| 研究課題/領域番号 |
17F17365
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| 研究機関 | 東京大学 |
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研究代表者 |
党 超鋲 東京大学, 大学院新領域創成科学研究科, 准教授 (30401227)
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| 研究分担者 |
SONG MENGJIE 東京大学, 新領域創成科学研究科, 外国人特別研究員
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| 研究期間 (年度) |
2017-11-10 – 2020-03-31
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| キーワード | Heterogeneous nucleation / droplet solidification / freezing front / trapped air bubbles / temperature distribution / frost and ice |
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| 研究実績の概要 |
As the prerequisite of frosting and icing, solidification of water droplets was firstly investigated in this project. To fundamentally understand the solidification process of a three-dimensional water droplet, a Hele-Shaw cell was designed and used to observe the solidification characteristics of two-dimensional water droplets. These investigated characteristics include heterogeneous nucleation, droplet morphology, freezing front evolution, trapped air bubbles, and temperature distribution inside a droplet, etc. The recalescence stage costs ten milliseconds, and the growing velocity could reach a peak of about 0.45 m/s, half of adult walking rate. The recalescence stage of a two-dimensional water droplet is clearly shown as the temperature sudden increase. The dendrite is still growing but melting at the initial and progress periods of the freezing stage, respectively. The freezing front evolution of three types of two-dimensional water droplets are different. For a hydrophilic one, it is always a concave, with the middle part is a horizontal straight line. For a hydrophobic one, it firstly is convex and then repeats that of the hydrophilic one. The freezing front of the transient two-dimensional water droplet is changed from a concave to a horizontal straight line, and then to concave, just like the front superposition of two hydrophilic ones. The trapped air bubbles in a frozen water droplet are at equivalent diameters of 1-100 μm, and more than half of the dissolved air in liquid water droplet trapped in the ice crystal during its solidification.
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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
2: おおむね順調に進展している
理由
The research work was carried out in accordance with the anticipated plan. It is always around the topic of frosting characteristics investigation. For example, the solidification of water droplet is the prerequisite of frosting and icing, which is the most important stage in the whole frosting process. Therefore, a Hele-Shaw cell was specially designed and used to observe the solidification characteristics of two-dimensional water droplets. The heterogeneous nucleation, freezing morphology, frontal migration, bubble formation, temperature distribution, and the heat and mass transfer process as the change of freezing rate in freezing process of droplets have been studied. Moreover, the energy transfer process in an air source heat pump unit during defrosting with melted frost locally drainage was analyzed. The effects of energy stored in the metal of indoor and outdoor coils on defrosting were quantitatively given. While some minor adjustments have been made for the next step research plan. Firstly, as an added section, the coalescence rate of droplets in condensation process and the ice bridge spreading rate in freezing process will be accurately measured with three-dimensional measurement method, with the droplet size specially considered. Secondly, in the physical order, frosting is early and more fundamental than the defrosting stage. And thus, the defrosting performance improvement work on the cold pate surface based on the Constructal Theory in our previous research plan will be delayed.
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| 今後の研究の推進方策 |
The following two sections will be investigated. (1) Accurately measure the coalescence rate of condensed micro-droplets and the freezing propagation rate of inter-droplet on different wettability cold surfaces. Currently, the two rates were calculated by using high-speed camera. The contact area of droplet and the solid surface is analyzed with top view pictures, and the height of droplet calculated using the measured contact angle. Firstly, the measurement error of contact angle makes the height of water droplet not accurate, and thus influence the calculations of the two rates. Secondly, as the scale of water droplet decrease, the measurement errors of the two rates in the optical theory would be enlarged. This problem directly influences the analysis of the heat and mass transfer mechanism. (2) Investigate the frosting characteristics on different wettability cold surfaces. To accurately measure and obtain the dynamic thickness of the frost layer and the morphology of frost crystal on the cold plate surface, and thus avoid the errors in all reported numerical and theoretical studies, the solidification process of a micro-droplet on the cold surface at different wettability and surrounding environment will be investigated firstly. It is also a fundamental problem for uneven frosting study. To solve this long-standing problem, the frosting characteristics should be investigated and thus rebuild dynamic frosting models. After these models being developed and validated, the other frosting parameters or characteristics can be predicted.
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