共查询到20条相似文献,搜索用时 15 毫秒
1.
Julia Ott Benjamin Völker Yixiang Gan Robert M. McMeeking Marc Kamlah 《Acta Mechanica Sinica》2013,29(5):682-698
Optimization of composition and microstructure is important to enhance performance of solid oxide fuel cells (SOFC) and lithium-ion batteries (LIB). For this, the porous electrode structures of both SOFC and LIB are modeled as a binary mixture of electronic and ionic conducting particles to estimate effective transport properties. Particle packings of 10000 spherical, binary sized and randomly positioned particles are created numerically and densified considering the different manufacturing processes in SOFC and LIB: the sintering of SOFC electrodes is approximated geometrically, whereas the calendering process and volume change due to intercalation in LIB are modeled physically by a discrete el- ement approach. A combination of a tracking algorithm and a resistor network approach is developed to predict the con- nectivity and effective conductivity for the various densified structures. For SOFC, a systematic study of the influence of morphology on connectivity and conductivity is performed on a large number of assemblies with different compositions and particle size ratios between 1 and 10. In comparison to percolation theory, an enlarged percolation area is found, es- pecially for large size ratios. It is shown that in contrast to former studies the percolation threshold correlates to varying coordination numbers. The effective conductivity shows not only an increase with volume fraction as expected but also with size ratio. For LIB, a general increase of conductivity during the intercalation process was observed in correlation with increasing contact forces. The positive influence of cal- endering on the percolation threshold and the effective conductivity of carbon black is shown. The anisotropy caused by the calendering process does not influence the carbon black phase. 相似文献
2.
Physical mechanisms and influencing factors on the effective stress coefficient for rock/soil-like porous materials are investigated, based on which equivalent connectivity index is proposed. The equivalent connectivity index, relying on the meso-scale structure of porous material and the property of liquid, denotes the connectivity of pores in Representative Element Area (REA). If the conductivity of the porous material is anisotropic, the equivalent connectivity index is a second order tensor. Based on the basic theories of continuous mechanics and tensor analysis, relationship between area porosity and volumetric porosity of porous materials is deduced. Then a generalized expression, describing the relation between effective stress coefficient tensor and equivalent connectivity tensor of pores, is proposed, and the expression can be applied to isotropic media and also to anisotropic materials. Furthermore, evolution of porosity and equivalent connectivity index of the pore are studied in the strain space, and the method to determine the corresponding functions in expressions above is proposed using genetic algorithm and genetic programming. Two applications show that the results obtained by the method in this paper perfectly agree with the test data. This paper provides an important theoretical support to the coupled hydro-mechanical research. 相似文献
3.
Saeid Sadeghnejad Mohsen Masihi Akbar Shojaei Mahmoudreza Pishvaie Peter R. King 《Transport in Porous Media》2012,92(2):357-372
The connectivity of high conductivity pathways in geological formations depend on the spatial distribution of geological heterogeneities
that may appear on various length scales. Appropriate modeling of this is crucial within in hydrology and petroleum systems.
The approach taken in this study is to use percolation theory to quantify the connectivity, hydraulic conductivity, and breakthrough
time behavior between an injector and a producer within such systems. In particular, a three-dimensional overlapping sandbody
model is considered which assumes that the geological formation can be split into either conductive flow units (i.e., good
sands) or non-conductive units (i.e., poor sands). The results are the master curves for the formation connectivity as well
as the hydraulic conductivity and breakthrough time. The percolation approach is then validated against Burgan offshore reservoir
dataset which reveal good matches when compared with the results obtained from computationally expensive conventional methods. 相似文献
4.
O. Yu. Dinariev D. N. Mikhailov 《Journal of Applied Mechanics and Technical Physics》2011,52(1):82-95
The class of models of porous media based on the concept of an ensemble of pores with a certain distribution of the main geometrical
parameters (e.g., pore size) is studied. The case of the saturation of the pore space with a single-phase multicomponent fluid
mixture is studied with and without taking into account the transfer of electric charges. Transfer laws are derived from the
condition of decreasing free energy. The hydrodynamic connectivity of pores (and electrical conductivity) is described by
two kernels: one kernel describes the connectivity of pores in space, and the other describes the connectivity of pores in
the elementary macrovolume. The frequency dependences of the dynamic permeability determined in laboratory experiments and
the electrical conductivity of the porous medium were determined using the concept of an ensemble of pores. The relationship
between the models considered and relaxation filtration models is established. 相似文献
5.
