Nanoparticles are particles with a diameter between 1 and 100 nm. Nanofluids are the dispersions of metallic nanoparticles (10 -100 nm) in typical coolants such as water and ethylene glycol etc. By adding higher thermal conductivity metallic nanoparticles (e.g. copper – 400 W/m K) into lower thermal conductivity fluids (e.g. water – 0.63 W/m K), the overall thermal conductivity of the nanofluid is improved. Higher thermal conductivity fluids have higher heat transfer coefficients in pumped systems, resulting in smaller heat exchangers; in the case of automotive engines this further translates into smaller engines and more efficient use of the fuel.
Preventing agglomeration, where the nanoparticles clump together, is a critical need in nanofluid development. Agglomeration is particularly unattractive in commercial applications as agglomerated solutions can erode heat exchanger surfaces and pumps, clog pipes causing increased pressure drop, and exhibit degraded thermal properties. One way to alter the inherent propensity of non-coated particles to aggregate is to alter the surface chemistry of the particles. Surfactants can temporarily solve this problem by blocking the surface chemical groups responsible for irreversible bonding between particles and by providing a steric layer of repulsion. However, surfactants tend to degrade at elevated temperatures and form non-permanent bonds with the particle surfaces.
Advanced Cooling Technologies, Inc. has developed stabilized nanoparticle suspensions to improve the thermal properties of oil-based lubricants. Using the plasma enhanced chemical vapor deposition (PECVD) technique, ACT has encapsulated copper oxide nanoparticles with a stabilizing coating and dispersed the coated nanoparticles in polyester based turbine oils. The thermal conductivity of the respective nanofluids at various concentrations was evaluated by the transient hot wire method.
At 0.8% by volume, the dispersion of nanoparticles increased the thermal conductivity of the turbine engine oil by as much as 4.5%; see Figure 20. Furthermore, the stabilized thermal conductivity throughout the duration of the transient hot-wire test indicates stability of the nanofluid. The heat transfer coefficient of the turbine engine oil nanofluid was evaluated in a custom-made pumped-loop and demonstrated a 5% increase in heat transfer coefficient; see Figure 2.