1,721,173 research outputs found
EFFECTS OF BUBBLE SHAPE ASSUMPTION ON SINGLE BUBBLE GROWTH BEHAVIOR IN NUCLEATE POOL BOILING
No full text11scopu
Geometrical parametric study of drop impingement onto heated surface with micro-pillar arrays
No full textIn this study, the effects of wall temperature, micro-pillar array size, and micro-pillar height on the outcomes of the impingement of a single water drop impingement onto a heated surface with micro-pillar arrays were investigated. The micro-pillar arrays were designed with four different diameters and spacing (D, S: 5, 10, 15, and 20 mu m) and three different heights (H: 20, 30, and 40 mu m). For each type of surface, we classified the outcomes of drop impingement regimes into contact boiling, explosive lift-off, transition rebound, and Leidenfrost rebound by high-speed visualization. Micro-pillar arrays can result in the explosive lift-offbehavior and increase the Leidenfrost temperature. The explosive lift-off occurs when the liquid-vapor interface is pushed up by vapor pressure. Based on the force balance analysis, the behaviors of the liquid-vapor interface were analyzed based on the competing wetting (capillary pressure) and non-wetting (vapor pressure) pressures to understand the explosive lift-off triggering mechanism. (C) 2020 Elsevier Ltd. All rights reserved.11Nsciescopu
A study of nucleate bubble growth on microstructured surface through high speed and infrared visualization
No full textWe studied bubble growth on a microstructured surface during nucleate boiling using optical high-speed and infrared (IR) cameras. The effects of structured surfaces on bubble growth and dynamics were examined and their role analyzed with the use of simple models. A smooth, bare surface was prepared, and four microstructured test sample surfaces were fabricated with microscale gaps ranging from 5 to 80??m. The optical high-speed camera was used to observe the bubble growth profile with high temporal resolution; the IR camera was focused on the underside of the sample for direct visualization of the boiling process. Overall, the microstructured surfaces produced more bubbles, a lower frequency and nucleation site density than the bare surface for the low heat flux range (100?300?kW/m2), corresponding to the isolated bubble growth regime. The liberated bubble size was dependent on the size of the microstructure gap. Analysis of the high-speed images revealed that the liquid between the microstructures did not evaporate during bubble growth; however, during the initial growth stage, there was a brief period in which the liquid at the nucleation site evaporated. The large surface area and relatively high number of nucleation points contributed to enhanced bubble growth on the structured surfaces. ? 2017 Elsevier Ltd114sciescopu
Heat transfer characteristics of an ex-vessel molten core cooling system based on two-phase natural circulation
No full textPassive cooling based on natural circulation is utilized in ex-vessel core catcher system of an advanced nuclear reactor to handle severe accident scenario. The core catcher coolant channel has a unique geometry which consists of heated downward-facing slightly inclined and vertical surfaces. A full height experimental facility with natural circulation driven flow to model ex-vessel core catcher system was designed using scaling analysis. In this study, the cooling capability and heat transfer characteristics of the ex-vessel core catcher system was carried out. Two-phase flow parameters and wall temperatures were measured under a uniform heat flux condition. Two-phase flow structures were identified by high-speed camera visualization along with measurements of two local parameters, void fraction and re-wetting time. The wall temperature and local heat transfer coefficient distribution along the cooling channel were obtained by direct measurements of the heater surface and liquid temperature. The cooling performance of the core catcher system was analyzed based on the experimental results. The results indicated that the core catcher coolant system provides adequate cooling and maintains the integrity of the core catcher plate for prototype heat flux conditions.11Nsciescopu
평판형 히터를 이용한 알루미늄과 타이타늄 산화물 나노유체의 풀비등 임계열유속에 관한 실험적 연구
No full textPool boiling heat transfer and critical heat flux (CHF) of water-based nanofluids with alumina and titania nanoparticles of 0.01% by volume were investigated on a disk heater at saturated and atmospheric conditions. The experimental results showed that the boiling in nanofluids caused the considerable increase in CHF on the flat surface heater. It was revealed by visualization of the heater surface subsequent to the boiling experiments that a major amount of nanoparticles deposited on the surface during the boiling process. Pool boiling of pure water on the surface modified by such nanoparticle deposition resulted in the same CHF increases as what boiling nanofluids, thus suggesting the CHF enhancement in nanofluids was an effect of the surface modification through the nanoparticle deposition during nanofluid boiling. Possible reasons for CHF enhancement in pool boiling of nanofluids are discussed with surface property changes caused by the nanoparticle deposition.11Nscopuskc
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