Summary

The present work studies the phenomena of heat transfer and moisture migration in the neighborhood of power cables. The heat generated by electrical currents must be dissipated by the medium that involves the cable; otherwise the cable may be damaged by super-heating. The thermal gradient causes moisture migration far away from the cable decreasing the thermal conductivity of the soil near the cable. Thus, the soil that should dissipate the heat begins to work as a thermal insulation. The main motivation for this work is to formulate and predict the drying processes that occur in the soil, which has an important role in the design of these cables. The model by Philip and de Vries for the simultaneous heat and mass transfer in an unsaturated porous media is used for describing this phenomenon. In this model, the vapor and liquid fluxes are written in terms of the difusivities due to gradients of termperature and moisture content. The governing equations are then recovered in such a way that the influences of the thermal and moisture content gradients are explicit in the heat and mass transport processes. In the present work, all diffusive and thermophysics properties of the medium are functions of temperature and moisture content. Two soils are used in the numerical simulation. The first one is a sand silt that has already been used as a cable burying material. This soil is chosen because all expressions for the properties are available in the literature. The second one is currently used as burying material by some cable factories. The characterization of this soil is made by the determination of three properties: the suction potential, the relative permeability and the effective thermal conductivity. The curve of suction potential versus moisture content for this soil is experimentally measured. The experimental methods for the other two properties are also described. The parameters experimentally obtained are then used in the theoretical development. The governing equations of the problem, namely, energy and mass conservation equations are numerically solved using the finite volume method. Bicylindrical coordinates are used to simulate the buried cable. The temperature and moisture content profiles are obtained as a function of time for several cases of generated power, cable diameters and initial moisture contents of the soil. These values give a large collection of results that can be used to predict the drying processes near the cable surfaces

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