Experiments were performed to study forced convective heat transfer of de-ionized water (DI water) and aqueous nanofluids flowing in a microchannel. An array of temperature nanosensors, called “Thin Film Thermocouples (TFT)”, was utilized for performing the experimental measurements. TFT arrays were designed (which included design of photomask layout), microfabricated, packaged and assembled for testing with the experimental apparatus. Heat removal rates from the heated surface to the different testing fluids were measured by varying the coolant flow rates, wall temperatures, nanoparticle material, nanoparticle morphology (shape and nanoparticle size) as well as mass concentrations of nanoparticles in the coolants. Anomalous thermal behavior was observed in the forced convective heat transfer experiments. Precipitation of the nanoparticles on the heat exchanging surface was monitored using Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray spectroscopy (EDX). Isolated precipitation of nanoparticles is expected to cause formation of “nanofins” leading to enhancement of surface area and thus resulting in enhanced convective heat transfer to the nanofluid coolants. However, excessive precipitation (caused due to the agglomeration of the nanoparticles in the nanofluid coolant) causes scaling (fouling) of the heat exchanging surfaces and thus results in degradation of convective heat transfer. This study shows that the surface morphology plays a crucial role in determining the efficacy of convective heat transfer involving suspensions of nanoparticles in coolants (or nanofluids). Flow visualization and quantitative estimation of near-wall temperature profiles were performed using quantum dots and fluorescent dyes. This non-contact measurement technique for temperature and flow profiles in microchannels using quantum dots is expected to make pioneering contribution to the field of experimental flow visualization and to the study of micro/nano-scale heat transfer phenomena, particularly for forced convective heat transfer of various coolants, including nanofluids. Logical extensions of this study were explored and future directions were proposed. Preliminary experiments to demonstrate feasibility showed significant enhancement in the flow boiling heat flux values for nanofluids compared to that of pure solvent (DIW). Based on the novel phenomena observed in this study several other topics for future research were suggested, such as, using Surface Plasmon Resonance (SPR) platforms to monitor precipitation of nanoparticles on microchannel surfaces in real time (e.g., for generating surface isotherms).

Miniaturization of electronic devices for enhancing their performance is associated with higher heat fluxes and cooling requirements. Surface modifi cation 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 percent. In addition, silicon nanotubes etched on the silicon substrates have shown heat flux enhancement by as much as 120 percent. The heat flux augmentation is attributed to the combined effects of increase in the surface area due to the protruding nanotubes (nano- n eff ect), disruption of vapor lms and modi fication of the thermal/mass di ffusion boundary layers. Since the e ffects of disruption of vapor lms and modifi cation of the thermal/mass di ffusion boundary layers are similar in the above experiments, the difference in enhancement in heat transfer is the consequence of dissimilar nano- n eff ect. 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 e ffect of the molecular scale chemistry and structure can help to simulate the performance of a nano fin in diff erent kinds of coolants. So in this work to elucidate the eff ect 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 diff erent 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 e ffective thermal interfacial resistance between the nano-fi n 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.