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

本课题的目的是利用近临界流体特有的传热特性，建立一种主动传热控制理论。本研究项目所取得的成果概要如下：在亚超临界和超临界二氧化碳中，使用分级加热的水平铂丝进行了传热实验。在此基础上，分析了快速瞬态传热特性，为主动传热控制提供了依据。在超临界压力下，浮力羽流的上边缘出现气泡状结构，并向上发展。作为上述瞬态传热特性的应用，对超临界CO2中脉冲加热水平金属丝的瞬态传热进行了实验研究。结果表明，利用近铬层的瞬态传热特性，可以控制加热表面温度 ...更多信息 理想流体控制因素是脉冲加热的频率和流体性质。为了阐明跨临界流体中沸腾和类沸腾现象的基本机理，本文进行了亚临界和超临界二氧化碳中水平细丝稳态传热实验。随着压力从亚临界到超临界的增加，换热特性和对流模式逐渐发生变化。在临界点没有观察到不连续的转变，因此我们认为从亚临界沸腾到超临界沸腾的跨临界转变是连续的。的tansion压力，这已被预期在临界点，似乎是略高于临界压力，沸腾过渡从核沸腾到膜沸腾伴随着临界热流密度，即使在他的临界压力观察。然后，将亚临界沸腾转变理论应用于该超临界流体，估算了超临界界面张力的大小，并对具有温度梯度的简单流体体系进行了分子动力学模拟，阐明了界面张力在超临界压力下保持不变的微观机理。即使在超临界压力下也能再现类似界面的现象。基于所得的密度分布，超临界界面张力估计的统计力学技术。将预测值与上述实验的估计值进行比较，观察到相当一致。这些发现提供了一个全新的视角，可以清楚地解释在轻微超临界压力下观察到的沸腾现象。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