A NUMERICAL MODEL FOR COUPLED HEAT AND MOISTURE FLOW THROUGH POROUS MEDIA (FINITE-ELEMENT)

The flow of heat and moisture around buried power cables is studied through the creation of a transient, two-dimensional, Finite Element numerical model. The model is menu driven and operates on a microcomputer. The formulation basis is the Philip and DeVries (1957) concept of coupled heat and moisture flow, but formulated in terms of pressure (p) rather than moisture content ((theta)) as the driving force for moisture flow. The pressure formulation ensures continuity across interfaces between different soil types, allows saturated conditions to be modeled and incorporates the independent variations of density with temperature necessary for modeling non-isothermal systems. However, the non-linear material properties, which form the flux and capacity terms in the respective formulations, vary more drastically with p than they do with (theta) and can lead to numerical instability in the solutions.;The work also includes a summary of the results of four full-scale field tests of simulated power cables in which temperature and thermal resistivity were periodically measured in the vicinity of the heat source. Thermal instability was observed in three of the tests.;Evaluations of the model were made by comparing simulation results to single flow (heat or moisture) and coupled flow (heat and moisture) problems. The single flow comparisons were very good for heat flow and good for unsaturated moisture flow solutions. The coupled flow capabilities were evaluated by comparison with laboratory and field test results. The model accurately predicted temperatures at the heat source prior to drying but could not accurately model the times for drying to initiate; the model predictions were consistently longer than actual, indicating that its predictions of the rate of moisture movement away from the heat source were too low. Parameter studies indicated sensitivity of the results to the soil permeability, the thermal vapor flux term, the shape of the moisture-suction curve, especially near the dry end, and whether the gravity term was included in the moisture flow formulation.