Solid Skeleton Research Articles

Peculiarities of Calculating the Thermal Conductivity of Moist Autoclaved Aerated Concrete

The pore space of autoclaved aerated concretes (ACCs), regardless of their apparent density, is represented by large pores formed as a result of gas formation and fine capillary pores. With the free absorption of liquid moisture, only the capillary pores are filled. Large pores contain vapor-air mixture. Considering such a bimodal pore size distribution, it is proposed to determine the thermal conductivity of moist ACC in two stages. First, an inhomogeneous ternary system that consists of a solid skeleton with a fine porous structure containing gas and liquid moisture should be considered. Then a binary system is taken into account, the first component of which is the mentioned ternary system, and the other component of which consists of isolated gas inclusions in large pores. To determine the thermal conductivity of the ternary system, the dependences constructed using the theory of generalized conductivity were used. The thermal conductivity of the binary system was calculated using well-known formulas. It was found that the results of thermal conductivity calculations based on the proposed two-step method deviate from the experimental data in the range from −7.29 to +5.75%, with an average deviation of −0.71%. With the one-step calculation method (assuming a unimodal pore distribution), the spread of analogous data ranges from −30.72 to −21.98%, with an average deviation of −26%.

Mesoscopic Mechanism of Permeability of Coal Rock Mass with Increasing Temperature in the Range of Room Temperature to 350°C

Engineering research on geothermal development, underground gasification of coal, and heat-injection-enhanced coal bed gas extraction is gaining more and more attention from the international community, and therefore the study of permeability of coal rock bodies under the effect of temperature has become almost a hot spot in the research of rock mechanics and seepage mechanics. However, the relationship between temperature and permeability that has been seen in the literature is different, and the mechanism explanation varies greatly. In this paper, the following conclusions were obtained from an experimental study of the fine-scale structural evolution of coking coal and fine sandstone specimens, using an experimental research method of simultaneous image observation of the pore structure evolution of coal rock samples by online heating. (1) With the increase of temperature, the inner areas of coal and rock mass with both solid particles and pure pores are affected by temperature. The microscopic experiment shows that the gray level of the image changes greatly, that is, the changes in pores are also large. These pores are the roar pores in the coal and rock mass. (2) With the increase in temperature, the solid skeleton of the coal specimen will produce expansion deformation. On the one hand, this expansion deformation will increase the pores between some skeletons and increase the overall porosity of the specimen. On the other hand, it will also reduce the pore area and reduce the overall porosity of the specimen due to the intrusion of the solid skeleton into the adjacent pores. These two phenomena occur at the same time with the increase in temperature. The dominant mode is determined by the type of coal. The physical structure and temperature of the section are affected jointly. (3) When the temperature increases, the porosity of coking coal samples increases first and then decreases, and 180°C is the turning point. The fine sandstone sample shows the law of decreasing first and then increasing, and 210°C is the turning point. (4) When the temperature increases, the smaller the porosity of coal and rock samples, the specimen shows the intrusion of the solid skeleton into adjacent pores, that is, the porosity decreases. After the turning temperature, the porosity increases with the increase of temperature.

Phase transition in monodisperse granular materials: How to model it by using a strain hardening visco‐elastic‐plastic constitutive relationship

AbstractDuring heating (loading characterised by a progressive increase in strain rate) and cooling (loading characterised by a progressive decrease in strain rate) numerical tests, performed by using Discrete Element codes, granular materials experience phase transition phenomena, named in this paper ‘dry liquefaction’ (when from solid the material starts behaving like a fluid) and ‘dry resolidification’ (freezing, when the material from fluid starts behaving like a solid). The aim of this paper consists in reproducing phase transition phenomena by using a strain hardening visco‐elastic‐plastic model based on the critical state concept and kinetic theories of granular gases. The authors demonstrate that crucial is the role of isotropic softening/hardening, which describes the size of the elastic domain and the capability of the solid skeleton of storing elastic energy according to permanent force chains. The main ingredients of the model are: (i) the additivity of quasi‐static and collisional stresses, (ii) the energy balance equation governing the evolution of the granular temperature, interpreted this latter as an additional internal variable for the system for the collisional contribution, (iii) the mixed isotropic and kinematic hardening characterising the quasi‐static incremental constitutive relationship. The model has been both calibrated and validated on Discrete Element Method (DEM) numerical results, obtained by testing dry assemblies of monodisperse spheres under true triaxial loading conditions.

