The Influence of Vapor Attachment Kinetics on Snow Effective Properties

<p>Proper modelling of heat and mass transfer in snow is a prerequisite for understanding snow metamorphism and simulating the mass and energy budget of a snowpack and the underlying ground. The transfer of heat and water vapor in snow can be described with macroscopic conservation equations, which include effective coefficients such as the snow thermal conductivity or the snow water vapor diffusion coefficient. Here, we investigate the impact of the surface kinetics of water vapor sublimation and deposition at the microscopic scale on these macroscopic equations, restraining ourselves to the limiting cases of slow and fast kinetics. In particular, we show that under the assumption of fast kinetics the thermal behavior of snow is similar to that of a regular inert medium, but with an enhanced conduction in the pores, due to latent heat transported with water vapor. Besides, faster kinetics increases the effective water vapor diffusion coefficient, which nonetheless remains less than that in free air. M<span>ost (but not all) available experimental investigations suggest that in snow, fast surface kinetics prevails, so that our results have numerous implications for the proper simulation of heat and mass transfer in snow.</span></p>

Macro-Scale Numerical Simulation of Moisture Transmission in Zeoli-Based Moisture Conditioning Material

By random growth method, this paper constructs isotropic porous media, anisotropic-1 porous media, and anisotropic-2 porous media, which have the same porosity but different micropore morphologies, and explores how the pore morphology affects the water vapor diffusion in the pores of porous media. The results show that: the random growth method can effectively reconstruct various porous moisture conditioning materials, and control their porosity and pore morphology; the equilibrium water vapor concentration and stabilization time of water vapor diffusion can effectively demonstrate the pore connectivity of porous media and the dynamic migration features of materials in the pores; the greater the change in the equilibrium water vapor concentration, the faster the stabilization of water vapor diffusion, and the better the pore connectivity of porous media.

Macroscopic water vapor diffusion is not enhanced in snow

Water vapor transport in dry snowpacks plays a significant role for snow metamorphism and the mass and energy balance of snowpacks. The molecular diffusion of water vapor in the interstitial pores is usually considered to be the main or only transport mechanism, and current detailed snow physics models therefore rely on the knowledge of the effective diffusion coefficient of water vapor in snow. Numerous previous studies have concluded that water vapor diffusion in snow is enhanced relative to that in air. Various field observations also indicate that for vapor transport in snow to be explained by diffusion alone, the effective diffusion coefficient should be larger than that in air. Here we show using theory and numerical simulations of idealized and measured snow microstructures that, although sublimation and deposition of water vapor onto snow crystal surfaces do enhance microscopic diffusion in the pore space, this effect is more than countered by the restriction of diffusion space due to ice. The interaction of water vapor with the ice results in water vapor diffusing more than inert molecules in snow but still less than in free air, regardless of the value of the sticking coefficient of water molecules on ice. Our results imply that processes other than diffusion play a predominant role in water vapor transport in dry snowpacks.

WATER VAPOR TRANSPORT IN PLASTERS CONTAINING SUPERABSORBENT POLYMERS

A proper characterization of material properties represents an important step towards an efficient building design. Considering the present issues in the construction sector, moisture loads pose a risk not only to increased material deterioration but also to the health of building inhabitants. In this paper, modified plaster mixtures with superabsorbent admixture are designed in order to improve passive moderation of finishing layers against varying humidity conditions. The relationship between the amount of applied superabsorbent admixture and resulting water vapor transport properties is identified and the influence of temperature on water vapor transport is analyzed. The steady-state cup method is used for the determination of water vapor transport properties, namely the water vapor diffusion permeability, water vapor diffusion coefficient and water vapor diffusion resistance factor. The obtained data show temperature as a very significant factor affecting water vapor transport in the analyzed plasters. Considering the dry-cup method arrangement, relative humidity probes should be used for monitoring relative humidity under the sealed sample for a sufficiently precise determination of water vapor pressure gradient.

Macroscopic water vapor diffusion is not enhanced in snow

Water vapor transport in dry snowpacks plays a significant role for snow metamorphism and the mass and energy balance of snowpacks. The molecular diffusion of water vapor in the interstitial pores is usually considered as the main or only transport mechanism, and current detailed snow physics models therefore rely on the knowledge of the effective diffusion coefficient of water vapor in snow. Numerous previous studies have concluded that water vapor diffusion in snow is enhanced relative to that in air. Various field observations also indicate that for vapor transport in snow to be explained by diffusion alone, the effective diffusion coefficient should be larger than that in air. Here we show using theory and numerical simulations on idealized and measured snow microstructures that, although sublimation and condensation of water vapor onto snow crystal surfaces do enhance microscopic diffusion in the pore space, this effect is more than countered by the restriction of diffusion space due to ice. The interaction of water vapor with the ice results in water vapor diffusing more than inert molecules in snow, but still less than in free air, regardless of the value of the accommodation coefficient of water on ice. Our results imply that processes other than diffusion, probably convection, play a preponderant role in water vapor transport in dry snowpacks.