Three-dimensional numerical study of slab-on-grade heat transfer

A detailed, hourly three-dimensional finite difference model was used to conduct parametric studies of heat transfer from slab-on-grade floors. Studies considered effects of geometry, climate, soil properties, and boundary conditions. The results indicate that the widely used F₂ method for heating load calculations may err by 50% or more because its central assumption (that heat transfer is proportional to perimeter length) is erroneous. The ratio of floor area to perimeter length (A/P) was found to be an appropriate length scale for correlating average heat flux from L-shaped and rectangular floors. A simplified analysis of daily averaged heat transfer based on A/P scaling could be the basis for an improved design method useful for both load and energy calculations. Thermal conductivity and ground surface boundary conditions proved to be important parameters affecting heat transfer, while thermal diffusivity and far-field boundary conditions were relatively unimportant. The shading of adjacent soil by a building's shadow caused significant changes in heat transfer rate under some conditions.