Evaluation Criterion of Buoyancy and Thermal Acceleration Behavior of Supercritical CO2 in a Horizontal Circle Tube

According to the complex thermophysical properties of supercritical CO2, the heat transfer experiments on a smooth horizontal tube with a 2 mm inner diameter were carried out. The test pressure was 7.4 - 8 MPa, the mass flux was about 600 - 1 000 kg/(m2·s), and the maximum heat flux was 94.2 kW/m2. Through experiments, the wall temperature and local heat transfer coefficient were obtained. The non-dimensional buoyancy and thermal acceleration behavior under different experimental conditions were analyzed. The results show that the peak heat transfer coefficient of the fluid increases gradually with the heat flux and that there exists great deviation in the variation trend for different non-dimensional buoyancy. The Kim's buoyancy correlation can better judge the value of buoyancy. Three different non-dimensional thermal accelerations indicate that the effect of thermal acceleration at low heat flux is not significant, while at high heat flux the acceleration effect presents a first increasing then decreasing tendency. When the buoyancy factor and thermal acceleration factor are less than 5.6×10-7 and 0.000 5, respectively, the effects of both buoyancy and thermal acceleration can be ignored. By taking into account both buoyancy and thermal acceleration, a new Nusselt correlation can be established. The average error and root mean square error of the correlation is -2.92% and 10.96%, respectively. © 2018, Editorial Office of Journal of Xi'an Jiaotong University. All right reserved.