Theoretical Model For Buoyancy-Induced Heat Transfer Deterioration Under Supercritical Pressure

In an organic Rankine cycle system which utilizes the low-grade heat sources in various industrial processes, operating the heating process at supercritical pressure provides a possibility to increase the system efficiency because more work can be recovered. Under this background, it is necessary to investigate the supercritical heat transfer characteristics of the organic working fluids. In this study, the characteristics of supercritical heat transfer of fluid R245fa, which is commonly applied in the commercial ORC power plant, and R1233zd(E), which is recognized as an alternative working fluid owing to its low-global-warming-potential value, were investigated experimentally under the heating condition in a vertical tube. The test tube has an inner diameter of 4 mm and a length of 1040 mm. The heat flux ranging from 15 to 100 kW/m2 for R245fa and 20 to 80 kW/m2 for R1233zd(E), and mass flux from 400 to 800 kg/m2s for R245fa and 400 to 600 kg/m2sfor R1233zd(E). The experiments were conducted at a pressure of 4.0, 4.5 and 5.0 MPa for R245fa and 3.93 and 4.40 MPa for R1233zd(E). The local heat transfer coefficients were calculated from the measurement parameters. The abrupt deteriorations of heat transfer were found in moderate heat flux and low mass flux at three operating pressure for R245fa, while such a characteristics were not found in the experiment of R1233zd(E). This abrupt deterioration can be attributed to the buoyancy, which was induced by severe variation of density when fluid temperature experienced pseudocritical temperature. When the mass flux is large, there is no abrupt deterioration of heat transfer, and the heat transfer coefficient changes smoothly.For all the experimental conditions, the heat transfer coefficients of R1233zd(E) were higher than that of R245fa