Phase Change In Porous Media Research Articles

Morphology Characteristics of the Liquid–Vapour Interface in Porous Media

The evolution of the liquid–vapour interface plays a crucial role in multiphase flow, heat and mass transfer, and fluid phase change in porous media. A thorough investigation of the interface under varying degrees of saturation is necessary and crucial to fully understanding the key mechanism of soil water evaporation. The pore voids and fluids are characterized using X-ray microtomography and image processing. Salt solutions usually replace pure water for better contrast and image development. Machine learning algorithms were employed to identify and extract the different phase and their interface accurately. Then, variations in the geometrical and topological features of the interface at varying saturation during evaporation were analysed to quantitatively describe the connectivity of the liquid phase and the morphological change in the liquid–vapour interface. Topological analysis reveals that normalized Euler characteristic numbers quantify the complementary connectivity of liquid and vapour phase. The curvatures of the liquid–vapour interface of the samples under various saturations classify the liquid–air interface curvature of samples under various saturations for quantitatively describing the migration progress and quantity distribution of typical interface along with drying.

Characterization of Liquid‐Vapor Interfaces in Pores During Evaporation

AbstractThe evolution of the liquid‐vapor interface plays a key role in multiphase flow, heat and mass transfer, and fluid phase change in porous media. In the soil water evaporation process, the vaporization occurs only on the liquid‐vapor interfaces rather than the apparent soil surface. Yet, the interfaces evolve with high distortion and great complexity along the drying path. Hence the microscale characteristics of interfaces especially geometrical and topological features in soil water evaporation are barely investigated. In this work, we scanned glass bead samples using X‐ray micro tomography and scrutinized the development of liquid‐vapor interfaces with different degrees of saturation. The liquid‐vapor interfaces are identified by morphological operations and extracted using the watershed segmentation technique. The liquid phase disperses into individual ganglia and distributes wildly in the pores as the saturation decreases, leading to low specific interface areas at saturated and dry state but a maximum value at a threshold saturation around 30%. The topological analysis reveals that the liquid and vapor phases present complementary connectivity behaviors quantified by normalized Euler characteristic numbers. The local mean curvature distribution of each typical individual interface cluster quantitatively describes the intricate progression of interface geometry and morphology along with drying. The overall mean curvature evolution of the sample separates the negative curvature component and confirms the capillary pressure increase during the pore water evaporation. The interfacial area and curvature analysis provide a cornerstone to determine the authentic interfacial evaporation rate for the soil under drying.

Salt weathering of sandstone under dehydration and moisture absorption cycles: An experimental study on the sandstone from Dazu rock carvings

AbstractSalt crystallization and phase change in porous media cause significant deterioration of cultural heritage made of stones and bricks. In this study, laboratory experiments on salt weathering of sandstone from the Dazu rock carvings (DRC) in Chongqing, southwest China, were carried out by exposing them to cycles of temperature and humidity change. Mirabilite, the main component of the salts in DRC that readily undergoes phase change, was utilized for the study. Three groups of Dazu sandstone (DS) samples with high (2.4 mol/L), moderate (1.5 mol/L) and low concentrations (1.0 mol/L) of sodium sulphate were prepared; and two conditions, high‐temperature with low‐humidity and low‐temperature with high‐humidity, were designed to simulate the fluctuations of temperature and humidity in the Dazu area. After several cycles, the dehydration and moisture absorption of DS under these two conditions forces the phase change of sodium sulphate between thenardite (Na2SO4) and mirabilite (Na2SO4·10H2O), causing different levels of damage. A combination results of sample length, weight, microstructure, and mechanical property variations show that the damage effect of the thenardite formed during dehydration is more significant than that of mirabilite in moisture absorption. During these cycles, there is noticeable salt migration and the local concentration of salts is influenced by the microstructure of the stone and the distribution of clay minerals. Samples with high salt content show disintegration of the stone into powder, while those with moderate salt concentration show flaking and salt accumulation on the surface. These types of damage correspond with observations of natural weathering in the field. The experiment shows that no obvious damage takes place when the salt content is lower than 0.67%. These results contribute to a better understanding of the effects of salt weathering on sandstone and provide references for choosing correct prevention and protection measures for the world heritage site DRC.

