Visualization experiments and piston effect of heat transfer for supercritical carbon dioxide

The characteristics of supercritical heat transfer and flow pattern visualization are investigated experimentally. The local heat transfer coefficient and visual scenario of supercritical carbon dioxide are presented. The experimental results reveal that the heat transfer peak occurs when the wall temperature is above the pseudo -critical temperature and the in-tube fluid is lower than the pseudo-critical temperature. The phenomenon of critical opalescence is observed under a pressure of 7.4 MPa and a temperature of 305.8 K. The piston effect is characterized by a rapid alternation of a sky blue and milky white color, which may be related to Rayleigh scattering due to drastic density fluctuation. A novel piston effect of strongly uneven aggregation is introduced to examine the mechanism of the supercritical heat transfer peak near the pseudo-critical temperature. The ratio of density and volumetric coefficient of expansion can be represented for the piston effect to obtain the modified Jackson's correlation, which has an average error of -0.7% and root-mean-square error of 10.4%.