Efficiently Lowering the Freezing Point in Heat Transfer Coolants Using Carbon Nanotubes

CONVENTIONAL heat transfer fluids such as water, mineral oil, and ethylene glycol play an important role in many industries including power generation, chemical production, air conditioning, transportation, and microelectronics. It has been demonstrated recently that the heat transfer properties of these conventional fluids can be significantly enhanced by dispersing nanometer-sized solid particles and fibers (i.e., nanoparticles) in fluids. These new heat transfer fluids are known as nanofluids. Nanoparticles of various materials have been used to make heat transfer nanofluids, including copper, aluminum, copper oxide, alumina, titania, and carbon nanotubes (CNT) [1-3]. Of these nanoparticles, carbon nanotubes show the greatest promise due to their excellent chemical stability and extraordinary thermal conductivity. The melting point of carbon nanotubes is very high and the decomposition temperature is more than 600 deg C. In an ideal circumstance, single-walled nanotubes (SWNT) have the thermal conductivity value of 3000-6000 W/m ? K, and multiwalled nanotubes (MWNT) have the thermal conductivity of 2000-3000 W/m ? K. The structure of a single-walled carbon nanotube can be described as a single graphene sheet rolled into a seamless cylinder whose ends either open, or are capped by either half fullerenes or more complex structures including pentagons. Multiwalled carbon nanotubes comprise an array of such nanotubes that are concentrically nested like rings of a tree trunk with a typical distance of approximately 0.34 nm between layers.