Moisture movement in porous materials under temperature gradients

TLDR

The prior simple theory of water‑vapor diffusion in porous media under temperature gradients ignored vapor–liquid–solid interactions and the difference between pore‑air and bulk temperature gradients. This study develops a theory of moisture movement in porous materials under temperature gradients that explains large apparent vapor transfer, moisture‑dependent net transfer, and latent‑heat transfer by distillation. The authors derive equations for coupled moisture and heat transfer, incorporating vapor, liquid, and solid phases, and discuss four moisture‑dependent diffusivities, yielding an approximate analysis that predicts orders of magnitude and general behavior consistent with experiments. The theory shows that experimental methods attributing transfer to vapor actually measured series‑parallel flow through liquid islands in a vapor continuum, thereby reconciling previously discordant observations.

Abstract

A theory of moisture movement in porous, materials under temperature gradients is developed which explains apparently discordant experimental information, including (a) the large value of the apparent vapor transfer, (b) effect of moisture content on net moisture transfer, and (c) the transfer of latent heat by distillation. The previous simple theory of water vapor diffusion in porous media under temperature gradients neglected the interaction of vapor, liquid and solid phases, and the difference between average temperature gradient in the air‐filled pores and in the soil as a whole. With these factors taken into account, an (admittedly approximate) analysis is developed which predicts orders of magnitude and general behavior in satisfactory agreement with the experimental facts. An important implication of the present approach is that experimental methods used to distinguish between liquid and vapor transfer have not done so, since what has been supposed to be vapor transfer has actually been series‐parallel flow through liquid ‘islands’ located in a vapor continuum. Equations describing moisture and heat transfer in porous materials under combined moisture and temperature gradients are developed. Four moisture‐dependent diffusivities arising in this connection are discussed briefly.