**Effect
of heated zone size on micro and nano scale convective heat transfer**

R.
A. Pulavarthy, M. T. Alam, M. A. Haque*

Mechanical & Nuclear
Engineering, Penn State University, University Park, PA 16802

**Summary:** It is the heater size, not just that of the specimen, that
controls the size dependence of free convective heat transfer at micro and nanoscales

Recent studies on free convective heat transfer from micro and
nanoscale structures show that the heat transfer coefficient (h) is size
dependent. 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. This finding is particularly significant in
microelectronic applications where localized heating (hot spots) in small areas
is very common.

(a)
MEMS heater, h = 3200W/m^{2}-K
(b) meso-scale RTD heater, h = 110 W/m^{2}-K
(c) Large thin foil heater, h = 10 W/ m^{2}-K

The same conclusion can be achieved from a different perspective,
such as the characteristic length scale dependence of the cooling rate. It is
well known that smaller systems (heater or specimen) have very small thermal
mass and get cooled very fast by heat loss due to the natural convection. For
such specimens, the heater size can be very important. When they are physically
attached to bulky heaters, their cooling dynamics are governed by the cooling
rate of the heater.