Study on Heat-Transfer Control Utilizing the Rapid-Transient Heat-Transfer Characteristics of Trans-Critical Fluids

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The aim of the present research project is to establish an theory of the active heat-transfer control using peculiar heat-transfer characteristics of near-critical fluids. The outline of the results obtained in this research project is given below.Heat-transfer experiment has been carried out using a step-heated horizontal platinum wire in sub-and supercritical carbon dioxide. Based on the results, rapid transient heat-tranfer characteristics were analyzed which are useful as a basis of the active heat-transfer control. At supercritical pressure, bubble-like structures were observed in the upper edge of the buoyancy plume which is developing upward.As an application of the above transient heat-transfer characteristics, transient heat transfer from a pulse-heated horizontal wire in supercritical carbon dioxide was investigated experimentally. It was clearly shown that the heated-surface temperature can be controlled by utilizing the transient heat-ternsfer characteristics of the near-cr … More itical fluid. The controlling factors are frequency of the pulse heating and fluid properties. This technique can be applied easily as an active heat-transfer control.In order to clarify the basic mechanism of boiling and boiling-like phenomena in the trans-critical fluid, experiments on steady heat tranfer from a fine horizontal wire in sub-and supercritical carbon dioxide have been performed. As pressure increases from subcritical to supercritical pressure, heat transfer charactenistics and convection patterms change gradually. No discontinuous transition was observed at the critical point, thus we conclude that the trans-critical transition from subcritical boilng to suercritical boilng-like phenomena is continuous. The tansion pressure, which has been expected to be at the critical point, seems to be slightly above the critical pressure ; boiling transition from nucleate boiling to film boiling accompanied by the critical heat flux were observed even at he critical pressure. Then, a theory of subcritical boiling transition, which is used to predict the critical heat as a function of interfacical tension, was applied to the present supercritical case and the magnitude of the superctitical interfacial tension was estimated.To clarufy the microscopic mechanism of the interfacial tension that remains even at supercritical pressures, molecular dynamics simulation was performed for a system of simple fluid with a temperature gradient. The interface-like phenomenon was reproduced even at supercritical pressures. Based on the resulted density profile, the supercritical interfacial tension was estimated by a statistical mechanical technique. The predicted Value was compared with the estimation by the above experiments, and fair agreement was observed. These findings brigns a entirely new insight which explains cleary the boiling-like phenomena observed at a slightly supercritical pressure. Less