Enhanced oxygen discharge with structured mesh channel in proton exchange membrane electrolysis cell

The oxygen discharge in the proton exchange membrane electrolysis cell (PEMEC) has a great influence on the performance because it hinders the liquid water supply from channel to the catalyst layer (CL). In this study, a novel structured mesh channel is proposed to enhance the oxygen discharge capacity in PEMEC, in which the solid skeleton structure facilitates oxygen bubble splitting, and the structural capillary force effect promotes the oxygen detach the liquid/gas diffusion layer (L/GDL) surface. The gas and liquid two-phase flow characteristics in structured mesh channel and conventional straight hollow channel are compared in detail using the volume of fluid (VOF) model, which is subsequently integrated with a three-dimensional (3D) multiphase full-cell model through data exchange to simulate the influence of oxygen in channel on the PEMEC performance. Note that the reliability of this integration method is validated against the experimental oxygen distribution in parallel and serpentine flow fields and the electrochemical performance simultaneously. It is also found that reducing the skeleton surface hydrophobicity (i.e. contact angle) gradually from the channel top to L/GDL surface helps improve the oxygen discharge capacity and hence the PEMEC performance.

Chloride diffusivity of pure cement paste

A two-step scheme is presented for estimating the chloride diffusivity of pure cement paste. In the scheme, gel pores are included in the solid skeleton, composed of calcium hydroxide, unhydrated cement and calcium silicate hydrate, while capillary pores and the solid skeleton constitute pure cement paste. In combination with percolation theory, the effective medium method was modified to formulate the chloride diffusivities of the solid skeleton and pure cement paste sequentially. Detailed analysis was conducted to calibrate the percolation exponent for the high-diffusivity phase and the chloride diffusivity of the pore solution. Comparisons with various test results were made to verify the proposed scheme. Unlike existing analytical methods, the physical properties, volume fractions and morphological relationships of various phase constituents and the shapes, percolation behaviour and different roles of gel and capillary pores are all taken into account simultaneously. The calibrated chloride diffusivity of the pore solution was found to be considerably close to that derived by molecular dynamics simulation, suggesting that the scheme represents a more realistic pure cement paste. The scheme can reasonably evaluate the chloride diffusivity of pure cement paste at medium and long curing ages.

Amplification of seismic wave motions in partially fluid-filled poroviscoelastic media

This paper presents a simple and an efficient formulation of wave motions in partially saturated poroviscoelastic soil media. The viscoelastic property of the solid skeleton is incorporated into the elastic moduli via a simple relationship between the skeleton damping and the relaxation time established based on the Kelvin-Voigt model. The proposed formulation is adaptable to various soil configurations. It has been successfully applied to some vibrational problems of nearly-saturated soil columns made of different materials and carrying different loadings. The consideration of the skeleton damping, which does not receive a sufficient attention in previous works unlike the Biot flow - induced damping, significantly reduced the motion amplification amplitudes as well as the accelerations and Fourier spectra amplitudes at the surface of actual sites. As a useful application of the present formulation, the motions obtained at the ground surface of actual sites have been applied as inputs to a wind turbine to obtain its response in the time domain. The maximal relative displacements of the turbine were substantially reduced due to the skeleton damping. The vertical response of the turbine still negligible compared to the horizontal response.

Mechanical properties of unidirectional nanoporous gold under compression

Dealloyed nanoporous (NP) metals are expected to be stronger by more than one order of magnitude than conventional porous metals of same porosity. However, the strengths of random-structured NP metals in practice are limited by the low mechanical efficiency of their network structure. In this paper, we report that an anisotropic nanoporous gold (NPG) with unidirectional pore channels can also be self-organized in dealloying. This material combines a mechanically efficient topology structure with ultra-strong nanoscale solid skeleton. In consequence, its out-of-plane mean strength (461 ± 52 MPa) is more than twice that of the strongest NPGs reported in previous studies. Because of the structural anisotropy, unidirectional NPGs deform by the formation and expansion of kink bands under compression, unlike the uniform densification of most previous NPGs. The pore-channel misalignment and other defects such as the voids in pore walls may have reduced macroscopic strength, which accounts for a large scatter in the measured strength and a gap between theoretical and experimental data. We anticipate that the structure and mechanical properties of unidirectional NPGs will be further improved in the future by optimizing dealloying parameters. A combination of high strength, unidirectional pore channels, and excellent thermal and electrical conductivity of metallic pore-wall phase might enable novel applications of this material in many areas.

