Effect of heated zone size on micro and nanoscale convective heat transfer

Recent studies on free convective heat transfer from micro and nanoscale structures show that the heat transfer coefficient is size dependent. The heat transfer coefficient is found to increase at smaller scales, which is ascribed to the higher surface area to volume ratio. Also, the mode is changed from advection to conduction due to the decaying influence of the gravitational field. Interestingly, it is tacitly assumed that the size effect is due to the specimen, since not a single study on the effect of the size of heat source on the convective heat transfer exists in the literature. In this study, we provide unambiguous experimental evidence of the predominance of heater size on the heat transfer coefficient. For micro-heaters, the heat transfer coefficient is measured to about 3200 W/m2 K. This value drops to about 110 W/m2 K for a millimeter scale heater and to 10 W/m2 K for a macroscopic heater; all for a nanoscale thin film specimen. This finding is particularly significant in microelectronic applications where localized heating (hot spots) in small areas is very common.