilizing influence on the system while increasing stress relaxation parameter Λ1 and porosity-modified conductivity ratio γ exhibit an opposite trend.

Conjugate Heat Transfer in an Enclosure With Internal Contaminant and Heat Source Using Lattice Boltzmann Method

2010 ◽  
Author(s):  
Salah Hosseini ◽  
Vahid Abdollahi ◽  
Amir Nejat
Keyword(s):  
Heat Transfer ◽  
Mass Transfer ◽  
Fluid Flow ◽  
Lattice Boltzmann Method ◽  
Lattice Boltzmann ◽  
Solid Phase ◽  
Internal Heat ◽  
Fluid Phase ◽  
Conductive Heat ◽  
Boltzmann Method

Conjugate convective-conductive heat transfer in an enclosure is simulated. Internal heat and contaminant sources are included in the fluid flow domain, causing mass transfer within the cavity. The two-dimensional governing equations for natural heat and mass convection in the fluid phase and heat conduction in the solid phase are solved employing lattice Boltzmann method. The effects of Rayleigh number and buoyancy ratio variations on the fluid flow, heat and mass transfer characteristics are studied.

scholarly journals Effect of Magnetohydrodynamics on Heat Transfer Behaviour of a Non-Newtonian Fluid Flow over a Stretching Sheet under Local Thermal Non-Equilibrium Condition

Fluids ◽  
2021 ◽  
Vol 6 (8) ◽  
pp. 264
Author(s):  
Konduru Sarada ◽  
Ramanahalli J. Punith Gowda ◽  
Ioannis E. Sarris ◽  
Rangaswamy Naveen Kumar ◽  
Ballajja C. Prasannakumara
Keyword(s):  
Heat Transfer ◽  
Stretching Sheet ◽  
Magnetic Parameter ◽  
Solid Phase ◽  
Fluid Phase ◽  
High Heat ◽  
Temperature Profiles ◽  
High Heat Transfer ◽  
Two Dimensional Flow ◽  
Non Equilibrium

A mathematical model is proposed to describe the flow, heat, and mass transfer behaviour of a non-Newtonian (Jeffrey and Oldroyd-B) fluid over a stretching sheet. Moreover, a similarity solution is given for steady two-dimensional flow subjected to Buongiorno’s theory to investigate the nature of magnetohydrodynamics (MHD) in a porous medium, utilizing the local thermal non-equilibrium conditions (LTNE). The LTNE model is based on the energy equations and defines distinctive temperature profiles for both solid and fluid phases. Hence, distinctive temperature profiles for both the fluid and solid phases are employed in this study. Numerical solution for the nonlinear ordinary differential equations is obtained by employing fourth fifth order Runge–Kutta–Fehlberg numerical methodology with shooting technique. Results reveal that, the velocity of the Oldroyd-B fluid declines faster and high heat transfer is seen for lower values of magnetic parameter when compared to Jeffry fluid. However, for higher values of magnetic parameter velocity of the Jeffery fluid declines faster and shows high heat transfer when compared to Oldroyd-B fluid. The Jeffery liquid shows a higher fluid phase heat transfer than Oldroyd-B liquid for increasing values of Brownian motion and thermophoresis parameters. The increasing values of thermophoresis parameter decline the liquid and solid phase heat transfer rate of both liquids.

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