Hysteresis is observed in sorption-induced swelling in various soft nanoporous polymers. The associated coupling mechanism responsible for the observed sorption-induced swelling and associated ...hysteresis needs to be unraveled. Here we report a microscopic scenario for the molecular mechanism responsible for hysteresis in sorption-induced swelling in natural polymers such as cellulose using atom-scale simulation; moisture content and swelling exhibit hysteresis upon ad- and desorption but not swelling versus moisture content. Different hydrogen bond networks are examined; cellulose swells to form water-cellulose bonds upon adsorption but these bonds do not break upon desorption at the same chemical potential. These findings, which are supported by mechanical testing and cellulose textural assessment upon sorption, shed light on experimental observations for wood and other related materials.
Droplet impact has been imaged on different rigid, smooth, and rough substrates for three liquids with different viscosity and surface tension, with special attention to the lower impact velocity ...range. Of all studied parameters, only surface tension and viscosity, thus the liquid properties, clearly play a role in terms of the attained maximum spreading ratio of the impacting droplet. Surface roughness and type of surface (steel, aluminum, and parafilm) slightly affect the dynamic wettability and maximum spreading at low impact velocity. The dynamic contact angle at maximum spreading has been identified to properly characterize this dynamic spreading process, especially at low impact velocity where dynamic wetting plays an important role. The dynamic contact angle is found to be generally higher than the equilibrium contact angle, showing that statically wetting surfaces can become less wetting or even nonwetting under dynamic droplet impact. An improved energy balance model for maximum spreading ratio is proposed based on a correct analytical modeling of the time at maximum spreading, which determines the viscous dissipation. Experiments show that the time at maximum spreading decreases with impact velocity depending on the surface tension of the liquid, and a scaling with maximum spreading diameter and surface tension is proposed. A second improvement is based on the use of the dynamic contact angle at maximum spreading, instead of quasi-static contact angles, to describe the dynamic wetting process at low impact velocity. This improved model showed good agreement compared to experiments for the maximum spreading ratio versus impact velocity for different liquids, and a better prediction compared to other models in literature. In particular, scaling according to We1/2 is found invalid for low velocities, since the curves bend over to higher maximum spreading ratios due to the dynamic wetting process.
Accurate Computational Fluid Dynamics (CFD) simulations of atmospheric boundary layer (ABL) flow are essential for a wide variety of atmospheric studies including pollutant dispersion and deposition. ...The accuracy of such simulations can be seriously compromised when wall-function roughness modifications based on experimental data for sand-grain roughened pipes and channels are applied at the bottom of the computational domain. This type of roughness modification is currently present in many CFD codes including Fluent 6.2 and Ansys CFX 10.0, previously called CFX-5. The problems typically manifest themselves as unintended streamwise gradients in the vertical mean wind speed and turbulence profiles as they travel through the computational domain. These gradients can be held responsible—at least partly—for the discrepancies that are sometimes found between seemingly identical CFD simulations performed with different CFD codes and between CFD simulations and measurements. This paper discusses the problem by focusing on the simulation of a neutrally stratified, fully developed, horizontally homogeneous ABL over uniformly rough, flat terrain. The problem and its negative consequences are discussed and suggestions to improve the CFD simulations are made.
Drying of porous media is governed by a combination of evaporation and movement of the liquid phase within the porous structure. Contact angle hysteresis induced by surface roughness is shown to ...influence multi-phase flows, such as contact line motion of droplet, phase distribution during drainage and coffee ring formed after droplet drying in constant contact radius mode. However, the influence of contact angle hysteresis on liquid drying in porous media is still an unanswered question. Lattice Boltzmann model (LBM) is an advanced numerical approach increasingly used to study phase change problems including drying. In this paper, based on a geometric formulation scheme to prescribe contact angle, we implement a contact angle hysteresis model within the framework of a two-phase pseudopotential LBM. The capability and accuracy of prescribing and automatically measuring contact angles over a large range are tested and validated by simulating droplets sitting on flat and curved surfaces. Afterward, the proposed contact angle hysteresis model is validated by modeling droplet drying on flat and curved surfaces. Then, drying of two connected capillary tubes is studied, considering the influence of different contact angle hysteresis ranges on drying dynamics. Finally, the model is applied to study drying of a dual-porosity porous medium, where phase distribution and drying rate are compared with and without contact angle hysteresis. The proposed model is shown to be capable of dealing with different contact angle hysteresis ranges accurately and of capturing the physical mechanisms during drying in different porous media including flat and curved geometries.