Modeling and Numerical Investigation of Transient Two-Phase Flow with Liquid Phase Change in Porous Media.

Two-phase flow with phase change in microstructure or nanostructure is an important issue in many fronts and critical applications nowadays, but with a lack of comprehensive understanding of the mechanism. This paper numerically investigates the transient behavior of two-phase flow with liquid phase change in the porous media, which consists of a series of connected pores at micro and nanoscale with the transient form of the semi-mixed model and self-compiled programs. Transient variation and spatial distribution of structure temperature, thermal non-equilibrium characteristic, phase change location and fluid-driven pressure are obtained and analyzed, and effects of initial system temperature, structure parameter and material property on the transient behaviors of two-phase flow and fluid-structure coupling heat transfer are discussed. The numerical simulations indicate that the two-phase flow with phase change in porous media is complex and ever-changing before reaching a steady state and affected by the above-mentioned three kinds of parameters significantly. Particularly, distinct phenomena of transient heat transfer deterioration and vapor block are discovered, and it is revealed that the transient heat transfer deterioration and vapor block are more serious in a porous matrix with smaller porosity and made of materials with higher heat capacity and density.

Fully Implicit Dynamic Pore‐Network Modeling of Two‐Phase Flow and Phase Change in Porous Media

AbstractDynamic pore‐network model (PNM) has been widely used to model pore‐scale two‐phase flow. Numerical algorithms commonly used for dynamic PNM including IMPES (implicit pressure explicit saturation) and IMP‐SIMS (implicit pressure semi‐implicit saturation) can be numerically unstable or inaccurate for challenging flow regimes such as low capillary number (Ca) flow and unfavorable displacements. We perform comprehensive analyses of IMPES and IMP‐SIMS for a wide range of flow regimes under drainage conditions and develop a novel fully implicit (FI) algorithm to address their limitations. Our simulations show the following: (1) While IMPES was reported to be numerically unstable for low Ca flow, using a smoothed local pore‐body capillary pressure curve appears to produce stable simulations. (2) Due to an approximation for the capillary driving force, IMP‐SIMS can deviate from quasi‐static solutions at equilibrium states especially in heterogeneous networks. (3) Both IMPES and IMP‐SIMS introduce mass conservation errors. The errors are small for networks with cubic pore bodies (less than 1.4% for IMPES and 1.2% for IMP‐SIMS). They become much greater for networks with square‐tube pore bodies (up to 45% for IMPES and 46% for IMP‐SIMS). Conversely, the new FI algorithm is numerically stable and mass conservative regardless of the flow regimes and pore geometries. It also precisely recovers the quasi‐static solutions at equilibrium states. The FI framework has been extended to include compressible two‐phase flow, multicomponent transport, and phase change dynamics. Example simulations of two‐phase displacements accounting for phase change show that evaporation and condensation can suppress fingering patterns generated during invasion.

Study of the thermal field of a mixture of soil and PCM materials with simulation of the warming effect during a phase change

Phase-change materials (PCM) possess an ability to accumulate and generate a lot of heat during the substance change from one phase to another. Such materials can be used with soil as an alternative type of thermal regulation of the subgrade during construction in cold regions. This paper reviews the impact of two types of PCM (PCMf-hybrid liquid and PCMm- microencapsulated paraffin) on the soil temperature. An experiment was conducted with three groups of samples: (1) samples with PCMm or PCMf added at a ratio of 2% and 4% of the dry weight of the soil; (2) samples reinforced with 4% of cement with an addition of PCMm or PCMf; and (3) samples with PCMf added after drying and re-adding water up to the optimal amount. PCMf and PCMm have been tested in DSC at various scanning speed levels. The results showed that, compared to that of the PCMm sample, PCMf has a greater enthalpy both during freezing and thawing, as well as the temperature peak during freezing is closer to zero. The study of temperature changes in soil samples with PCMm, PCMf, and cement added at the change of the ambient temperature T0=10°C→(-10°C)→(10°C) showed that the soil samples with various PCM additions have a lower temperature change rate than the soil samples without PCM additions. The results of the Scanning Electron Microscope (SEM) microfilming confirmed satisfactory components concentration in the soil mixture. There is also a noticeable number of craters and less porosity in the samples containing liquid PCMf and cement. A two-dimensional model of PCMm microcapsule per unit volume of soil was analyzed with the help of Comsol Multiphysics software (a powerful software product for modeling heat transfer and phase change in porous medium). It was determined that, during the phase change, the PCMm microcapsule has a thawing effect on the soil. During the examination of a three-dimensional model of eight PCMm capsules per unit volume of soil, it was noted that the thawing effect extends to 18.2% of the calculated area.

