ABSTRACT
Greek Symbols .............................................................................................. 210 Subscripts ...................................................................................................... 210 Coordinates ................................................................................................... 210
References .............................................................................................................. 210
Advances in the large-scale synthesis of a variety of nanoparticles, nanowires, and nanotubes over the last decade have led to new interest in the design of nanoparticle suspensions, or nanofluids, having tailored thermophysical properties. Such engineered, two-phase systems may have utility as enhanced heat-transfer fluids, for example, having greater thermal conductivities and heat-transfer coefficients than conventional fluids. Experimental and theoretical investigations of nanofluids in the heat-transfer community were stimulated by initial reports of large increases in the thermal conductivity of very-low-volume-fraction nanoparticle and nanotube suspensions, for example, copper nanoparticles in ethylene glycol [1], multiwalled carbon nanotubes (MWNTs) in oil [2], and silver nanoparticles in water and toluene [3], among others. Nanofluids were reported to deviate from classical effective medium theory for the thermal conductivity of composite materials in terms of (1) anomalously large increases in thermal conductivity, (2) possible dependence on particle size and shape, and (3) dependence on temperature. With respect to the first two reported anomalies, Nan et al. were among the first to point out that the reported increases in thermal conductivity were within the bounds of existing theory when the aspect ratio of highly anisotropic nanoparticles such as carbon nanotubes (CNTs) is taken into account [4]. For suspensions of spherical nanoparticles, Prasher et al. show the importance of particle-cluster morphology, which, with the formation of large-aspect-ratio particle clusters, can substantially increasing the effective thermal conductivity of a suspension [5]. For several types of well-dispersed nanoparticles (alumina nanorods and nanoparticles, gold nanoparticles, silica nanoparticles, and Mn-Zn ferrite nanoparticles) in oil and water, recent experimental results from the International Nanofluid Property Benchmark Exercise (INPBE) indicate that the measured thermal conductivities are in agreement with classical effective medium theory (EMT) [6].
