Investigation of field synergy principle for convective heat transfer with temperature-dependent fluid properties

The field synergy principle is widely used in the analyses and optimizations of convective heat transfer, but its original derivations are based on fluids with constant thermal properties. The applicability of the principle for fluids with temperature-dependent properties needs clarification. A theoretical analysis of the problem is conducted. It is found that a synergy principle of mass flux vector and specific enthalpy gradient is more fundamental. Because temperature gradients are parallel to specific enthalpy gradients, the original field synergy principle of velocity and temperature fields is still valid. Heat transfer of supercritical CO2 and molten salt in different finned channels is numerically studied, and different mean synergy angles are compared. It is found that the domain integration mean synergy angles calculated with specific enthalpy gradient or temperature gradient have the same trends. For the volume-weighted mean angles, the absolute value of dot product of velocity and temperature gradient should be used in calculating the local synergy angles. Otherwise, the trend of the volume-weighted mean angles can be contrary to the field synergy principle. Meanwhile, the accuracy of the mesh and the temperature gradient near the heat transfer surfaces is important for the numerical applications of the field synergy principle.