Natural convective boiling in vertical annular tubes filled with porous medium

Experimental and theoretical studies were carried out on the natural convective boiling heat transfer and critical heat flux (CHF) in uniformly heated vertical annular tubes filled with a porous medium and submerged in saturated water and R11 liquid. The heat transfer experimental results were compared with the case without a porous medium. It was shown that heat transfer is greatly enhanced by the porous medium in the region of low heat flux. By adopting a simple mixing flow model, a generalized approximate relationship was derived for predicting the CHF. The prediction agrees relatively well with the CHF experimental data. © 2000 Scripta Technica, Heat Trans Asian Res, 29(6): 447–458, 2000     

Subcooled flow boiling and microbubble emission boiling phenomena in a partially heated microchannel

A simultaneous visualization and measurement study has been carried out to investigate subcooled flow boiling and microbubble emission boiling (MEB) phenomena of deionized water in a partially heated Pyrex glass microchannel, having a hydraulic diameter of 155 μm, which was integrated with a Platinum microheater. Effects of mass flux, inlet water subcooling and surface condition of the microheater on subcooled flow boiling in microchannels are investigated. It is found that MEB occurred at high inlet subcoolings and at high heat fluxes, where vapor bubbles collapsed into microbubbles after contacting with the surrounding highly subcooled liquid. In the fully-developed MEB regime where the entire microheater was covered by MEB, the mass flux, the inlet water subcooling and the heater surface condition have only small effects on the boiling curves. The occurrence of MEB in microchannel can remove a large amount of heat flux, as high as 14.41 MW/m² at a mass flux of 883.8 kg/m² s, with only a moderate rise in wall temperature. Therefore, MEB is a very promising method for cooling of microelectronic chips. Heat transfer in the fully-developed MEB in the microchannel is presented, which is compared with existing subcooled flow boiling heat transfer correlations for macrochannels.     

Analytical determination of permeability of porous fibrous media with consideration of electrokinetic phenomena

Flow behavior in porous fibrous media with consideration of electrokinetic phenomena is investigated based on a linearized Poisson–Boltzmann equation and Navier–Stokes equation. An analytical solution of effective permeability of porous fibrous media as functions of porosity, dimensionless local averaging net charge density and dimensionless electric resistance number is derived in this paper. The influences of the electrokinetic phenomena can be measured by the dimensionless electric resistance number, which is proportional to the square of the liquid dielectric constant, the solid surface Zeta potential and inversely proportional to the liquid dynamic viscosity, electric conductivity and the square of the maximum pore radius. The analytical results show that when the dimensionless electric resistance number is small and the porosity is large, the dimensionless total flow rate shows a nearly uniform distribution. When the dimensionless electric resistance number is large, the resistant effects of the electrical double layer (EDL) become so significant that the superficial velocity decreases. The effective permeability of the porous fibrous media decreases correspondingly. Furthermore, the theoretical predicted effective permeability values are compared with experimental data, and good agreement is observed between the two. It shows that the mathematical model for the effective permeability of porous fibrous media with consideration of electrokinetic phenomena is satisfactory.     

The rate of heat transfer for boiling on porous surfaces

It is shown that the constant, C_sf, introduced by Rohsenow⁶ for correlating pool boiling heat transfer data, may be used in order to characterize plates which have been made porous.     

The onset of convection in a tridisperse porous medium

This paper develops a theory of mass, momentum, and heat transfer in a tridisperse porous medium. Coupling between three different scales present in this medium is accounted for by introducing momentum and interphase heat transfer coupling coefficients. The developed theory is then applied to solve the classical Rayleigh–Bénard problem, for the onset of convection in a horizontal layer uniformly heated from below, for this new type of a porous medium. The formulation uses the Darcy law, which now results in three different filtration velocities in three porosity scales present in this medium. The linear stability analysis leads to an expression for the critical Rayleigh number as a function of three volume fractions, two permeability ratios, two thermal capacity ratios, two thermal conductivity ratios, two inter-phase heat transfer parameters and two inter-phase momentum transfer parameters. The dependence of the critical Rayleigh number on these parameters is investigated.     