The effective thermal conductivity is an important element in understanding the thermal response to heating of a paper coating,
e.g. during drying in heatset web-offset, and thus it not only affects the drying efficiency but also affects print quality
detriments like web fluting. This study examines both the effective thermal conductivity of liquid-saturated ground calcium
carbonate coating structures as well as the vaporisation behaviour from these structures. The liquids used for saturation
were mineral oil and water in order to resemble ink and fountain solution, respectively, both of which are present in the
traditional heatset web-offset process. The effective thermal conductivities of liquid-saturated coating structures are discussed
in regard to the corresponding unsaturated systems by using a Lumped Parameter Model. It is shown that the liquid saturation
has a dominant effect in determining the effective thermal conductivity. Since this effect is not fully captured by the model,
other mechanisms like an apparent pigment–pigment connectivity increase by liquid bridging and the role of liquid in changing
the contact resistance during the measurement of thermal conductivity are discussed. In addition, the transformation of three-dimensional
structures to an equivalent two-dimensional modelling is evaluated. The vaporisation behaviour of mineral oil and water is
studied by a thermogravimetric analysis. By following the changes in maximum evaporation temperatures and evaporation rates,
the addition of binder is seen to lead to a reduction in the vaporisation rate of both liquids. Since there is little to no
interaction between the liquids and the binder, the confinement caused by the geometry change induced by binder addition is
identified as the mechanism resulting in elevated vapour pressure within the structure. 相似文献
6.
A new stochastic method of reconstructing porous media 总被引:1,自引:0,他引:1
We present a new stochastic method of reconstructing porous medium from limited morphological information obtained from two-dimensional
micro- images of real porous medium. The method is similar to simulated annealing method in the capability of reconstructing
both isotropic and anisotropic structures of multi-phase but differs from the latter in that voxels for exchange are not selected
completely randomly as their neighborhood will also be checked and this new method is much simpler to implement and program.
We applied it to reconstruct real sandstone utilizing morphological information contained in porosity, two-point probability
function and linear-path function. Good agreement of those references verifies our developed method’s powerful capability.
The existing isolated regions of both pore phase and matrix phase do quite minor harm to their good connectivity. The lattice
Boltzmann method (LBM) is used to compute the permeability of the reconstructed system and the results show its good isotropy
and conductivity. However, due to the disadvantage of this method that the connectivity of the reconstructed system’s pore
space will decrease when porosity becomes small, we suggest the porosity of the system to be reconstructed be no less than
0.2 to ensure its connectivity and conductivity. 相似文献
7.
8.
This paper investigates the correlation between the geometry of crack networks and the altered transport properties of cement-based porous materials. Cracks were artificially introduced into slice specimens to obtain bidimensional (2D) crack networks, and the network was characterized by the crack density, orientation, connectivity and crack opening aperture. For the permeability, the water vapor sorption isotherms were measured and an algorithm was established to solve the intrinsic permeability of cracked specimens with the help of moisture transport modeling and the data of drying tests. The electrical conductivity of cracked specimens was measured using an alternative current method. The study on the specimens with percolated cracks shows that: (1) the pertinent geometry parameters for altered transport properties include average-based crack density, crack opening and local crack connectivity; (2) the water permeability of cracked specimens is correlated to the combination \(b^{1.7}\rho f\) and electrical conductivity to \(b^{0.45}\rho f\); (3) the different exponents on the crack opening/length ratio reflect the resistance of tortuosity of crack paths to the water and current flow and this resistance is stronger for current flow. 相似文献
9.
The strong-contrast formulation is used to predict the effective conductivity of a porous material. The distribution, shape and orientation of the two phases are taken into account using two- and three-point probability distribution functions. A new approximation for the three-point probability function appropriate for two-phase media is proposed and discussed. Computed results for the effective conductivity using the strong-contrast formulation are compared to the Voigt and the Hashin-Shtrikman upper-bound estimates. These results show that the predicted effective conductivity is lower than both Voigt and Hashin-Shtrikman bounds. Compared to previous results using the weak-contrast formulation, the strong-contrast formulation seems to provide a better estimate for the effect of the microstructure on the conductivity. 相似文献
10.
