• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 춘계학술대회
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• pp.1512-1517
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• 2003

Augmentation of CHF by ultrasonic is experimentally studied under subcooling pool boiling condition. Experiment is carried out for downward-facing plate with and without the ultrasonic. The working fluid is distilled water. Experimental apparatus is composed of a bath, power supply, test section, ultrasonic generator, DAQ system. Experiment is performed with the subcooling temperature of $5^{\circ}C$, $20^{\circ}C$, $40^{\circ}C$ and the inclined angle of $0^{\circ}$, $10^{\circ}$, $20^{\circ}$, $45^{\circ}$, 90. From the experimental results, it is found that ultrasonic effect enhances CHF of the downward-facing plate. As increasing the degree of subcooling, the rate of CHF increase is enhanced. As increasing the inclined angle, the rate of CHF increase decreases. Also, we can see that the heat transfer mechanism of CHF augmentation is closely connected with the dynamic behavior of bubble generation and departure.

• 대한기계학회논문집B
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• 제33권10호
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• pp.729-736
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• 2009

Pool 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.

• Nuclear Engineering and Technology
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• 제56권7호
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• pp.2474-2479
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• 2024

In-Vessel Retention (IVR) is a key technology to retain the molten core in the reactor vessel during severe accidents of Pressurized-water reactors (PWRs). In order to gain the safety margin of IVR, it is crucial to enhance the critical heat flux (CHF) of the reactor vessel, which is submerged in a water pool. To enhance the CHF, we have designed and additive-manufactured porous grid plates with a 3-D printer for design flexibility. We measured the CHF for the porous grid plate on the boiling heat transfer surface and found that the CHF was enhanced by 50 % more than that of the bare surface. The CHF enhanced more with a narrower grid pitch and a lower grid height. The visual observation study revealed that the vapor film was formed at the bottom of the grid plate.

• Nuclear Engineering and Technology
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• 제47권4호
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• pp.398-406
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• 2015

In this study, R-123 flow boiling experiments were carried out to investigate the effects of nanoparticle deposition on heater surfaces on flow critical heat flux (CHF) and boiling heat transfer. It is known that CHF enhancement by nanoparticles results from porous structures that are very similar to layers of Chalk River unidentified deposit formed on nuclear fuel rod surfaces during the reactor operation period. Although previous studies have investigated the surface effects through surface modifications, most studies are limited to pool boiling conditions, and therefore, the effects of porous surfaces on flow boiling heat transfer are still unclear. In addition, there have been only few reports on suppression of wetting for decoupled approaches of reasoning. In this study, bare and $Al_2O_3$ nanoparticle-coated surfaces were prepared for the study experiments. The CHF of each surface was measured with different mass fluxes of $1,600kg/m^2s$, $1,800kg/m^2s$, $2,100kg/m^2s$, $2,400kg/m^2s$, and $2,600kg/m^2s$. The nanoparticle-coated tube showed CHF enhancement up to 17% at a mass flux of $2,400kg/m^2s$ compared with the bare tube. The factors for CHF enhancement are related to the enhanced rewetting process derived from capillary action through porous structures built-up by nanoparticles while suppressing relative wettability effects between two sample surfaces as a highly wettable R-123 refrigerant was used as a working fluid.

• 한국가시화정보학회:학술대회논문집
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• 한국가시화정보학회 2006년도 추계학술대회 논문집
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• pp.125-130
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• 2006

The characteristics of boiling heat transfer and critical heat flux (CHF) behavior of nano-fluids were studied by using various sized silver and alumina nanoparticles. The diameter of nanoparticles was from 2 nm to 250 nm for silver and from 20nm to 40nm for alumina. Pool boiling characteristics and CHF enhancement of nano-fluids with different sized nanoparticles were compared with those of pure water and each nano-fluids. The experiment was performed at atmospheric pressure and the temperature of the pool was maintained constantly by using a flat immersed heater. The concentration of nano-fluids was uniform in all experiments as 0.01g/liter. The results showed that the measured boiling curves were shifted to the right. It demonstrated that the occurrence of nucleate boiling regime in nano-fluids retarded, compared with that of pure water. Also, in nano-fluids, the boiling curves showed that CHF of nano-fluids is significantly enhanced and represented the effect of particle size on boiling characteristics.

• 설비공학논문집
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• 제28권3호
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• pp.95-102
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• 2016

A boiling heat transfer is used in various industry such as power generation systems, heat exchangers, air-conditioning and refrigerations. In the boiling heat transfer system, the critical heat flux (CHF) is the important factor, and it indicated safety of the system. It has kept up studies on the CHF enhancement. Recently, it is reported the CHF enhancement, when working fluid used the nanofluid with high thermal properties. But it could be occurred nanoflouling phenomenon from nanoparticle deposition, when nanofluid applied the heat transfer system. And, it is reported that the safety and thermal efficiency of heat transfer system could decrease. Therefore, it is compared and analyzed to the CHF and the boiling heat transfer coefficient on effect of artificial nanofouling (coating) in oxidized multi-wall carbon nanotube nanofluids. As the result, the CHF of oxidized multi-wall carbon nanofluids and the CHF of artificial nanofouling in the nanofluids increased to maximum 99.2%, 120.88%, respectively. A boiling heat transfer coefficient in nanofluid increased to maximum 24.29% higher than purewater, but artificial nanofouling decreased to maximum -7.96%.

