Experimental Study on Convective Heat Transfer of Supercritical R134a in Vertical Circular Tubes

In order to investigate the convective heat transfer at supercritical pressures, experimental research was conducted on heated Freon R134 a flowing upward through an I.D 25 mm vertical circular tube under supercritical pressure condition. The experimental data covers a wide range of conditions. The pressure is at 4.5MPa and 4.7MPa. The mass flow flux varies from 400 to 700 kg/（m²·s） and the heat flux ranges from 30 to 60 k W/m². Both enhanced and deteriorated heat transfer were analyzed and the parametric sensitivity was also carried out. The heat transfer performance was evidently enhanced near the pseudo-critical point. The deteriorated heat transfer appeared at lower mass flow velocity or at higher heat flux under a certain ratio of q/G=0.06 k J/kg. At a mass flux of 500 kg/（m²·s） two types of deteriorated heat transfer were observed in the experiment: the first type appeared at the near entrance region of the tube and existed within different range of fluid inlet temperature; The second type appeared at any section inside the tube than entrance but only within a certain enthalpy range. Heat transfer can be enhanced by increasing mass flow velocity, decreasing heat flux or decreasing pressure, while the variance of the heat transfer deterioration is opposite.