A new formulation for nondimensionalization heat transfer of phase change in porous media: An example application to closed cell porous media

The problem of solid/liquid phase change in a cavity assisted by a closed cell porous medium is analyzed by pore scale and volume average approaches. The volume average governing equations are non-dimensionalized by using the porous media stagnant thermal diffusivity and Sparrow number appears automatically. The study consists of four parts as the derivation of new formulation based on Sparrow number, proving the validation of volume average method based on the given formulation for the closed cell porous media, discussion on the change of interfacial heat transfer coefficient with time and finally presenting a thermal equilibrium chart in terms of Sparrow number and thermal diffusivity ratio for prediction of local thermal equilibrium state. A good agreement between the pore scale and volume average results is observed. Based on the established thermal equilibrium chart, it is found that the possibility of local thermal equilibrium is high for the large values of Sparrow number (such as 500) and for the thermal diffusivity ratio around 1. The established equilibrium chart is verified by prediction of local thermal equilibrium state for two different closed cell porous media and then the validation of predication is proved by using the results of pore scale study.

Model comparison, thermal nonequilibrium characteristic and boundary conditions discussion of two-phase flow with phase change in porous media

This article comprehensively compares three models of two-phase flow with phase change in porous media: the widely used separated phase model (SPM) and two-phase mixture model (TPMM), and the new developed semi-mixed model (SMM), with the identification of the reasonability of different boundary conditions (BCs) at cold and hot sides. Differences and common features from the physical, as well as from the numerical point of view, are shown in detail. Independent numerical algorithms are, respectively, developed and implemented to solve the three models. The study indicates that the numerical results obtained by SPM and SMM are always similar, but TPMM shows significant difference in temperature and phase distributions of fluid due to the inveracious increase of the effective diffusion coefficient in numerical simulation. The effect of different BCs at cold side emerges and is delivered to the hot side only when the porous matrix is thin, while one type of BC at hot side is infeasible at small porosity. At last, in view of the huge latent heat of phase change, the thermal nonequilibrium characteristic between solid and fluid for two-phase flow is quantitatively evaluated.

Separate-phase model and its lattice Boltzmann algorithm for liquid-vapor two-phase flows in porous media.

In this article, we formulate a set of the separate-phase governing equations at the representative-elementary-volume scale and develop its double-distribution function lattice Boltzmann (LB) algorithm to describe liquid-vapor two-phase flows with or without phase change in porous media. Different from those previous studies, the mathematical description in this article involves the Darcy force, viscous force, and pressure gradient, and the resulting LB simulations can well describe two-phase flows and mass transfer throughout porous media under the compounding effects of these forces. The LB algorithm was validated by simulating single-phase flows in porous media. Its results are in good agreement with those available analytical solutions. We also applied it to model water flows through a semi-infinite porous region bounded by a heated solid wall, where liquid-vapor phase change takes place. The numerical simulations recover the previous results in the limit of the zero Darcy number. Significantly, it reveals much richer two-phase flow and mass transfer characteristics in porous media adjacent to solid walls. The separate-phase model and its lattice Boltzmann algorithm in this article are effective means to gain more profound and clearer understandings of complex two-phase transport processes in a porous system.

