ABSTRACT
Nanofluids, suspensions of nanoparticles in a base liquid, are promising thermal fluids for achieving effective heat transfer enhancement. Nanofluids provide much higher thermal conductivity enhancement with significant increase in viscosity when compared to suspensions of micron-sized particles and eliminate problems of sedimentation, channel clogging, and tube wall erosion, which limited the applications of conventional suspensions for heat transfer enhancement. Since the first studies reporting the high thermal conductivity of nanofluids in 1995 [1], there has been a tremendous amount of experimental and theoretical research to gain insight into the relevant parameters
5.1 Introduction .......................................................................................................................... 125 5.1.1 Advantages of Nanofluids over Conventional Suspensions ...................................... 126 5.1.2 Nanofluid Preparation Techniques ............................................................................ 127 5.1.3 Stability and Dispersion Characteristics of Nanofluids ............................................ 127
5.2 Density .................................................................................................................................. 128 5.3 Specific Heat Capacity .......................................................................................................... 128 5.4 Thermal Conductivity ........................................................................................................... 129
5.4.1 Measurement Techniques ......................................................................................... 129 5.4.2 Effect of Particle Material and Base Liquid ............................................................. 129 5.4.3 Effect of Particle Volume Fraction ........................................................................... 130 5.4.4 Effect of Particle Size ............................................................................................... 131 5.4.5 Effect of Particle Shape ............................................................................................ 131 5.4.6 Effect of Temperature ............................................................................................... 132 5.4.7 Conclusions ............................................................................................................... 134
5.5 Mechanisms of Thermal Conductivity Enhancement .......................................................... 134 5.5.1 Classical Models for Suspensions of Particles in a Liquid ....................................... 134 5.5.2 Brownian Motion of Nanoparticles .......................................................................... 135 5.5.3 Liquid Layering around Nanoparticles ..................................................................... 137 5.5.4 Clustering of Nanoparticles ...................................................................................... 138 5.5.5 Combined Effects of Ballistic Phonon Transport and Clustering ............................ 141 5.5.6 Combined Effects of Brownian Motion and Clustering ........................................... 142
5.6 Viscosity ............................................................................................................................... 142 5.6.1 Classical Models for Suspensions of Particles in a Liquid ....................................... 142 5.6.2 Effects of Nanofluid Parameters ............................................................................... 143 5.6.3 Theoretical Models for Nanofluids ........................................................................... 145
5.7 Combined Effect of Enhanced Thermal Conductivity and Increased Viscosity on Heat Transfer ................................................................................................................... 146
5.8 Conclusions ........................................................................................................................... 147 References ...................................................................................................................................... 148
and governing mechanisms of thermal conductivity enhancement and viscosity increase in nanofluids. In order to give an idea about the rate of expansion of this field, the number of publications about nanofluids per year in the past decade is shown in Figure 5.1 [2].