Use of recycled gypsum in the cement-based stabilization of very soft clays and its micro-mechanism

This paper presents an experimental study and micro-mechanism discussion on gypsum role in the mechanical improvements of cement-based stabilized clay (CBSC). A soft marine clay at two initial water contents (i.e. 50% and 70%) was treated by reconstituted cementitious binders with varying gypsum to clinker (G/C) ratios and added metakaolin to facilitate the formation of ettringite, followed by the measurements of final water contents, dry densities and strengths in accordance with ASTM standards as well as microstructure by mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Results reveal that the gypsum fraction has a significant influence on the index and mechanical properties of the CBSC, and there exists a threshold of the G/C ratio, which is 10% and 15% for clays with 50% and 70% initial water contents, respectively. Beyond which adding excessive gypsum cannot improve the strength further, eliminating the beneficial role. At these thresholds of the G/C ratio, the unconfined compressive strength (UCS) values for clays with 50% and 70% initial water contents are 1.74 MPa and 1.53 MPa at 60 d of curing, respectively. Microstructure characterization shows that, besides the common cementation-induced strengthening, newly formed ettringite also acts as significant pore infills, and the associated remarkable volumetric expansion is responsible, and may be the primary factor, for the beneficial strength gain due to the added gypsum. Moreover, pore-filling ettringite also leads to the conversion of relatively large inter-aggregate to smaller intra-aggregate pores, thereby causing a more homogeneous matrix or solid skeleton with higher strength. Overall, added gypsum plays a vital beneficial role in the strength development of the CBSC, especially for very soft clays. • Gypsum fraction has a significant influence on the index and mechanical properties of the CBSC. • Optimal G/C ratio determined in CBSC was obviously greater than that of mortar or concrete. • Ettringite-induced filling and cementation are responsible for the improved mechanical performance of the CBSC. • The abnormal beneficial role of gypsum is closely associated with the increase in the density. • Design guideline for cementitious binders that are only applicable to soft clays was re-established.

A finite strain poroviscoelastic model based on the logarithmic strain

In this research, we present a theory of finite deformation poroviscoelasticity. The proposed theory is based on the logarithmic strain and a quadratic free energy function. As an illustration, we apply the theory to the study of the consolidation of a finite porous layer. To show the viscous effect of solid skeleton, we compare the results obtained from poroviscoelasticity with those from poroelasticity. Moreover, we provide the finite element formulation for the one-dimensional problem considered, which can however, be extended straightforward for two- and three-dimensional problems.

Dynamics of plates resting on layered transversely isotropic poroelastic media under moving loads

This paper investigates the dynamic interaction between a plate and multi-layered transversely isotropic (TI) hysteretically damped poroelastic media under a moving rectangular load. Following Kirchhoff's thin plate theory, the plate deflection is derived utilizing the Fourier series expansion and the integral transform technique. The governing equation for the layered TI hysteretically damped poroelastic media is formulated through the standard hysteretic damping model and Biot's theory. The solution for the plate-soil dynamic interaction in the wavenumber domain considering the plate-soil interface condition is obtained by the extended precise integration method (PIM) for the layered TI saturated porous media. This solution can be further converted into the spatial domain by the inverse transform technique. We have verified the efficiency and accuracy of the proposed method. Several numerical examples are presented to investigate the influences of the plate thickness, plate modulus ratio, surface layer, speed of the moving load, and hysteretic damping of the solid skeleton on the dynamic behaviors of the plate on layered TI poroelastic media.

Equations for Calculating the Thermal Conductivity of Capillary-Porous Materials with over Sorption Moisture Content

This article is the result of the authors’ work on the method of calculating the effective thermal conductivity of moist capillary-porous materials used in wall partitions. The proposed methodology was developed based on the theory of generalized conductivity and geometric modeling of the structure. Materials are considered as heterogeneous ternary systems consisting of a solid skeleton, gas and liquid, and all components are simultaneously taken into account in the calculation. In this work, additional equations are constructed that allow calculation of the effective thermal conductivity of capillary-porous materials with over sorption moisture, thus extending the scope of application of the method to the entire possible range of changes in moisture content. The details of calculating the thermal conductivity of moist capillary-porous materials are demonstrated using experimental data for wall ceramics samples. It is shown that the proposed equations are capable of predicting the thermal conductivity of moist capillary-porous materials with sufficient accuracy. Knowledge of the thermal conductivity of materials in conditions of their actual moisture is fundamental in the sustainable design of new buildings as well as thermo-renovation and dump protection of historic ones.

Drying-induced stresses in poroelastic drops on rigid substrates.