• 한국원자력학회:학술대회논문집
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• 한국원자력학회 1997년도 추계학술발표회논문집(1)
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• pp.528-533
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• 1997

A CANFLEX(CANdu FLEXible fuelling) 43-element bundle has developed for a CANDU-6 reactor as an alternative of 37-element fuel bundle. The design has two diameter elements (11.5 and 13.5㎜) to reduce maximum element power rating and buttons to enhance the critical heat flux(CHF), compared with the standard 37-element bundle. The freon CHF experiments have performed for two series of CANFLEX bundles with and without buttons with a modelling fluid as refrigerant H-l34a and axial uniform heat flux condition. Evaluating the effects of buttons of CANFLEX bundle on CHF and Critical Channel Power(CCP) with the experimental results, it is shown that the buttons enhance CCP as well as CHF. All the CHF's for both the CANFLEX bundles are occurred at the end of fuel channel with the high dryout quality conditions. The CHF enhancement ratio are increased with increase of dryout quality for all flow conditions and also with increase of mass flux only lot high pressure conditions. It indicates that the button is a useful design lot CANDU operating condition because most CHF flow conditions for CANDU fuel bundle are ranged to high dryout quality conditions.

• Nuclear Engineering and Technology
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• 제48권4호
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• pp.932-940
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• 2016

Pool boiling heat transfer of water saturated at atmospheric pressure was investigated experimentally on Cu surfaces with high-temperature, thermally-conductive, microporous coatings (HTCMC). The coatings were created by sintering Cu powders on Cu surfaces in a nitrogen gas environment. A parametric study of the effects of particle size and coating thickness was conducted using three average particle sizes (APSs) of $10{\mu}m$, $25{\mu}m$, and $67{\mu}m$ and various coating thicknesses. It was found that nucleate boiling heat transfer (NBHT) and critical heat flux (CHF) were enhanced significantly for sintered microporous coatings. This is believed to have resulted from the random porous structures that appear to include reentrant type cavities. The maximum NBHT coefficient was measured to be approximately $400kW/m^2k$ with APS $67{\mu}m$ and $296{\mu}m$ coating thicknesses. This value is approximately eight times higher than that of a plain Cu surface. The maximum CHF observed was $2.1MW/m^2$ at APS $67{\mu}m$ and $428{\mu}m$ coating thicknesses, which is approximately double the CHF of a plain Cu surface. The enhancement of NBHT and CHF appeared to increase as the particle size increased in the tested range. However, two larger particle sizes ($25{\mu}m$ and $67{\mu}m$) showed a similar level of enhancement.

• Design & Manufacturing
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• 제13권1호
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• pp.42-47
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• 2019

Nucleate boiling heat transfer is one of the most important phenomenon in the various industries. Especially, critical heat flux (CHF) refers to the upper limit of the pool boiling heat transfer region. Therefore, many researchers have found that CHF can be significantly increased by adding very small amounts of nanoparticles. In this study, the CHF and heat transfer coefficient were tested under the pool boiling state using copper and multi wall carbon nanotube nanoparticles. The results showed that two different types of nanoparticles deposited on the surface of two specimens made of the same material increased the heat flux in the nanoparticles with high conductivity, and there was no difference in the critical heat flux when the same material nanoparticles were deposited on the two different specimen surfaces.

• Nuclear Engineering and Technology
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• 제44권4호
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• pp.429-436
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• 2012

Critical heat flux (CHF) is the thermal limit of a phenomenon in which a phase change occurs during heating (such as bubbles forming on a metal surface used to heat water), which suddenly decreases the heat transfer efficiency, thus causing localized overheating of the heating surface. The enhancement of CHF can increase the safety margins and allow operation at higher heat fluxes; thus, it can increase the economy. A very interesting characteristic of nanofluids is their ability to significantly enhance the CHF. Nanofluids are nanotechnology-based colloidal dispersions engineered through the stable suspension of nanoparticles. All experiments were performed in round tubes with an inner diameter of 0.01041 m and a length of 0.5 m under low pressure and low flow (LPLF) conditions at a fixed inlet temperature using water, 0.01 vol.% $Al_2O_3$/water nanofluid, and SiC/water nanofluid. It was found that the CHF of the nanofluids was enhanced and the CHF of the SiC/water nanofluid was more enhanced than that of the $Al_2O_3$/water nanofluid.