h adiabatic and umax′

Abstract

In all electronics cooling situations, and many other practical situations, the surface temperature may vary rapidly in the streamwise direction. In these cases, defining the heat transfer coefficient using the adiabatic temperature of the surface instead of the mixed mean temperature of the coolant results in significant benefits. The resulting coefficient, called hadiabatic, is well behaved, being a function only of the geometry and flow characteristics. Calling attention to Tadiabatic, as opposed to Tmean, helps designers identify the root cause of overheating problems and more quickly reach good solutions. The theoretical and practical bases for hadiabatic are presented. Examples of its use in electronics cooling are described to show the operational advantages this approach offers. Turbulence strongly affects heat transfer. A simple, turbulence-based correlation is presented that yields an estimate of the heat transfer coefficient good enough for preliminary design estimates and often as accurate as can be relied on from CFD calculations using present codes.