Experimental Investigations on Heat Transfer of Supercritical CO₂ Flowing Through A Helically Coiled Tube

Heat transfer characteristics of supercritical CO₂ flowing upward through a uniform-heated helically- coiled tube were experimentally investigated under the conditions of q=0~25 kW/m², G=10~262 kg/（m²·s） and Pin =8~9 MPa. Effects of mass flux, heat flux and inlet pressure on the longitudinal heat transfer performance were analyzed. It was found that the heat transfer coefficients first increased and then decreased. The maximum values of heat transfer coefficients always occurred at the temperature range of Tb ＜Tpc ＜Tw. For the heat transfer coefficients ascent stage, the heat transfer capability enhancement per unit volume caused by the increased heat capacity, together with the thinner thermal boundary layer induced by the decreased viscosity, dominated the heat transfer performance at the near-wall region. While for the stage that CO₂ experienced the state transition from quasi-liquid to qusi-vapour, the sharp decreases of heat capacity and thermal conductivity became the dominant factor for the heat transfer deterioration. For the transcritical convective heat transfer with large physical property variations, the Nusselts number, standing for the ratio of heat convection to conduction, could not represent the real heat transfer capacity any more.