We develop a theory for drying-induced stresses in sessile, poroelastic drops undergoing evaporation on rigid surfaces. Using a lubrication-like approximation, the governing equationsof three-dimensional nonlinear poroelasticity are reduced to a single thin-film equationfor the drop thickness. We find that thin drops experience compressive elastic stresses but the total in-plane stresses are tensile. The mechanical response of the drop is dictated by the initial profile of the solid skeleton, which controls the in-plane deformation, the dominant components of elastic stress, and sets a limit on the depth of delamination that can potentially occur. Our theory suggests that the alignment of desiccation fractures in colloidal drops is selected by the shape of the drop at the point of gelation. We propose that the emergence of three distinct fracture patterns in dried blood drops is a consequence of a nonmonotonic drop profile at gelation. We also show that depletion fronts, which separate wet and dry solid, can invade the drop from the contact line and localize the generation of mechanical stress during drying. Finally, the finite element method is used to explore the stress profiles in drops with large contact angles.

Effect of flow-independent viscosity on the propagation behavior of Rayleigh wave in partially saturated soil based on the fractional standard linear solid model

Based on the mixture theory, considering the flow-independent viscosity related to solid skeleton, the present work investigates the propagation characteristics of Rayleigh wave in partially saturated viscoelastic soil. Firstly, the complex history-dependent viscoelastic behavior of solid skeleton is characterized by the fractional standard linear solid constitutive model. The generalized governing equations of motion for the partially saturated viscoelastic soil are established theoretically. Secondly, the analytical solution of Rayleigh wave fields in the partially saturated viscoelastic soil is obtained using the wave function expansion method. Finally, the influences of flow-independent viscosity characterized by fractional viscoelastic parameter on the propagation behaviors of Rayleigh wave are implemented analytically and then discussed in detail. The results show that the fractional index, stress relaxation time and strain relaxation time in the fractional standard linear solid constitutive model have significant influence on the phase velocity and attenuation coefficient of Rayleigh wave. The fractional index, stress relaxation time and strain relaxation time have similar effects on the P1 and S waves included in body waves. Additionally, the influences of the fluid hydraulic conductivity and liquid saturation on the wave propagation cannot be ignored.

Experimental Study of 3D Micro-CT on Meso-Structure Evolution of Coal Samples with Different Coal Grades under the Action of Temperature

In the three-dimensional micro-CT experiment system, the room temperature is set to 300°C when different coal samples (lignite, anthracite, lean coal and gas coal) are observed for mesoscopic observation. The evolution regularity of mesoscopic structure is analyzed according to the CT scan of coal samples under different temperatures and three sections of scanning images, and by ImageJ image processing software, image processing, and analysis of the characteristics of the profile, the following conclusions are obtained: (1) Coal specimen will have an overall expansion deformation along with the rise of temperature. The sample expansion can be divided into two types: outward expansion and inward expansion. Outward expansion means that the expansion of the skeleton extends outward from the adjacent pores, while inward expansion means that the solid skeleton intrudes into the adjacent pores. When the temperature rises, the outward expansion and inward expansion occur simultaneously. The dominant expansion mode is influenced by the type of coal sample and the temperature value. (2) With the increase of temperature, coal and anthracite coal specimen pore fissure structure shows an expansion tendency before contraction, while gas coal and lean coal show reverse patterns; in addition to the above the reason of the difference vitrinite differences, one must also consider selected specimen original porosity and mechanical physical properties, such as a combination of other factors. (3) In the temperature range of 100–200°C, when the temperature increases at the same rate, lignite porosity increases the most, followed by gas coal, lean coal, and anthracite. (4) There are certain differences in the variation trends of the pixel proportions of the three sections of the coal specimen, and the temperature values of the three curves at the maximum pixel point are also different, which indicates that the expansion of each point in the coal specimen with the change of temperature is not completely synchronous, and the physical and mechanical properties of the sample are heterogeneous.

A finite element-thin layer element coupling method for near-field wave propagation in fluid-saturated porous media based on u-U dynamic formulation

A finite element-thin layer element coupling method for the near-field wave propagation analysis in the fluid-saturated porous media (FSPM) is developed based on the Biot u-U dynamic formulation. The case of the wave propagation normal to the boundary of calculating domain is investigated. The wave propagation equations are decoupled by the variables-separating method. The finite element method is adopted for the space discretization and numerical analysis of the finite domain, and the thin layer element method is applied to discretize the infinite domain and construct the consistent artificial boundary condition. This method is validated and then applied to the analysis to the near-field wave propagation problem of FSPM. Parametric sensitivity studies are conducted to analyze the effect of the mechanical parameters, including permeability coefficient, porosity, and Young's modulus of solid skeleton, on the dynamic response of FSPM. The study results indicate the effectiveness and efficiency of the developed method to the near-field wave propagation analysis in FSPM.

