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Model for Energy Transport in Nanofluids
Keywords: 2 phase cooling, Brownian motion nanoparticles, Improved thermal management
The conventional heat transfer enhancement techniques can hardly meet the challenges of ever increasing demand of heat removal in processes involving electronic chips, heat exchangers, automotives, manufacturing, refrigeration and air-conditioning, aircraft, space applications, laser applications and similar high energy devices. The factors which limit the usual techniques are many. Among them one major limitation is the poor thermal characteristics of conventional heat transfer fluids. They are less efficient in conducting heat compared to metals. Then the idea of increasing the thermal conductivity of fluids by 2 phase cooling (suspension of solid particles in fluid otherwise called nanofluid) came in to light. However, the suspended particles of order micro to millimeters posed many problems like clogging in small flow passages, abrasive action causing erosion and settling of particles under gravity. Thus, even though these slurries have higher conductivities, they are impracticable as heat transfer fluids. The bottleneck of these slurries was replaced by particles having nanometer dimensions. This nanoparticle liquid combination resulted in high thermal conductivity which has already been proved by many researches experimentally. To the date there is no clear theoretical explanation for this unconventional behavior. To understand the underlying phenomenon a theoretical model which accounts for the effect of nanolayer formation around a particle, Brownian motion of the nanoparticles and the size distribution of nanoparticles were used in this study to calculate the effective thermal conductivity of the Nanofluids. The study divided the heat transfer in nanofluids into two portions heat transferred by stationary particles and heat transferred by moving particles. Each portion was studied individually and then together. The study came up with an expression that successfully explained the enhanced thermal conductivity of nanofluids and came to some important conclusions. It was found that Kinetic theory is not enough to explain the enhanced thermal conductivity of nanofluids. The contribution of the Brownian motion of nanoparticles towards the overall thermal conductivity of nanofluids is very small. Then study concentrated on size distribution of nanoparticles which have been proven to be an important factor and it gave satisfactory results. The values of different nanofluid combinations were calculated and they agreed successfully with published experimental data. The present model was tested against several nanofluid combinations. So far no study has reported the results of these many kinds of nanofluid combinations. To understand the properties that influence the thermal conductivity of nanofluids, parametric studies of a number of nanofluids were carried out. From the parametric study, it was observed that Brownian motion is significant when the particle diameter is less than 10 nm. So this advanced technology of suspending nanoparticles in base fluids might provide answers to improved thermal management. Improved understanding of complex nanofluids will have an even broader impact.
Vijaya Kumar Kollipara, Research Assistant
Southern Illinois University
Carbondale, IL

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