Heat transfer augmentation in rectangular micro channel covered with vertically aligned carbon nanotubes

Abstract

An experimental heat transfer investigation was carried out to examine the influence of carbon nanotubes (CNTs) layer deposits on the convective heat transfer performance inside rectangular microchannels. Successful synthesis of vertically aligned CNTs was achieved using a catalytic vapor deposition (CVD) process on a silicon sample substrate. By varying the synthesis time, on average 6μm and 20μm thick layers of CNTs were made with surface roughness of (Sa=1.062μm, Sq=1.333μm) and (Sa=0.717μm, Sq=0.954μm) respectively. The external surface area of the samples increased 7 times compared to the bare silicon chip. The heat transfer performance of each sample was measured inside two rectangular microchannels with cross-section of 125μm×9mm and 200μm×9mm. For the 125μm channel height, the 6μm and 20μm thick layer of CNTs resulted in 12% and 26% increase in pressure drop respectively. The pressure drop obtained from the 200μm channel height show a similar trend with an increase of 6% and 16.4% for 6μm and 20μm CNTs layer thickness respectively. An average heat transfer enhancement of 19% and 74% is obtained inside the 125μm height microchannel with 6μm and 20μm CNTs layer thickness respectively. Whereas, the average heat transfer enhancement of 22% and 62% are obtained inside the 200μm channel with respective CNTs layer thicknesses of 6μm and 20μm. Enhancements are attributed to an increase in surface area and effective thermal conductivity inside the thermal boundary layer. However, the frictional heating (viscous dissipation) of a particular nanostructured sample increases with a decrease in channel height. This difference in channel size results in stronger competition between heat transfer enhancement potential that can be achieved by the deposited surface and the decrease in Nusselt number due to viscous dissipation.