Comparison of Heat Transfer Characteristics of Flow of Supercritical Carbon Dioxide and Water inside a Square Microchannel

Water is a common coolant for many heat transfer applications, yet, it has many disadvantages relative to other liquid coolants, such as high corrosiveness, high electrical conductance, and high pressure drop characteristics. Carbon dioxide slightly above its supercritical point — known as supercritical carbon dioxide (sCO
2
) — exhibits excellent thermo-physical properties, such as high specific heat, thermal conductivity, and high heat capacity than many other gases, such as air. Also, sCO
2
is less corrosive and experience less pressure drop than water. In this study, heat transfer characteristics of sCO
2
and water were compared for a 300-μm square microchannel. Computational studies were made for both laminar and turbulent flow regimes, and it was found that at a mass flux of 5,555 kg/m
²
s, sCO
2
experienced a pressure drop of 27.81 kPa compared to 51.77 kPa for water. Nusselt numbers were compared with available empirical correlations, such as Dittus- Boelter and Gnielinski, for both water and sCO
2
, and it was found that they failed to accurately predict the Nusselt number for sCO
2
obtained through CFD simulation. At higher mass fluxes the local Nusselt number for sCO
2
were higher than those for water as the flow became turbulent for sCO
2
due to its low dynamic viscosity.