| Summary: | 碩士 === 國立臺北科技大學 === 能源與冷凍空調工程系碩士班 === 104 === With the development of technology and the gradual modernization of the industry in Taiwan, more and more energy was demanded. But now we were confronted with the problem of energy shortage. To save energy and improve the utilization of energy, our country strongly advocated the energy conservation and emission reduction, and the most important research direction which was how to improve the efficiency of the device for thermal energy and power. Because of the corresponding research being less, especially the fluid flow and heat transfer for supercritical fluid under the conditions of the thermal power plant had become a bottleneck restricting. At present, people required their heat exchange equipment for small volume, light weight, and effective thermal efficiency. To provide the corresponding theoretical basis on optimization design of the heat exchange equipment, this thesis used numerical simulation method to mixed convection of supercritical carbon dioxide in inclined tubes and helical pipes. The effects of thermal buoyancy induced by physical property variations on the fluid flow and heat transfer were analyzed. Additionally, the heat transfer enhancement methods in inclined and helical tubes were then be explored and proposed. In the present thesis, the conditions of constant wall temperature and mass flow rate in inclined tubes and helical tubes were adopted. The predicted results show when the supercritical carbon dioxide is heated along the flow direction of the inclined tube, the strong buoyancy force generated by the temperature difference between the fluid and the fluid would occur (1) near the entrance of the inclined tube, (2) for positive inclined angle, and (3) for inlet fluid temperature close to 308K at 8 MPa. Besides, the most severe heat exchange occurred at (1) the position of strong buoyancy, (2) secondary flow and the main flow being 90 degree angle, and (3) the fluid flow rate being slow. For mixed convection inclined tubes, the supercritical carbon dioxide is influenced by the thermal buoyancy and the flow is symmetric. While for mixed convection in helical pipes, the fluid flow was affected by the floating lift and centrifugal force. Therefore, the flow distribution is more chaotic than that of the asymmetric fluid. Simulation results disclose that the supercritical carbon dioxide in the helical tube heated along the direction of flow, turbulence intensity increases with increasing temperature and velocity. So, the inlet Reynolds number and wall temperature would influence the turbulence intensity and the effects of inlet Reynolds number are relative important.
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