EFFECTS OF ORTHOTROPIC THERMAL-CONDUCTIVITY OF SUBSTRATES IN NATURAL-CONVECTION COOLING OF DISCRETE HEAT-SOURCES

Substrate thermal conductivity plays a critical role in heat removal i n natural convection cooling of discrete heat sources. It is well know n that printed circuit boards used in electronic systems display stron gly anisotropic thermal conduction behavior. In the present investigat ion, conjugate conduction, natural convection, and radiation heat tran sfer from a flush heat source mounted on an orthotropic substrate in a n air-filled cubic enclosure are numerically examined. Two types of bo undary conditions are investigated at the back of the substrate: insul ated and prescribed temperature. The results are presented as surface temperatures, Nusselt numbers, and thermal resistances. Heat transfer budgets are presented to illustrate the combined effects of orthotropi c thermal conductivity and surface radiation. The anisotropy in therma l conductivity is found to influence heat transfer in very different w ays for the two boundary conditions considered.