Coolant
An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corrosion of the cooling system.
Sulfur hexafluoride is used for cooling and insulating of some high-voltage power systems (circuit breakers, switches, some transformers, etc.).
Steam can be used where high specific heat capacity is required in gaseous form and the corrosive properties of hot water are accounted for.
Anhydrous ammonia is frequently used in large commercial systems, and sulfur dioxide was used in early mechanical refrigerators.
Carbon dioxide (R-744) is used as a working fluid in climate control systems for cars, residential air conditioning, commercial refrigeration, and vending machines.
The phase change may not occur at the cooled interface, but on the surface of the liquid, to where the heat is transferred by convective or forced flow.
Betaine is a similar coolant, with the exception that it is made from pure plant juice, and is not toxic or difficult to dispose of ecologically.
[1] Polyalkylene glycol (PAG) is used as high temperature, thermally stable heat transfer fluids exhibiting strong resistance to oxidation.
A cold fuel flows over some parts of the engine, absorbing its waste heat and being preheated before combustion.
Liquid fusible alloys can be used as coolants in applications where high temperature stability is required, e.g. some fast breeder nuclear reactors.
For certain applications the stems of automotive poppet valves may be hollow and filled with sodium to improve heat transport and transfer.
A new class of coolants are nanofluids which consist of a carrier liquid, such as water, dispersed with tiny nano-scale particles known as nanoparticles.
Purpose-designed nanoparticles of e.g. CuO, alumina,[6] titanium dioxide, carbon nanotubes, silica, or metals (e.g. copper, or silver nanorods) dispersed into the carrier liquid enhance the heat transfer capabilities of the resulting coolant compared to the carrier liquid alone.
The experiments however did not prove so high thermal conductivity improvements, but found significant increase of the critical heat flux of the coolants.
[8] Some significant improvements are achievable; e.g. silver nanorods of 55±12 nm diameter and 12.8 μm average length at 0.5 vol.% increased the thermal conductivity of water by 68%, and 0.5 vol.% of silver nanorods increased thermal conductivity of ethylene glycol based coolant by 98%.
[9] Alumina nanoparticles at 0.1% can increase the critical heat flux of water by as much as 70%; the particles form rough porous surface on the cooled object, which encourages formation of new bubbles, and their hydrophilic nature then helps pushing them away, hindering the formation of the steam layer.
This approach is common in spaceflight, for ablative atmospheric reentry shields and for cooling of rocket engine nozzles.
