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Institut für Wasserbau - IWS

Selected Topics and International Network Lectures

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Dienstag
30.01.2007
16:00 Uhr
Professor Dani Or
Laboratory of Soil & Environmental Physics (LASEP)
School of Architectural, Civil and Environmental Engineering (ENAC)
Ecole Polytechnique Federale de Lausanne (EPFL)

Textural Contrasts, Pore Size Distribution, and Characteristic Lengths Affecting Evaporation Rates from Porous Media – Measurements and Models

Evaporation from soil and other porous materials plays an important role in the hydrologic cycle as well as in a wide range of engineering applications. Dynamics of evaporative drying rates reflect complex interplay between soil pore space, transport properties and boundary conditions. An initially high drying rate induced by atmospheric or air flow over wet soil surface, may gradually decrease as mass flow between a receding drying front becomes limiting. Subsequent water would then be driven by vapor diffusion through soil pores at significantly slower rates. Theoretical and experimental results indicate that the depth of the drying front depth at which a transition from mass flow (high) to diffusion controlled (slow) evaporation takes place, is related to pore size distribution of a homogeneous porous medium, and to presence (and nature) of textural discontinuities within heterogeneous media. Dual and multi-capillary models are used to describe the competing forces and the origins of dominant characteristic lengths for evaporation. Measurements of drying front position and morphology within sand-filled Hele-Shaw cells were made with neutron transmission technique. New insights concerning water content distribution new the evaporating surface and dynamics of drying front in different coarse-textured materials are compared with theoretical predictions. The extent of polydispersity of pore sizes (either as pore size distribution, textural contrasts, or roughness within pore spaces) is key to sustaining high (and constant) evaporation rates via mass flow, and to determining length and time scales for abrupt changes in evaporation characteristics.