Study On Thermal Properties Of Silver Nanofluids And Its Heat Transfer Enhancement In Micro-pin Fin Heat Sinks

With the development of technology, the integration degree of electronicdevices is higher and higher. The heat flow per unit area increases quickly. The sizerestriction and working environment of the heat exchange device is increasinglystringent, which requires much higher enhanced heat transfer technology. To solvethis problem, on the one hand, we can try to develop the heat exchange equipmentwith small size, light weight and good heat transfer performance to meet the highthermal flux. The heat sink with the micro-pin fin is one of them, which has beenproved to be a very effective way for thermal control of microelectronic devices. Onthe other hand, the common working fluid (water, oil, alcohol, etc.) has been difficultto meet some special conditions of heat transfer and cooling requirements. The fluidwith higher thermal conductivity should be developed as soon as possible. Theintroduction of nanofluids to heat transfer enhancement is a breakthrough of thisapproach.In this paper, combining the theoretical and experimental methods, we studiedtwo kinds of heat transfer enhancement: nanofluids and micro-pin fin heat sinks. The"drop" shaped micro-pin fin heat sinks with optimized structure were design. Thepreparation and thermal properties of nanofluids were analyzed. Using nanofluids asworking fluid, the flow and heat transfer characteristics of "drop" shaped micro-pinfin heat sinks were also investigated. These researches may provide basic data for thedevelopment of high efficient micro heat exchanger. The main contents include thefollowing aspects:Water-based silver nanofluids were prepared using "one-step" way, where thepolyvinylpyrrolidone (PVP) was used as surfactants. The nanoparticle size innanofluids we prepared has a narrow range (3-7nm) and the average particle size issmall (4.8nm). The addition of surfactant can improve the stability of nanofluids,however, the types and characteristics of surfactant will also influence the thermalproperties of nanofluids and the base fluid.The effects of surfactant type and mass fraction, temperature, pH value andother factors on the thermal conductivity and viscosity of the base fluid andnanofluids were investigated experimentally. The results show that the thermalconductivity of the base fluid changes obviously as the addition of surfactant,however, when the amount of surfactants reaches a certain value, the thermalconductivity of the base fluid trends to be a constant. Compared with the non-ionicsurfactants, ionic surfactants are more sensitive to temperature. With increasing oftemperature, the thermal conductivity of the ionic surfactant solution is graduallyclose to the water. Both Strong acid or strong alkaline conditions are not conducive tothe improvement of the thermal conductivity. When the mass fraction of non-ionicsurfactant (PVP) is4.0%, the viscosity of the solution is twice higher than the value of water. The ionic surfactants have little influence on the viscosity of the base fluid,especially at low mass fraction.The prediction model of nanofluids thermal conductivity and experimentalvalues were comparatively analyzed. The impact factors of the thermal conductivityof nanofluids were studied experimentally. Without considering the effects of smallparticle size, the traditional two-phase macroscopic models are difficult to predict thethermal conductivity of nanofluids.Compared with the millimeter and micron-level particle suspensions, nanofluidsthermal conductivity needs to take full account of the increase contact area betweenparticles and liquid brought by the decreasing of particle size, the adsorption of liquidmolecules on particle surface and micro-movement, micro-diffusion andmicro-convection of nanoparticles caused by Brownian motion. With the increasing ofvolume fraction of nanoparticles, the thermal conductivity of nanofluids increasedrapidly, especially in the higher volume fraction. With the volume fraction of0.012%,the ratio of the thermal conductivity of nanofluids reaches up to1.15. With increasingof temperature, the thermal conductivity of nanofluids increases more obviously, andwith the decreasing of particle size, the influence of the role of Brownian motionincreases.Using deionized water as the working fluid, combining the numerical simulationand experiment, the flow and heat transfer characteristics of drop-shaped micro-pinfin heat sinks were studied. The drop-shaped micro-pin fin with appropriate trail anglemay reduce the resistance loss by avoiding the vortex shedding after the tail of pin fin,instead of the weak heat transfer area with solid and also expand the heat exchangersurface to the fluid mainstream area, so as to enhance the heat transfer.The results show that: at corresponding Re, the drop-shaped micro-pin fin withthe tail angle α=60°has the best performance in drag reduction effect. Thestreamlined structure of drop-shaped micro-pin fin can improve the flow of the taildistribution, and also postpone the change of flow from laminar flow to transitionregion flow. The smaller of the tail angle α, the delayed effect is more obvious.Theexperiment of heat transfer shows that the optimal tail angle α of drop-shapedmicro-pin fin changes with the changing of flow rate. As the increasing of flow rate,tail angle α trends to decrease. However, the drop-shaped micro-pin fin with the tailangle α=60°has the best heat transfer enhancement, among the present Re range andexperimental condition.Two experimental systems of fluid flow across the micro-pin fin heat sinks andsubmerged jet impingement heat sinks were set up. The flow and heat transfercharacteristics of nanofluids in two systems were analyzed experimentally. Theexperimental data from flowing across the micro-pin fin heat sinks show that theaddition of surfactants increases the viscosity of fluid. The increment of viscosity willexacerbate the boundary layer separation and vortex shedding of the pin fin tail, thus increasing the pressure drop of the flow. The difference of nanofluid pressure drop atdifferent volume fractions of nanoparticles is quite small. The existence of thenanoparticles has significantly improved the heat transfer performance of nanofluids,but the high viscosity of nanofluids inhibits the effect of heat transfer enhancement ina certain degree. Compared with deionized water, the combined effect of thenanofluids on heat transfer enhancement can be embodied, only when the volumefraction of silver particles reaches to0.012%.The experimental data from submerged jetting impingement heat sinks showthat compared with base fluid (water and surfactant), the heat transfer coefficient ofnanofluids increases averagely by6.23%,9.24%and17.53%, with the silvernanoparticle weight fraction of0.02%,0.08%and0.12%, at the same jet velocityrespectively. The heat transfer coefficient is enhanced by6.61%with the silvernanoparticle weight fraction of0.12%, compared with water. In addition to the effectof increasing of thermal conductivity, the micro-movement, micro-diffusion andmicro-convection of nanoparticles caused by Brownian motion also play an importantrole to the increasing of convective heat transfer coefficient.