Substrate thermal conductivity plays a critical role in the heat removal in natural convection cooling of discrete heat sources.  It is removal in natural convection cooling of discrete heat sources.  It is well known that printed circuit boards (PCB) used in electronic systems display strongly anisotropic thermal conduction behavior.  In the present investigation, conjugate conduction, natural convection and radiation heat transfer from a flush heat source mounted on an orthotropic substrate in an air filled cubic enclosure is numerically examined.  Two types of boundary conditions are investigated at the back of the substrate: (i) insulated and (ii) prescribed temperature.  The results are presented as surface temperatures, Nusselt numbers and thermal resistances.  Heat transfer budgets ate presented to illustrate.  Heat transfer budgets are presented to illustrate the combined effects of orthotropic thermal conductivity and surface radiation.  The anisotropy in thermal conductivity is found to influence the heat transfer in very different ways for the two boundary conditions considered.
 

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