Fluid–fluid phase separation in a soft porous medium

Various biological and chemical processes lead to the nucleation and growth of non-wetting fluid bubbles within the pore space of a granular medium, such as the formation of gas bubbles in liquid-saturated lake-bed sediments. In sufficiently soft porous materials, the non-wetting nature of these bubbles can result in the formation of open cavities within the granular solid skeleton. Here, we consider this process through the lens of phase separation, where thermomechanics govern the separation of the non-wetting phase from a fluid–fluid–solid mixture. We construct a phase-field model informed by large-deformation poromechanics, in which two immiscible fluids interact with a poroelastic solid skeleton. Our model captures the competing effects of elasticity and fluid–fluid–solid interactions. We use a phase-field damage model to capture the mechanics of the granular solid. As a model problem, we consider an initial distribution of non-wetting fluid in the pore space that separates into multiple cavities. We use simulations and linear-stability analysis to identify the key parameters that control phase separation, the conditions that favour the formation of cavities, and the characteristic size of the resulting cavities.

Evaluation of the thermal conductivity of sandstone solid skeleton using the concept of equivalent microstructure

Evaluation of the thermal conductivity of sandstone solid skeleton using the concept of equivalent microstructure

Modelling unsaturated soils with the Material Point Method. A discussion of the state-of-the-art

Many natural hazards involve large deformations of unsaturated soils, e.g. rainfall-induced landslides, embankment collapses due to wetting, seepage-induced instabilities of dams and levees, etc. The study of these phenomena requires accounting for the complex hydro-mechanical interactions between solid skeleton and pore fluids and modelling large deformations to predict the post-failure behaviour, which poses significant computational challenges. In recent years, several hydro-mechanical coupled MPM formulations were developed to model saturated and unsaturated soils. These approaches are slightly different in terms of governing equations, integration schemes and have been implemented in different MPM software; thus, they benefit from various computational features. The purpose of this paper is to present an overview of the available MPM approaches to model unsaturated soils discussing differences and similarities of the formulations and their impact on the results under different conditions in a range of geotechnical applications. In addition, the effect of partially saturated conditions on the critical time step in explicit numerical integration schemes is studied for the first time. Different analytical expressions are derived and compared with the numerical results.

Modeling fracture propagation in a rock‐water‐air system with the assumed enhanced strain method

AbstractThis article presents a fully coupled finite element method for the single cohesive fracture propagation in a deformable porous medium (rock) interacting with two immiscible pore fluids, that is, water and air. The cohesive fracture is represented with the assumed enhanced strain method which can be cast into a classical plasticity framework. The governing equations for the solid skeleton deformation, pore water flow, and pore air flow are derived based on the mixture theory. The coupled nonlinear global system is then solved by the standard Galerkin finite element method. The numerical implementation is verified against the numerical and analytical solutions previously reported in the literature. The proposed model is capable of capturing fluid transfer between the fracture and the host rock due to the pressure difference. The numerical examples demonstrate that the behavior of fracture propagation can be significantly influenced by the capillary pressure inside fracture and vice versa. Since the fracture is represented at the level of Gauss points, the proposed numerical model can be easily implemented on any standard finite element code base.

Simulation of heat and fluid flow in porous medium and fractures by material point method

PurposeThe material point method (MPM)is a particle-based numerical method suitable for solid–liquid simulation and large deformation problems. However, MPM is generally used in solid deformation at present, to develop a multi-physics coupling MPM; the purpose of this study is to extend the MPM to simulate the heat and fluid flow and address the thermal-hydrological (TH) coupling problems.Design/methodology/approachThe porous medium was discretized into two sets of Lagrangian points, and the motion of fluid points follows the Darcy’s law. Two sets of heat transport equations were established for the heat conduction and heat exchange in the pore fluid and solid skeleton. Fractures were considered by adding the porosity gradient term in the governing equations; also a transition function was introduced to smoothen the fracture boundary.FindingsFour cases of heat and fluid flow in porous medium and fractures were presented to verify the feasibility of the proposed method. And the effects of fractures on heat and fluid flow were investigated. Additionally, a case of geothermal extraction was solved and the importance of the interstitial convective heat transfer coefficient was analyzed.Originality/valueThe proposed method extends the conventional MPM, using two sets of material points and two sets of heat transport equations to simulate the heat and fluid flow and address the TH coupling problems, which can be applied in both porous medium and fractures.