Textural Contrasts Pore Size Distribution and CharacteristicLengths Affecting Evaporation Rates from Porous Media - Measurements andModels
30.01.07, 16:00 – 17:30 Uhr
Professor Dani Or Laboratory of Soil & Environmental Physics (LASEP) School of Architectural, Civil and Environmental Engineering (ENAC) Ecole Polytechnique Federale de Lausanne (EPFL)
Evaporation from soil and other porous materials plays an important roleinthe hydrologic cycle as well as in a wide range of engineeringapplications.Dynamics of evaporative drying rates reflect complex interplay betweensoilpore space, transport properties and boundary conditions. An initiallyhighdrying rate induced by atmospheric or air flow over wet soil surface,maygradually decrease as mass flow between a receding drying front becomeslimiting. Subsequent water would then be driven by vapor diffusionthroughsoil pores at significantly slower rates. Theoretical and experimentalresults indicate that the depth of the drying front depth at which atransition from mass flow (high) to diffusion controlled (slow)evaporationtakes place, is related to pore size distribution of a homogeneousporousmedium, and to presence (and nature) of textural discontinuities withinheterogeneous media. Dual and multi-capillary models are used todescribethe competing forces and the origins of dominant characteristic lengthsforevaporation. Measurements of drying front position and morphology withinsand-filled Hele-Shaw cells were made with neutron transmissiontechnique.New insights concerning water content distribution new the evaporatingsurface and dynamics of drying front in different coarse-texturedmaterialsare compared with theoretical predictions. The extent of polydispersityofpore sizes (either as pore size distribution, textural contrasts, orroughness within pore spaces) is key to sustaining high (and constant)evaporation rates via mass flow, and to determining length and timescalesfor abrupt changes in evaporation characteristics.