Porous media containing gas-filled inclusions embedded in a solid phase constitute an important class of natural or artificial materials of both theoretical and practical interest. In these materials, thermal conductivity is one of the most important properties. In a variety of situations of practical interest, when the characteristic size of gas-filled inclusions is comparable with the mean free path of gas molecules and when the slip flow regime is considered, the behavior of gas near solid surfaces cannot be described by classical thermal conductivity equations. In fact, the boundary conditions at the solid surfaces must be modified by considering that the temperature and normal heat flux simultaneously suffer a discontinuity. The first purpose of the present work is to develop an efficient and accurate micromechanical model capable of estimating the effective conductivity of porous materials while taking into account the discontinuities of the temperature and normal heat flux across solid surfaces and the non-spherical form of gas-filled inclusions. The second purpose of the present work is to study the dependencies of the effective conductivity on the size and shape of gas-filled inclusions. By applying the micromechanical model based on the differential scheme and by using the solution results obtained for auxiliary dilute problem accounting for modified boundary conditions on surface solids, the closed-form expression for the effective conductivity is obtained. Numerical results are provided to illustrate the dependence of the effective conductivity on the size and shape of gas-filled inclusions in the case of randomly oriented inclusions. 相似文献
11.
A methodology is proposed for predicting the effective thermal conductivity of dilute suspensions of nanoparticles (nanofluids) based on rheology.The methodology uses the rheological data to infer microstructures of nanoparticles quantitatively,which is then incorporated into the conventional Hamilton-Crosser equation to predict the effective thermal conductivity of nanofluids.The methodology is experimentally validated using four types of nanofluids made of titania nanoparticles and titanate nanotubes dispersed in water and ethylene glycol.And the modified Hamilton-Crosser equation successfully predicted the effective thermal conductivity of the nanofluids. 相似文献
12.
The magnesium–magnesium hydride–hydrogen-system (Mg–MgH2–H2) offers, because of its combined hydrogen and heat storage capacity, the possibility to design hydride heat pumps and heat stores. For such industrial application systems based on cylindrically formed reactors filled with an active magnesium powder, the effective thermal conductivity limits the time in which the metal hydride alloy is charged and discharged with hydrogen. Determination of this transport coefficient is of fundamental importance for the optimum design of magnesium hydride reactors. The complex interrelation of the different transport mechanisms in a metal hydride packed bed and the hitherto undefined rule that the solid effective thermal conductivity behaves as a function of the hydrogen concentration, requires a reliable and simple-to-realize measuring method so as to determine the effective thermal conductivity of a magnesium hydride bed. In the present study, a report is given for the first time on the initiation of a measuring technique with oscillating change of temperature in a non-permeated packed bed of fine-grained material. The measurement of the effective thermal conductivity can ensue by tailoring the problem-specific mathematical result to the experimentally recorded temperature-time function. The effective thermal conductivity of the magnesium hydride bed varies between 2 and 8 W/(m K) in a temperature range of 523–653 K. 相似文献
13.
Predicting thermal conductivity of liquid suspensions of nanoparticles (nanofluids) based on rheology 总被引:1,自引:0,他引:1
A methodology is proposed for predicting the effective thermal conductivity of dilute suspensions of nanoparticles (nanofluids) based on rheology. The methodology uses the rheological data to infer microstructures of nanoparticles quantitatively, which is then incorporated into the conventional Hamilton-Crosser equation to predict the effective thermal conductivity of nanofluids. The methodology is experimentally validated using four types of nanofluids made of titania nanoparticles and titanate nanotubes dispersed in water and ethylene glycol. And the modified Hamilton-Zrosser equation successfully predicted the effective thermal conductivity of the nanofluids. 相似文献
14.
This paper numerically investigates the fluid flow behavior through single fractures with directional shear dislocations. Synthetic fractures are generated with directional shear dislocations, and the lattice Boltzmann method is used to simulate the fracture flow. With an ignorance of tortuosity effect, a notable overestimation of hydraulic conductivity is observed when the simplified local cubic law is used. During the closure process, the decreasing rate of conductivity is found to be highly related to the roughness of fractures. The conductivity of smoother fractures decreases faster than that of rougher fractures. By conducting simulations on fractures with a constant shear displacement, the effective conductivity is found to vary with the shear directions. The results show that the conductivity of rougher fractures is less sensitive to the shear directions than that of smoother fractures. As fracture surfaces come into contact, a sharp decrease in effective conductivity is observed and the decreasing trend flattens as the contact ratio continues to increase. A new model is proposed based on the bottleneck model to predict the conductivity of sheared fractures. By integrating the tortuosity and channeling effects into the original model, the proposed new model shows a better performance in predicting the conductivity, especially for fractures with rougher surfaces. 相似文献
15.
