Computational analysis of thermo-fluidic characteristics of a carbon nano-fin.

摘要

Miniaturization of electronic devices for enhancing their performance is associated with higher heat fluxes and cooling requirements. Surface modification by texturing or coating is the most cost-effective approach to enhance the cooling of electronic devices. Experiments on carbon nanotube coated heater surfaces have shown heat transfer enhancement of 60%. In addition, silicon nanotubes etched on the silicon substrates have shown heat flux enhancement by as much as 120%. The heat flux augmentation is attributed to the combined effects of increase in the surface area due to the protruding nanotubes (nano-fin effect), disruption of vapor films and modification of the thermal/mass diffusion boundary layers. Since the effects of disruption of vapor films and modification of the thermal/mass diffusion boundary layers are similar in the above experiments, the difference in enhancement in heat transfer is the consequence of dissimilar nano-fin effect. The thermal conductivity of carbon nanotubes is of the order of 6000 W/mK while that of silicon is 150 W/mK. However, in the experiments, carbon nanotubes have shown poor performance compared to silicon. This is the consequence of interfacial thermal resistance between the carbon nanotubes and the surrounding fluid since earlier studies have shown that there is comparatively smaller interface resistance to the heat flow from the silicon surface to the surrounding liquids.;At the molecular level, atomic interactions of the coolant molecules with the solid substrate as well as their thermal-physical-chemical properties can play a vital role in the heat transfer from the nanotubes. Characterization of the effect of the molecular scale chemistry and structure can help to simulate the performance of a nano-fin in different kinds of coolants. So in this work to elucidate the effect of the molecular composition and structures on the interfacial thermal resistance, water, ethyl alcohol, 1-hexene, n-heptane and its isomers and chains are considered. Non equilibrium molecular dynamic simulations have been performed to compute the interfacial thermal resistance between the carbon nanotube and different coolants as well as to study the different modes of heat transfer. The approach used in these simulations is based on the lumped capacitance method. This method is applicable due to the very high thermal conductivity of the carbon nanotubes, leading to orders of magnitude smaller temperature gradients within the nanotube than between the nanotube and the coolants. To perform the simulations, a single wall carbon nanotube (nano-fin) is placed at the center of the simulation domain surrounded by fluid molecules. The system is minimized and equilibrated to a certain reference temperature. Subsequently, the temperature of the nanotube is raised and the system is allowed to relax under constant energy. The heat transfer from the nano-fin to the surrounding fluid molecules is calculated as a function of time. The temperature decay rate of the nanotube is used to estimate the relaxation time constant and hence the effective thermal interfacial resistance between the nano-fin and the fluid molecules. From the results it can be concluded that the interfacial thermal resistance depends upon the chemical composition, molecular structure, size of the polymer chains and the composition of their mixtures. By calculating the vibration spectra of the molecules of the fluids, it was observed that the heat transfer from the nanotube to the surrounding fluid occurs mutually via the coupling of the low frequency vibration modes.