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Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,*,Thermal Conductivity of Nanofluids,Siw,Sin Chien,University of Pittsburgh,ENERGETICS,ME 3007,Oct 27,2021,Outline,Introduction,Objective,Experimental results,Simulation results,Discussion and Comments,Conclusion,Introduction,“Nanofluid is the name conceived to describe a fluid in which,nanometer-sized particles are suspended Choi 2021.,knanoparticles kbase fluids,X.Q.Wang&A.S.Mujumbar 2007,Introduction-Why?,Early days,researchers-heat transfer performance of fluid -,changing flow geometry -boundary conditions -,enhancing thermal conductivity of the fluid-dispersion of particles,Base fluid can be enhanced by suspending micro/larger size solid particles.-,large particles(micron size)higher density,settling down -induced additional flow resistance possible erosion,-,basic concept of dispersing solid particles in fluids to enhance thermal conductivity can be traced back to,Maxwell in the 19th Century,.,-,confined to mm-sized particles.,-major challenge is the rapid settling of these particles in fluids.,Nanoparticles,stay suspended much longer than micro-particles and,if below a threshold level and/or enhanced with surfactants/stabilizers,remain in suspension almost indefinitely.,-surface area per unit volume of,nanoparticles,is much larger(million times)than that of,microparticles,(the number of surface atoms per unit of interior atoms of,nanoparticles,is very large).,Criteria,Microparticles,Nanoparticles,Stability,Settle,Stable(remain in suspension almost indefinitely),Surface/volume ratio,1,1000 times larger than that of microparticles,Conductivity,Low,High,Clog in microchannel?,Yes,No,Erosion?,Yes,No,Pumping Power,Large,Small,Introduction,Table 1:Comparison of the Old and New Sarit K.Das et.a.l.2021,Introduction,Unprecedented growth in electronics,communication and computing technologies -Increasing power,decreasing size,higher heat capacity,demand for suitable for next generation of flow and heat transfer,(innovative cooling technology),Conventional method to increase heat flux rates:-extended surfaces such as,fins,and,micro-channels,-increasing flow rates increases pumping power,However,current design solutions already,push available technology to its limits,.,NEW Technologies and new,advanced fluids with potential to improve flow&thermal characteristics are of critical importance,Nanofluids,are promising solution to meet and enhance the challenges,Introduction,Thermal ConductivityEnhancement,Particle volume concentration,Base fluid material,Particle Size,Particle material,Temperature,pH,Objectives,Explore the experimental and simulation results of thermal conductivity enhancement reported by researcher in the recent years.,Compare the experimental and simulation results.,Identify the current issues in,nanofluids,.,Experiment Preparation of nanofluids,2 techniques,-The single step method The single step simultaneously makes and disperses the,nanoparticles,directly into a base fluid;best for metallic,nanofluids,.-,Nanoparticles,agglomeration is minimized(Pros),-,Only suitable for low vapor pressure fluid(Cons),-The two-step method(widely used),Nanoparticles,was first produced and dispersed into the base fluids.,-,Good for oxides,nanoparticles,(Pros),-,Not suitable for metallic,nanoparticles,(Cons),Figure 2:ZrO,2,in water that produced,with,Two Step method,Experiment Preparation of nanofluids,Figure 3:Cu nanoparticles in ethylene,glycol produced with,One Step method,Experimental Results Effect of Particle Size,Figure 4 Effect of particle size for Al,2,O,3,in water Chon C.H.et.al.(2006),Figure 5 Effect of particle size for Al,2,O,3,in ethylene glycol H.U.Kang et.Al.(2006),smaller particles,higher thermal conductivity-effective surface area increase as particle size decrease-increment due to Brownian motion-exponential increment as size getting smaller,Experimental Results Effect of Particle Shape,Figure 6 Effect of particle shape for SiC in water,Figure 7 Effect of particle shape for SiC in water,cylinder shape,nanoparticles,extended profile/geometry that can transport heat rapidly over significant distance.,-High aspect ratio,nanoparticles,may aligned parallel to the temperature gradient enhance,k,eff,cylindrical shape,cylindrical shape,Experimental Results Effect of Temperature,Figure 8 Effect of temperature for Al,2,O,3,in water,temperature increment reduce the viscosity of the,nanofluids,promote Brownian motion,same volume fractions,highest thermal conductivity highest temperature.,7%-10%increment for temp.range from 28,o,C 35,o,C.,Figure 9 Effect of temperature for Al,2,O,3,in water,Experimental Results Effect of Temperature,Figure 10 Effect temperature for MWCNT in water,Figure 11 Effect temperature for MWCNT in water,temperature increment reduce the viscosity of the,nanofluids,-,Inconsistency;,At 0.4%volume fraction different,k,eff,1.26,1.62,Experimental Results-Volume fraction,Figure 1
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