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The term “nanofluid” was introduced by Choi et al (1995) for stable dispersions of nanoparticles. These colloidal suspensions of nano-size-powders (of size 1 nm to 100 nm) in a base fluid are of technical interest for their potential as enhanced alternatives to ordinary cooling fluids (Yu et al, 2008). The first reference to nanofluids enhanced thermal properties was reported by Masuda et al (1993), in their studies of the thermal conductivity in liquid dispersions of ultra-fine particles. Nanofluids are the subject of growing research, and they are typically mixtures of solid nano-particles (metal oxides as alumina, silica, titanium dioxide, copper oxide, various carbides and nitrides, and carbon nanotubes or nanofibers), which are added in concentrations of up to 5% in continuous and saturated fluids (mainly typical coolants as water, ethanol and ethylene glycol, or lubricants).
Nanofluids are predicted to have higher thermal conductivity and heat transfer coefficients than those of the base fluids, because nanoparticles have much larger surface-to-volume ratio and higher mobility than those of larger solid particles (Machrafi et al, 2016). The potential of nanofluids as promising alternative for critical-cooling systems, and critical issues on nanofluid heat-transfer research have been recently reviewed by Kostic (2013).
Many concerns remain about the practical application of nanofluids, mainly because of their potential wear, erosion and corrosion effects on cooling system materials. Addition to fluids of solid particles in the micrometer-range (as silica) is known to lead to higher erosion rates on conduit materials, but the effect of adding smaller-size ones, as nanoparticles (of 1nm to 100nm) are largely unknown. Future use of nanofluids to replace conventional cooling fluids requires a better understanding of the likely surface-modification effects when such nanofluids come in contact with typical cooling-system materials.
The effects of multiple solid-particle impacts on solid surfaces are known to depend mainly on particle velocity and size, and experimental study is required to assess eventual erosion, impact and corrosion effects. Most of the available knowledge on multiple-particle impact was obtained from jet-projected particles for velocities that are orders of magnitude than those of actual cooling system flows, and involving particles in the millimeter or micrometer ranges. Early studies of multiple macroscopic-particle impact and erosion were carried out by Brainard and Salik (1980) for copper and aluminum specimens with 3.2-millimeter-diameter steel balls in air at normal incidence and speeds of up to 140 m/s; their results agree with the findings of Cousens and Hutchings’ work (1983). Rao and Buckley (1983) investigated that an “incubation period” and acceleration-deceleration erosion evolution occur on 6061-aluminum when treated by normal impingement-jets of a mix of spherical glass beads and angular crushed-glass particles. Gee and Hutchings (2002) reviewed the four common “dry-type” erosion test systems (e.g., gas jet; centrifugal accelerator; wind tunnel; and whirling arm tests), and included a discussion on the important variables in these erosion tests (i.e., particle impact velocity, particle impact angle, particle size, shape and material, and ambient temperature). The ASTM G76 - 07: “Standard Test Method for Conducting Erosion Tests by Solid Particle Impingement Using Gas Jets” (2007) covers testing of material loss by solid particle impingement with gas-carrier jet-type erosion equipment. There is abundant data from these particle-in-gas-jet erosion tests of metals, the work of Molina et al (2012) reviewed the mechanisms of observed gas-jet erosion and the strong effect of particle size and velocity in those tests.