Therein, k is the thermal conductivity of fluid; Dh is the hydraulic diameter of the channel defined as 4V
A with fluid volume V and wetted surface area A in the channel. According to this definition,hydraulic diameters of the cold (water) side and hot (CO2) side flow channels made of the S-shaped fin configuration are about 3.36 mm and 0.59 mm, respectively.The calculated heat load and Nusselt number are also presented in Tables 2 and 3.