16.
Edmund Dikow 《Transport in Porous Media》1988,3(2):173-184
A saturated flow problem with spatially varying conductivity is studied in a rectangular domain. An expansion of the flow equation with respect to small perturbations of the conductivity is given. Discrete spectra are used to calculate the expected flux across the outflow boundary and its variability. The results obtained are compared with results based on Monte Carlo studies. Another way to deal with heterogeneous soils is to replace the actual conductivity by a smooth, so-called, effective conductivity. A comparison is made between results based on that approach and our results. 相似文献
17.
18.
This paper presents a fully analytical model for the effective thermal conductivity of two-phase porous media with two-/three-dimensional closed cells, applicable to honeycombs and closed-cell foams. The present model combines an existing analytical expression derived based on the Laplace heat conduction equation with an analytical shape factor which corrects the deviation caused from a non-circular (or non-spherical) pore inclusion. Results demonstrate the validity of the present model capable of analytically estimating the effective thermal conductivity of closed-cell porous media. The simple yet accurate model provides the physical mechanisms of how effective thermal conductivity depends upon the shape of pores. 相似文献
19.
A THEORY OF EFFECTIVE THERMAL CONDUCTIVITY FOR MATRIX-INCLUSION- MICROCRACK THREE-PHASE HETEROGENEOUS MATERIALS BASED ON MICROMECHANICS 总被引:1,自引:0,他引:1
The effective thermal conductivity of matrix-inclusion-microcrack three-phase heterogeneous materials is investigated with
a self-consistent micromechanical method (SCM) and a random microstructure finite element method(RMFEM). In the SCM, microcracks
are assumed to be randomly distributed and penny-shaped and inclusions to be spherical, the crack effect is accounted for
by introducing a crack density parameter, the effective thermal conductivity is derived which relates the macroscopic behavior
to the crack density parameter. In the RMFEM, the highly irregular microstructure of the heterogeneous media is accurately
described, the interaction among the matrix-inclusion-microcracks is exactly treated, the inclusion shape effect and crack
size effect are considered. A Ni/ZrO2 particulate composite material containing randomly distributed, penny-shaped cracks is examined as an example. The main results
obtained are: (1) the effective thermal conductivity is sensitive to the crack density and exhibits essentially a linear relationship
with the density parameter; (2) the inclusion shape has a significant effect on the effective thermal conductivity and a polygon-shaped
inclusion is more effective in increasing or decreasing the effective thermal conductivity than a sphere-shaped one; and (3)
the SCM and RMFEM are compared and the two methods give the same effective property in the case in which the matrix thermal
conductivity λ1 is greater than the inclusion one λ2. In the inverse case of λ1 < λ2, the two methods agree as the inclusion volume fraction and crack density are low and differ as they are high. A reasonable
explanation for the agreement and deviation between the two methods in the case of λ1 < λ2 is made.
This work was supported by the National Natural Science Foundation of China and Chnese “863” High-Tech, Program. 相似文献
20.
The assumption of constant local coefficients is one of the first restrictions in most of the smoothing theories for transport in porous media. In this paper we present a formal analysis of the effects produced by nonconstant local transport coefficients on the nonlinear behavior of the effective transport properties. In particular, we use the volume averaging method to study heat transport in a two-component system considering the local thermal conductivities as analytical functions of the temperature. Within this approach we obtain a general expression for the effective nonlinear thermal conductivity dependence on the averaged temperature gradient. The important result is that the effective conductivity is obtained by a linearly bounded problem (the closure problem), just as if the conductivities were constants, by replacing the constant conductivities by the actual temperature dependent ones. As an example, we model the porous medium as cylindrical inclusions in a periodic array and solve the closure problem for the case of the one-equation model. We analyze the values of the second derivative of the thermal conductivity with respect to the temperature to establish the range where the nonlinear corrections must be considered to correctly describe the effective transport. 相似文献