Numerical study on in-tube laminar heat transfer of supercritical fluids

Numerical calculations were carried out for supercritical carbon dioxide flowing in miniature tubes with Re less than 1000. The heat transfer coefficient α and friction factor f were numerically studied for different values of the tube diameter, pressure, mass flux, and heat flux. When compared with the constant property flow, where Nu = 4.364 and f = 64/Re for a circular tube under a constant heat flux condition, a large divergence from the constant value was obtained for both Nu and f·Re in the vicinity of the pseudocritical temperature T_m. When cooled under a constant heat flux, Nu attained its peak value when T_b > T_m and its minimum value when T_b < T_m, while f·Re attained its peak value at T_b = T_m. With regard to the heating process, the reverse tendencies were confirmed. The variations of the specific heat with temperature were found to be the dominant factor for Nu. In addition, empirical correlations that considered the cross-sectional distribution of thermophysical properties were proposed to predict the values of Nu and f both in the near-pseudocritical temperature region and in the thermal entrance region of the tube. The proposed correlations were also verified by comparing the predicted results with numerical results obtained by using supercritical water.