•The advances and challenges of pore-scale modeling are summarized.•Porous structure imaging and computational reconstruction methods are reviewed.•Recent progresses in pore-scale numerical methods ...and schemes are introduced.•Pore-scale studies of transport processes in porous media are discussed.•Application of pore-scale modeling in geoscience and fuel cells are presented.
Porous media play important roles in a wide range of scientific and engineering problems. Recently, with their increasing application in energy conversion and storage devices, such as fuel cells, batteries and supercapacitors, it has been realized that transport processes and reactions occurring in the pores and at the interfaces of different constituents significantly affect the performance of the porous media, yet these pore-scale transport phenomena are not well described or even neglected in the conventional numerical models based on the representative element volume (REV). Pore-scale modeling is an efficient tool for the simulation of pore-scale transport and reactions in porous media because of its ability to accurately characterize these processes and to provide the distribution details of important variables which are challenging for current experimental techniques to provide either due to lack of in-situ measurement capability or due to the limited spatial and temporal resolution. In the present review, the advances and challenges of the state-of-the-art pore-scale modeling are summarized. The practical applications of pore-scale modeling in the fields of geoscience, polymer exchange membrane fuel cells (PEMFC) and solid oxide fuel cells (SOFC) are discussed. Notable results from the pore-scale modeling are presented, and the challenges facing the pore-scale model development are discussed. This in-depth review is intended to give a well-rounded introduction of critical aspects on which the pore-scale modeling can shed light in the development of relevant scientific and engineering systems.
In this work, a numerical model for isothermal liquid–vapor phase change (evaporation) of the two-component air–water system is proposed based on the pseudopotential lattice Boltzmann method. Through ...the Chapman–Enskog multiscale analysis, we show that the model can correctly recover the macroscopic governing equations of the multicomponent multiphase system with a built-in binary diffusion mechanism. The model is verified based on the two-component Stefan problem where the measured binary diffusivity is consistent with theoretical analysis. The model is then applied to convective drying of a dual-porosity porous medium at the pore scale. The simulation captures a classical transition in the drying process of porous media, from the constant rate period (CRP, first phase) showing significant capillary pumping from large to small pores, to the falling rate period (FRP, second phase) with the liquid front receding in small pores. It is found that, in the CRP, the evaporation rate increases with the inflow Reynolds number (Re), while in the FRP, the evaporation curves almost collapse at different Res. The underlying mechanism is elucidated by introducing an effective Péclet number (Pe). It is shown that convection is dominant in the CRP and diffusion in the FRP, as evidenced by Pe > 1 and Pe < 1, respectively. We also find a log-law dependence of the average evaporation rate on the inflow Re in the CRP regime. The present work provides new insights into the drying physics of porous media and its direct modeling at the pore scale.
► Energy simulations were performed for stand-alone and street canyon buildings. ► Higher space cooling demands in street canyons than for stand-alone buildings. ► Solar and thermal radiation ...exchange affects the cooling demand most. ► Convective heat fluxes and urban heat island effect contribute to higher demands. ► Urban heat island effects reduce also cooling potential by night-time ventilation.
An important part of the world's energy is used for space cooling and heating of buildings. Its minimization has great energy saving potential. An important part of the heat exchange between buildings and the ambient surrounding is due to convective and radiative heat flows. In this study detailed building energy simulation (BES) is used to analyse the effect of neighbouring buildings on these heat flows and their influence on the space cooling and heating demand of buildings. BESs were conducted for stand-alone buildings and buildings in street canyon. This study demonstrates the importance of accounting for the urban microclimate for the prediction of the energy demand of buildings. With the proposed model most of the thermal effects of the urban microclimate can be captured and quantified on street canyon scale. Due to multiple reflections more solar and thermal radiation is absorbed at the façades of buildings in street canyons than at façades of stand-alone buildings. These effects cause higher surface temperatures in street canyons leading to higher space cooling and lower space heating demands. Other reasons are the lower convective heat transfer coefficients in street canyons, the reduced removal of heat from street canyons and the urban heat island effect.