New Mixed Finite Element Methods for Natural Convection with Phase-Change in Porous Media

This article is concerned with the mathematical and numerical analysis of a steady phase change problem for non-isothermal incompressible viscous flow. The system is formulated in terms of pseudostress, strain rate and velocity for the Navier–Stokes–Brinkman equation, whereas temperature, normal heat flux on the boundary, and an auxiliary unknown are introduced for the energy conservation equation. In addition, and as one of the novelties of our approach, the symmetry of the pseudostress is imposed in an ultra-weak sense, thanks to which the usual introduction of the vorticity as an additional unknown is no longer needed. Then, for the mathematical analysis two variational formulations are proposed, namely mixed-primal and fully-mixed approaches, and the solvability of the resulting coupled formulations is established by combining fixed-point arguments, Sobolev embedding theorems and certain regularity assumptions. We then construct corresponding Galerkin discretizations based on adequate finite element spaces, and derive optimal a priori error estimates. Finally, numerical experiments in 2D and 3D illustrate the interest of this scheme and validate the theory.

Pore-scale modeling of phase change in porous media

Liquid-vapor phase change is simulated in a porous medium, using a diffuse-interface model and the van der Waals equation of state. The role of wettability and pore space geometry on phase separation and on the propagation of evaporation fronts is investigated.

Experimental study on the dynamic filtration control performance of N2/liquid CO2 foam in porous media

Liquid CO2 has been successfully used as a fracturing fluid and injected into rock formations to enhance oil and gas production. However, the difficulty in controlling the filtration of liquid CO2 in porous media limits its application. The goal of this study is to investigate the dynamic filtration control performance of N2/liquid CO2 foam with a fluorochemical (HFE) as stabilizer. A laboratory apparatus has been specially designed and built for the generation of N2/liquid CO2 foams and the measurement of their viscosity and filtration rate under high pressure (10–25MPa). The test results show that after liquid CO2 and HFE were mixed with N2, the N2/liquid CO2 foams were generated and showed significant viscosity improvement compared to liquid CO2. The foams exhibited better filtration control performance compared to liquid CO2 and liquid CO2+N2 systems. Although the addition of HFE did not result in the formation of filter cake, the foam showed a wall-building behavior, which could be explained by the CO2 phase change in porous media. As foam quality increased from 28% to 92%, the leakoff coefficient and spurt loss volume first decreased until the foam became unstable and changed into mist flow. The leakoff coefficient of foam increased with the increase in permeability of porous media. For foam with the quality of about 50–80%, a change of 2 orders of magnitude in permeability resulted in a change of 1 order of magnitude in leakoff coefficient. At high pressure difference, lower initial quality foam (∼30%) showed better filtration performance than higher initial quality foam (∼80%). When the pressure difference was high enough to cause the CO2 phase change from liquid to gas in the porous media, the initial foam damage after filtration became obvious, but the damage could be eliminated with time by gas return flow. Thus, by using N2/liquid CO2 foam in porous media, the fluid filtration behavior could be controlled without damage.

The Phase Change Process and Properties of Saline Soil During Cooling

Detrimental frost heave and salt expansion may occur in saline soils of cold regions. The severity of these processes depends on the amount of salt precipitation and ice crystal growth in the pore solution at different temperatures. By employing nuclear magnetic resonance, the liquid water fractions in different concentrations of sodium chloride and sodium sulfate saline soils were measured at different temperatures. Based on the theory of solution phase diagram and phase change in porous media, the differences and similarities of phase change between pore solution and free solution were analyzed. The phase change sequence and effects of salt on frozen soils are also discussed. The theoretical and experimental results confirm that the phase diagram can reflect the phase change of pore solutions roughly, and pore size distribution is the cause of differences between the pore solution and free solution. Sodium chloride can mitigate the frost heave of soil, and frost heave and salt expansion occur together when the sodium sulfate soil is frozen.

Thermo-hydro-salt-mechanical coupled model for saturated porous media based on crystallization kinetics

Crystallization is generally considered as the most destructive factor of porous material, but few studies have reported the coupling effects among fluid flow, heat transfer, crystallization and deformation in porous media. In this paper, the driving forces of phase transition were studied initially and then the crystallization kinetics models were established. Moreover, constitutive relations related to saline frozen soil were discussed, and the expression of unfrozen water content in non-equilibrium state was improved. Finally, a thermo-hydro-salt-mechanical coupled model for fully saturated saline frozen soil with phase change was proposed. Validation of the model is illustrated by comparisons between the simulation and experimental results. The predicted values of frost heave, temperature and moisture distribution are consistent with the experimental data, and it is demonstrated that the presented crystallization kinetics approach is valid for studying heat and mass transfer coupled problem with phase change in porous media.

