About 10-15 years ago, fluids, either directly (mainly Fluorocarbons), or indirectly (cold plates flushed with water) cooled the majority of mainframe computers. Nowadays, the expected increase in heat flux density causes a renewed interest in either passive or active liquid cooling, because the limits of air cooling come within reach. Other practical applications in which fluids play a role include phase change materials and heat pipes.
If we focus on maximizing the heat transfer coefficient, it is clear that the thermal conductivity of the fluid is the dominant parameter. In general, the thermal conductivity of solids drops by 15-40% at the melting point caused by the increased disorder. However, the difference for water is much larger. Ice at ‘normal’ temperatures has a thermal conductivity that is more than three times higher. More importantly for cooling purposes, the thermal conductivity of water is much higher than that of all other (non-metallic) fluids of practical interest to electronics cooling.
Regarding the dependency on temperature, most liquids exhibit a 10% decrease in thermal conductivity in the 0-100°C temperature range. However, as usual, water is an exception, showing an increase of 10%.
The table below provides some order of magnitude data for different classes of liquids. Many more data, also as a function of temperature, can be found in the VDI Warmeatlas, Springer Verlag Heidelberg, Germany.
| Fluid | Thermal conductivity (W/mK) |
| Water | 0.6 |
| Other polar liquids | 0.2-0.6 |
| Fluorinerts | 0.06-0.07 |
| Low viscosity oils | 0.11-0.15 |
| Other organic liquids | 0.12-0.20 |
| Molten salts | 1-4 |
| Molten metals | 10-100 |
