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물의 Leidenfrost 지점 온도에 대한 표면의 영향
김형대(Hyungdae Kim) 대한기계학회 2011 대한기계학회 춘추학술대회 Vol.2011 No.10
It is known that quenching phenomena are affected by the physico-chemical characteristics of the hot surface, such as surface roughness, wettability and porosity. However, while some general qualitative trends are known, there is a lack of quantitative data on the relative importance and magnitude of these effects. As conventional quenching tests of relatively large objects plunged into a pool cannot provide sufficient information to understand the separate effects of these micro/nano-scale surface characteristics, we have therefore conducted LFP tests of a single water droplet using custom-fabricated surfaces for which roughness, wettability and porosity were controlled accurately and separately at the nanoscale. This approach reveals that nanoporosity is the crucial feature in efficiently increasing the LFP temperature by initiating heterogeneous nucleation of bubbles during short-lived solid-liquid contacts, which results in disruption of the vapor film.
막비등 영역에서 액적-벽면 충돌 시 충돌각도가 열전달에 미치는 영향에 관한 실험적 연구
박준석,김형대,Park, Junseok,Kim, Hyungdae 한국액체미립화학회 2017 한국액체미립화학회지 Vol.22 No.3
Effects of collision angle on heat transfer characteristics of a liquid droplet impinging on a heated wall above the Leidenfrost point temperature were experimentally investigated. The heated wall and droplet temperatures were $506^{\circ}C$ and $100^{\circ}C$, respectively, and the impact angle varied from $20^{\circ}$ to $90^{\circ}$ while the normal collision velocity was constant at 0.27 m/s. The droplet collision behaviors and the surface temperature distribution were measured using synchronized high-speed video and infrared cameras. The major physical parameters influencing upon droplet-wall collision heat transfer, such as residence time, wall heat flux, effective heat transfer area, heat transfer amount, were analyzed. It was found at the constant normal collision velocity that the residence time, wall heat flux and effective heat transfer area were hardly not changed, resulting in the almost constant heat transfer amount.
적외선 입자추적유속계를 이용한 액체-기체 분리유동 시 계면속도 측정기법 개발 및 검증
김상은(Sangeun Kim),김형대(Hyungdae Kim) 대한기계학회 2015 大韓機械學會論文集B Vol.39 No.7
적외선 카메라를 이용한 입자추적유속계(IR-PTV)를 활용하여 물-공기 분리유동 시 계면속도를 측정하는 기법을 개발하였다. 3-5 ㎛ 파장대의 적외선은 물에 대해 20 ㎛ 이하의 침투 깊이를 가지므로 입자추적유속계 기법에 활용 시 물-공기 계면 근처에 존재하는 추적입자들의 이동속도를 선택적으로 측정할 수 있다. IR-PTV 기법의 측정 정확도를 검증하기 위하여 물에 잘 뜨는 스티로폼 입자를 이용하여 10° 기울어진 경사면에서 공기-물 분리유동 시 계면속도를 측정하여 비교한 결과 5% 이내의 오차를 보이면서 잘 일치하였다. 개발한 기법을 이용하여 획득한 실험결과로부터 공기 속도가 증가함에 따라 계면속도가 비례하여 증가하는 것을 관찰하였으며 이는 계면전단력의 증가에 의한 것으로 해석된다. A measurement technique of interfacial velocity in air-water separated flow by particle tracking velocimetry using an infrared camera (IR-PTV) was developed. As infrared light with wavelength in the range of 3-5 um could hardly penetrate water, IR-PTV can selectively visualize only the tracer particles existing in depths less than 20 um underneath the air-water interface. To validate the measurement accuracy of the IR-PTV technique, a measurement of the interfacial velocity of the air-water separated flow using Styrofoam particles floating in water was conducted. The interfacial velocity values obtained with the two different measurement techniques showed good agreement with errors less than 5%. It was found from the experimental results obtained using the developed technique that with increasing air velocity, the interfacial velocity proportionally increases, likely because of the increased interfacial stress.
불응축 기체가 존재하는 수직·수평 관내·외부에서의 응축에 대한 문헌연구
김상은(Sangeun Kim),김형대(Hyungdae Kim) 대한기계학회 2013 대한기계학회 춘추학술대회 Vol.2013 No.12
Importance of passive cooling systems operating without electricity during a station blackout accident of nuclear power plants is certainly recognized in the nuclear disaster at Fukushima. Condensation is a key phenomenon in designing and operating passive cooling systems. The presence of non-condensable gases can severely degrade effectiveness of film condensation heat transfer in heat exchanger tubes of passive cooling systems. In this study, an extensive review of previous experimental and theoretical studies on effects of non-condensable gases upon film condensation heat transfer in vertical and horizontal heat exchanger tubes of passive cooling systems was conducted. Key research findings and research gaps identified from this systematic review are presented. Areas for future studies are also suggested along with application to advanced passive cooling systems.
Andree Pusey,김형대(Hyungdae Kim) 한국가시화정보학회 2016 한국가시화정보학회지 Vol.14 No.2
This paper presents an experimental investigation to visualize cross-sectional two-phase flow structure and identify liquid-gas interface for condensation of steam at a low mass flux in a slightly inclined tube using the axial-viewing technique, which permits to look directly into flow during condensation of steam. In this technique, two-phase flow is viewed along the axis of a pipe by locating a high-speed video camera in front of a viewer that is fitted at the outlet of the pipe. A short section of the pipe is illuminated and is recorded through the viewer, which is kept free of liquid by mildly introducing air. Experiments were conducted in a pipe of 19.05 ㎜ in inner diameter at atmospheric pressure. Cross-sectional two-phase flow structure is obtained at a steam mass flux of 2.62 ㎏/㎡s as a function of steam quality in the range from 0.5 to 0.9. The results show that stratified-wavy flow is a unique flow pattern observed in the scope of the present study. Condensate film thickness, stratification angle and void fraction were measured from the obtained flow structure images. Finally, heat transfer coefficient was calculated using the measurement data and discussed in comparison with existing correlations.