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Drying of porous materials at pore scale using lattice Boltzmann and pore network models
Jianlin Zhao2, Feifei Qin2, Dominique Derome1, Jan Carmeliet2
1Universite de Sherbrooke, Canada; 2ETH Zurich, Switzerland
Drying at macroscale shows a first drying period with constant drying rate followed by second drying period showing a receding front. In order to improve the drying of materials, it is favorable to maintain the first drying period as long as possible. Large pores are known to be invaded first by the non-wetting fluid, namely air. When neighboring smaller pores remain filled by capillary pumping from larger pores, the liquid phase can remain connected to the surface maintaining high drying rates for longer periods.
Drying of porous materials has been studied by pore network models (PNM) using invasion percolation algorithms. It is known that the dynamics of the receding menisci in the pore structure can be better modelled by direct numerical methods at pore scale, such as lattice Boltzmann modelling (LBM). However, LBM still suffers from high computational costs.
In this presentation, we present a new hybrid method, where the dynamics of the liquid-vapor interfaces is modelled by LBM in the two-phase pores, while the single phase flow in the pores filled solely by vapor or liquid is solved by PNM. This hybrid method is validated by comparison with reference full LBM simulations. The hybrid method combines the advantages of both methods, i.e. accuracy and computational efficiency.
The hybrid LBM-PNM method is used to study the drying of porous media at pore scale. We analyze the influence of different pore structures on the capillary pumping effect in order to maximize the drying rate. We also study the effect of contact angle hysteresis on the drying rate. Finally, we show how the drying rate can be maximized in particularly when designing facade or pavement solutions that can mitigate higher temperatures in urban environments by evaporative cooling.
10:45am - 11:00am
Experimental and numerical study of non-isothermal hygroscopic behavior of spruce during adsorption and desorption
Xiaohai Zhou1,2, Guylaine Desmarais1, Dominique Derome3, Jan Carmeliet1
1ETH Zurich, Switzerland; 2Laboratory of Multiscale Studies in Building Physics, Empa, Dübendorf, Switzerland; 3Université de Sherbrooke, Sherbrooke, Canada
Vapor sorption in a hygroscopic porous material is associated with a change of the system energy, which could lead to a reduction in energy consumption and an improvement of indoor thermal comfort. There is a need to better understand the coupled vapor and heat transport during adsorption and desorption processes. In this study, spruce is used for adsorption and desorption experiments. Spruce samples were mounted inside a custom-built micro-wind tunnel. Temperature and humidity-controlled air flow was controlled in the micro-wind tunnel to induce moisture adsorption and desorption in the samples. Neutron radiography, a non-destructive imaging technique that uses thermal neutrons to probe the sample, was used to document the time- and space-resolved moisture content distributions in spruce samples during adsorption and desorption. Accurate wireless thermocouples were used to measure temperature in the sample at different locations. Large changes in moisture content and temperature were observed during both adsorption and desorption experiments. The temperature increase at the top part of the sample can be up to 8.0 °C during adsorption. Moisture and heat transport during the experiments were simulated with a state-of-the-art hygrothermal model. Both measured moisture content and temperature change were well simulated with the hygrothermal model. The vapor resistance factor in the longitudinal direction of the spruce samples is found to be extremely small. Temperature change in the samples is mainly due to latent heat of vaporization of water. The effect of differential heat of sorption latent heat is found to be much smaller. The influence of the vapor transport properties on temperature change is much larger than thermal properties. The temperature change during adsorption and desorption is very sensitive to the vapor permeability.
11:00am - 11:15am
Recent progress on hygroscopic materials for indoor moisture buffering
Xu Zhang1,2, Menghao Qin1, Kan Zu1
1Technical University of Denmark; 2Hebei GEO Universit
Abstract: Once in contact with the indoor air, hygroscopic materials can moderate the indoor humidity fluctuation by adsorbing or releasing water vapor, and then improve the moisture regulation and thermal management of buildings. It is desirable to explore the characterized properties of these materials about moisture buffering behavior. In this regard, we review various hygroscopic materials used for the built environment control. The hygrothermal properties of hygroscopic materials often can be characterized by some parameters, such as water vapor adsorption/desorption capacity, water vapor adsorption/desorption rate, water vapor diffusion coefficient and so on. In order to provide an insight on the existing research on humidity control materials, different research studies and the recent progress on humidity control materials have been summarized. The materials include traditional and conventional building materials, some natural materials and novel humidity control materials. Besides, the relevant parameters are considered as well as the improvement suggestions to enhance the application of humidity control materials in building environments. Finally, new multifunctional materials and intelligent moisture control materials together with the corresponding systems are collated to summarize the latest research trends. The overview of the application of hygroscopic materials can provide current and future researchers guidelines for the science-oriented design of moisture control system for new energy-efficient buildings.