Towards a Model for Reacting Flows with Phase Change in Porous Media: Model Formulation and Preliminary Results

Towards a Model for Reacting Flows with Phase Change in Porous Media: Model Formulation and Preliminary Results

A lattice Boltzmann model for heat and mass transfer phenomena with phase transformations in unsaturated soil during freezing process

In this paper, a mesoscopic numerical model for simulating the heat and moisture transport phenomena in frozen soil during freezing process is presented. The model includes a stochastic generation-growth method for reconstructing the soil structure with given macroscopic geometric parameters, a lattice Boltzmann method (LBM) model for solving multiphase fluid flow and a LBM model for solving heat conduction with phase change in porous media. By using the present model, freezing process in sandy loam soil is demonstrated. Two types of boundary condition, prescribed temperature and adiabatic boundary conditions are employed in the given cases. All the numerical results show good agreement with experimental results. Considering the mesoscopic nature of LBM, the proposed model is a potential alternative to traditional continuum models.

A Numerical Study for Fusing Process Including Moisture Phase Change in The Porous Media

A numerical study has been conducted for accurate prediction of temperature in the toner layer including moisture phase change in porous media (paper). The moisture contents, in previous study, have been found experimentally to influence the heat transfer behavior significantly in the fusing process. Heat and mass transfer equations were incorporated into fusing processes analysis to consider the energy loss due to the evaporation of moisture in the paper. The effects of moisture content, dwell-time, fusing temperature, paper grammage, and heating energy from heater on fusing behaviors were numerically investigated by using the present numerical model.

对流换热耦合相变渗流问题的格子玻尔兹曼方法研究

相变是传质传热中的一种典型情况。而多孔介质中物质的相变在工业生产和日常生活中十分常见。由于相变问题的非线性特点，需要格子Boltzmann方法这种能够模拟流体运动以及为复杂物理现象建模的数值方法来处理它。本文运用格子Boltzmann方法对纯相变以及多孔介质内物质的对流相变过程进行了数值模拟，并对数值结果加以研究和讨论。发现不同的达西数对多孔介质内的对流相变过程中有重要的影响。 Phase change is a basic problem in heat and mass transfer field, especially in porous media. In this work, as it’s nonlinear characteristics, Lattice Boltzmann method (LBM) is used to numericaly simulate the heat and mass trans- fer coupling with phase change in porous media, and results showed that different Darcy Numbers have some certain effects on phase change process in porous medium.

Water flow and heat transport including ice/water phase change in porous media: Numerical simulation and application

Abstract According to the basic law of water flow and heat transfer in porous media under freeze/thaw condition, based on the theories of continuum mechanics, thermodynamics and segregation potential, the governing equations for thermo-hydro (TH) coupling model are established. These equations include not only the effects of thermal conductivity, latent heat of phase change and the seepage velocity on temperature distribution, but also the effects of water flow in porous resulted by Soret effect and segregation potential on seepage velocity and water pressure distribution. Compared with the famous TH coupling laboratory test conducted by Mizoguchi, it was found that the presented TH coupled model predicts the freezing process in porous media well and satisfactorily reflects observed phenomena. Combining this basis and the construction of Galongla tunnel in Tibet of China, the effect of insulation materials is analyzed at first, and then research work is done to study the influence of position, thickness and types of insulation materials on heat preservation effect. The research results indicated that the application of insulation material with 6 cm thickness and 600 m long at the entrance, while 400 m long at the exit of Galongla tunnel can prevent the lining and surrounding rock from freezing–thawing damages effectively.

Investigation of moisture phase change in porous media using neutron radiography and gravimetric analysis

A new technique for observing the phase change of water vapor in an insulation material is presented. It combines neutron radiography with gravimetric analysis in order to obtain quantitative two-dimensional insight in a transient process due to a transient temperature boundary condition at one side of the insulation. The advantage of this approach is, that the neutron radiography provides high-resolution pictures while the process is slow enough to allow exposure times of nearly a minute. The article describes the experimental setup and the analysis method and shows results which will allow a future validation of transient mathematical models.