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

本研究的目的是建立一种利用近临界流体特殊的换热特性进行主动换热控制的理论。本研究项目所取得的研究成果概述如下。在亚临界和超临界二氧化碳中，利用水平铂丝进行了阶梯加热的传热实验。在此基础上，分析了快速非稳态换热特性，为主动换热控制提供了依据。在超临界压力下，在向上发展的浮力羽流的上缘观察到气泡状结构。作为上述瞬时换热特性的应用，对脉冲加热水平导线在超临界二氧化碳中的瞬时换热进行了实验研究。结果表明，利用近铬…的瞬时换热特性，可以控制被加热表面的温度。更多的液体物质。控制因素是脉冲加热频率和流体性质。为了阐明跨临界流体中沸腾和类沸腾现象的基本机理，进行了亚临界和超临界二氧化碳中水平细导线的稳态换热实验。当压力从亚临界压力增加到超临界压力时，换热特性和对流形态逐渐发生变化。在临界点没有观察到不连续的转变，因此，从亚临界沸腾到类超临界沸腾现象的跨临界转变是连续的。压强在临界点附近，似乎略高于临界点；即使在临界压强下，也观察到了沸腾从核沸腾到膜沸腾的转变，并伴随着临界热流密度。然后，将亚临界沸腾转变理论应用于超临界情况，并估算了超临界界面张力的大小。为了阐明超临界压力下界面张力的微观机理，对具有温度梯度的简单流体体系进行了分子动力学模拟。这种类界面现象即使在超临界压力下也能重现。根据得到的密度分布，用统计力学方法估算了超临界界面张力。将预测值与上述实验的估计值进行了比较，并观察到了较好的一致性。这些发现带来了一种全新的见解，解释了Cleary在略微超临界压力下观察到的沸腾现象。较少

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