Nanofluids for Heat Transfer – Potential and Engineering Strategies

In an age of increasing heat fluxes and power loads in applications as diverse as power electronics, renewable energy, transportation, and medical equipment, liquid cooling systems are necessary to enhance heat dissipation, improve energy efficiency, and lengthen device lifetime. To satisfy these increasing thermal management needs, the heat transfer efficiency of conventional fluids must be improved. Nanofluids are nanotechnology-based heat transfer fluids that are engineered by stably dispersing nanometer-sized solid particles (such as ceramics, metals, alloys, semiconductors, nanotubes, and composite particles) in conventional heat transfer fluids (such as water, ethylene glycol, oil, and mixtures) at relatively low particle volume concentrations. Nanofluids have been considered for applications as advanced heat transfer fluids for almost two decades, since they have better suspension stability compared to micron-sized solid particles, can flow smoothly without clogging the system, and provide enhanced thermal and physical properties. Nanofluids are in essence nanocomposite materials, with adjustable parameters including, but not limited to nanoparticle material, size, and shape, base fluid, surfactants and other additives. The thermal conductivity of heat transfer fluid is widely recognized as a main factor influencing the heat transfer efficiency. Low thermal conductivity of conventional fluids (i.e. 0.1-0.6 W/mK at 25oC) improves when solid particles with significantly higher thermal conductivity values (i.e. 10-430 W/mK for pure elements) are added. Therefore addition of small solid particles to liquids improves thermal conductivity of suspension, while still allowing for convection heat transfer mechanism of the fluid. The magnitudes of the effects reported in the literature are scattered from few percent (as predicted by effective medium theory (EMT) [1-3]) to hundred percents per nanoparticle volume concentration (i.e. abnormal enhancements [4-6]).