Mathematical modelling of silica deposition in a porous medium

The problem of silica deposition in packed columns is considered. Various deposition rate models are investigated and the method of characteristics is used to obtain an analytic solution for the special case where the flow rate through the column of porous material is constant. The theoretical results are compared with measured data from published experimental work. It is found that the dual regime model suggested by Fleming (1986) gives the best match of theory and experiment. The techniques developed can be used to investigate deposition near a reinjection well or in an approximately one-dimensional fractured zone connecting production wells and reinjection wells.     

One-domain approach for heat transfer between a porous medium and a fluid

The purpose of this work is to investigate the extents of the local thermal equilibrium (LTE) assumption at the fluid-porous medium boundary (i.e., in a heterogeneous region). The analysis is performed in terms of the one-domain approach. Therefore, we posed and solved the associated closure problems in order to compute the spatial dependence of the effective coefficients at the fluid-porous medium boundary. Steady-state comparisons with direct numerical simulations evidence that the LTE is, in general, justifiable everywhere in the system, i.e., in both homogeneous and heterogeneous regions.     

Gas Flow in a Stratified Porous Medium with Crossflow

IntroductionMany ..servoirs are stratified in nature due to thevarying energy level of the original depositionaIenvironment. The transporting medium softs the sourcematerial depositing one layer at a time in any givenenvironment. When the depositionaI energy levelschange, the subsequent overiying layers may be differentin composition and texture, fOrming a series of dissindlarunits of strata. Such fOrmations are calIed multilayersystems. In many cases some of the layers have such lowpermeab…     

Heat transfer in a conical cylinder with porous medium

In this paper, numerical study of heat transfer in a conical annular cylinder fixed with saturated porous medium is presented. The heat transfer is assumed to take place by natural convection and radiation. The inner surface of conical cylinder is maintained at uniform wall temperature. The governing partial differential equations are non-dimensionalised using suitable non-dimensional parameters and then solved by using finite element method. The porous medium is divided using triangular elements with uneven element size. A computer software is used to solve the coupled momentum and energy equations in an iterative manner. The results are discussed for various values of geometric and physical parameters of porous medium with emphasis on cone angle of the cylinder. It is seen that the cone angle plays a vital role in heat transfer from the hot surface to porous medium.     

Mathematical modelling of moisture desorption in a porous medium

This paper discusses heat and mass transfer in desorption drying. A basic equation system is derived to describe coupled heat and mass transfer in a porous medium with moisture desorption under temperature gradients and a vacuum environment. The desorption mushy zone model is used to obtain an exact solution for coupled heat and mass transfer with a moving desorption mushy zone in a porous half-space. The results are analysed numerically to demonstrate the effects of various parameters on desorption.     

Transport phenomena within the porous cathode for a proton exchange membrane fuel cell

A two-phase, one-dimensional steady model is developed to analyze the coupled phenomena of cathode flooding and mass-transport limiting for the porous cathode electrode of a proton exchange membrane fuel cell. In the model, the catalyst layer is treated not as an interface between the membrane and gas diffusion layer, but as a separate computational domain with finite thickness and pseudo-homogenous structure. Furthermore, the liquid water transport across the porous electrode is driven by the capillary force based on Darcy's law. And the gas transport is driven by the concentration gradient based on Fick's law. Additionally, through Tafel kinetics, the transport processes of gas and liquid water are coupled. From the numerical results, it is found that although the catalyst layer is thin, it is very crucial to better understand and more correctly predict the concurrent phenomena inside the electrode, particularly, the flooding phenomena. More importantly, the saturation jump at the interface of the gas diffusion layer and catalyst layers is captured, when the continuity of the capillary pressure is imposed on the interface. Elsewise, the results show further that the flooding phenomenon in the CL is much more serious than that in the GDL, which has a significant influence on the mass transport of the reactants. Moreover, the saturation level inside the cathode is determined, to a great extent, by the surface overpotential, the absolute permeability of the porous electrode, and the boundary value of saturation at the gas diffusion layer-gas channel interface. In order to prevent effectively flooding, it should remove firstly the liquid water accumulating inside the CL and keep the boundary value of liquid saturation as low as possible.     