The temperatures in cities are increased due to the urban heat island effect. Heat can be removed from urban areas by wind and buoyancy driven ventilation. Building geometries have a strong impact on ...the wind flow patterns and heat removal in urban areas. The microclimate is analysed for a generic urban area with 23 buildings. Six different urban topologies are studied, where only the building heights of the individual buildings are changed. The local air temperatures are studied with coupled CFD and building energy simulations. The results show that the building height topology has a minimal impact on the mean air temperatures, but influences local air temperatures and the size of local heat islands. An analysis based on a comparison of simulations with/without buoyancy, shows a strong impact of buoyancy on the mean and local air temperatures. Further, correlations are found showing that the normalized increase in local air temperature is linked to the local air volume flow rates and thermal diffusivities. With higher volume flow rates and thermal diffusivities more heat can be removed from the environment. Based on these correlations an approach to evaluate the local heat island formation risk using isothermal CFD simulations is proposed.
•Wind driven ventilation is an efficient ways of removing heat from urban areas.•Building height topologies and buoyancy strongly influence local air temperatures.•The formation of local heat islands (LHI) is studied with coupled CFD simulations.•Heat fluxes are determined from the CFD results to study the local heat removal.•An approach is proposed to predict the LHI formation with isothermal CFD simulations.
This paper presents a computational model coupling heat, water and salt ion transport, salt crystallization, deformation and damage in porous materials. We focus on crystallization-induced damage. ...The theory of poromechanics is employed to relate stress, induced by crystallization processes or hygro-thermal origin, to the material's mechanical response. A non-local formulation is developed to describe the crystallization kinetics. The model performance is illustrated by simulating the damage caused by sodium chloride crystallization in a porous limestone. The results are compared with experimental observations based on neutron and X-ray imaging. The simulation results suggest that the crystallization kinetics in porous materials have to be accurately understood in order to be able to control salt damage. The results show that the effective stress caused by salt crystallization depends not only on the crystallization pressure but also on the amount of salt crystals, which is determined by the spreading of crystals in the porous material and the crystallization kinetics.
•Model for heat and mass transport, salt crystallization, deformation and damage.•Effective stress depends on crystallization pressure and precipitated crystal amount.•Simulation of damage due to drying-induced NaCl crystallization in limestone.•Simulations confirm the experimental assessment of damage.•Controlling the crystallization kinetics is the key to control crystallization damage.
•Coupling mechanism of sorption and deformation is reviewed by molecular simulations.•Poromechanics model is developed based on the coupling mechanism at nanoscale.•Different types of sorption ...isotherms exhibited by different adsorbent-adsorbate systems are quantitatively explained by the developed model.•Influences of material properties on the coupled behavior of sorption and deformation are characterized.
The coupling of water sorption and deformation in soft nanoporous polymers is studied by means of statistical mechanics molecular simulation and the general framework of poromechanics. It is shown that the large amount of water adsorbed by soft nanoporous polymers under free swelling condition results from sorption-induced deformation, which generates more inter-chain space to accommodate water molecules. A poromechanical model is proposed to describe this coupled behavior from the molecular simulation data without any arbitrary fitting. More in detail, by taking into account the mechanical properties, sorption characteristics and structural information as a function of water loading, the model agrees with the molecular simulation and accurately captures the coupling mechanism. Using this model, it is also shown that distinct sorption and deformation behaviors can be observed depending on the material properties. On the one hand, small mechanical modulus, strong adsorbent-adsorbate interaction and significant coupling between sorption/deformation lead to Type II sorption isotherms (with larger sorption amount and sorption-induced deformation). On the other hand, Type I sorption isotherms with limited sorption amount and sorption-induced deformation are obtained for materials with opposite properties.
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