Comparison of the Heat Transfer Characteristics of Supercritical Pressure Water to That of Subcritical Pressure Water in Vertically-upward Tubes

Abstract

A systematic comparison was made between the forced convection heat transfer characteristics of the supercritical pressure water and that of the subcritical pressure water in vertically-upward tubes. It was found that, severe heat transfer deterioration did not occur in the vertically-upward internally-ribbed tube at supercritical pressures, and the variations in the inside-wall-temperature with the bulk fluid enthalpy experienced three stages, namely, the monotonically increasing stage, the fast changing stage and another monotonically increasing stage at the supercritical pressures; however, at subcritical pressures, there existed at least four stages for the variation of the internal tube wall temperature, i.e., the monotonically increasing stage, the basically unchanging stage, the sharply rising stage and another monotonically increasing stage. The heat transfer coefficients (HTC) in the subcritical two-phase region, in which the heat transfer deterioration did not occur, were much greater than that in the heat transfer enhancement region of supercritical pressure water. In the large specific heat region of supercritical pressure water, the lower the heat flux, the more obvious the heat transfer enhancement would be; however, in the subcritical two-phase region, the higher the heat flux, the greater the heat transfer coefficient would be. It was also found that the heat transfer deterioration of supercritical pressure water was similar in mechanism to the DNB (departure from nucleate boiling) at subcritical pressures.