An analytical approach for a Hiemenz flow in a porous medium with heat exchange

In this work, a Hiemenz flow established inside a semi-infinite low porosity medium with heat exchange is analyzed analytically. Heat is supplied to the system through two different wall conditions: a constant wall temperature, and a constant wall heat flux. Local thermal non-equilibrium is considered, and the two-equation model is used to consider heat exchange between gas and solid phases. The flow is analyzed through a non-Darcian model, in which viscous and convective terms are considered in the Darcy pressure equation. The results obtained point out the importance of the inner zone (close to the wall) analysis in the case of a constant wall heat flux. From the analysis, a new dimensionless parameter, κ, emerges, which gathers information of the transport phenomena in gas and solid phases, and it is responsible to determine the flow field.     

On a porous medium combustor for hydrogen flame stabilization and operation

In this study, the characteristics of hydrogen flame stabilization in porous medium combustor were investigated. The flame was observed in a quartz tube. The porous medium was oxide-bonded silicon carbide (OB-SiC) or aluminum oxide (Al₂O₃) with 60 PPI and 30 PPI pore size distributions. The results indicated that under a low equivalence operation, the flame would transform from surface combustion to interior combustion with an increased heating value. Under a high equivalence ratio, both interior combustion and flashback transition existed at the same time. The thermal conductivity of silicon carbide is higher than that of aluminum oxide. Thus, interior combustion region was more extensive under a low equivalence ratio operation with a high premixed gas velocity. Flashback was apparent for Al₂O₃ under high an equivalence ratio with low a premixed gas velocity. Consequently, hydrogen flame stability could be controlled by the pore size distribution and thermal conductivity of the porous media, input heating value and input equivalence ratio.     

Effects of a porous medium on the accelerated flow past a vertical porous surface

In the present work we investigate the effects of a porous medium on the accelerated flow past a vertical porous limiting surface. The solution of the problem is obtained in closed form by using the Laplace transform technique.     

Free convection in a wavy cavity filled with a porous medium

The steady-state free convection inside a cavity made of two horizontal straight walls and two vertical bent-wavy walls and filled with a fluid-saturated porous medium is numerically investigated in the present paper. The wavy walls are assumed to follow a profile of cosine curve. The horizontal walls are kept adiabatic, while the bent-wavy walls are isothermal but kept at different temperatures. The Darcy and energy equations (in non-dimensional stream function and temperature formulation) are solved numerically using the Galerkin Finite Element Method (FEM). Flow and heat transfer characteristics (isothermal, streamlines and local and average Nusselt numbers) are investigated for some values of the Rayleigh number, cavity aspect ratio and surface waviness parameter. The present results are compared with those reported in the open literature for a square cavity with straight walls. It was found that these results are in excellent agreement.     

Thermal instability in a porous medium layer saturated by a nanofluid

The onset of convection in a horizontal layer of a porous medium saturated by a nanofluid is studied analytically. The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis. The analysis reveals that for a typical nanofluid (with large Lewis number) the prime effect of the nanofluids is via a buoyancy effect coupled with the conservation of nanoparticles, the contribution of nanoparticles to the thermal energy equation being a second-order effect. It is found that the critical thermal Rayleigh number can be reduced or increased by a substantial amount, depending on whether the basic nanoparticle distribution is top-heavy or bottom-heavy, by the presence of the nanoparticles. Oscillatory instability is possible in the case of a bottom-heavy nanoparticle distribution.