Investigation on heat transfer and hydraulic resistance characteristics of supercritical fluids in tubes

Supercritical fluids have a wide range of applications in ultra-supercritical power units, water cooled reactors and aerospace fields with a good performance in flowing and heat transfer. When medium works in large specific-heat region, a dramatic change in physical properties of fluids occurs, resulting in a complicated change about the flow structure in tubes. In the present paper, the main conclusions are: At supercritical pressure, when the water works in large specific-heat region, the frictional resistance coefficient values decrease sharply, leading to a lower friction pressure drop and a smaller proportion in total pressure drop; simultaneously acceleration pressure drop values gradually increase. At low heat flux, the heat transfer coefficients are in good proportions to acceleration resistance coefficients. An appropriate increase in heat flux could effectively enhance the heat transfer coefficients. At high heat flux, being accompanied with heat transfer deterioration, a big velocity and viscosity gradient difference exist and laminarzation phenomenon occurs at the same time in tubes. The heat transfer mechanisms of supercritical water has been analyzed in this paper. Whether the heat transfer is improved or deteriorated primarily depends on the extent of the difference of physical properties of fluids in different radial positions.