• Nuclear Engineering and Technology
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• v.52 no.10
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• pp.2353-2360
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• 2020

An anodic electrophoretic deposition (EPD) technique is used to create a uniform TiO₂ thin film coating on boiling thin steel plates (1.1 mm by 90 mm). All of the effective parameters except time of the EPD method are kept constant. To investigate the effect of gamma irradiation on the critical heat flux (CHF), the test specimens were irradiated in a gamma cell to different doses ranging from 100 to 300 kGy, and then SEM and BET analysis were performed. For each coated specimen, the contact angle and capillary length were measured. The specimens were then tested in a boiling pool for CHF and boiling heat transfer coefficient. It was observed that irradiation significantly decreases the maximum pore diameter while it increases the porosity, pore surface area and pore volume. These surface modifications due to gamma irradiation increased the CHF of the nano-coated surfaces compared to that of the unirradiated surfaces. The heat transfer coefficient (HTC) of the nano-coated surfaces irradiated at 300 kGy increased from 83 to 160 kW/(㎡ K) at 885 kW/㎡ wall heat flux by 100%. The CHF of the irradiated (300 kGy) and unirradiated surfaces are 2035 kW/㎡ and 1583 kW/㎡, respectively, an increase of nearly 31%.

• 한국가시화정보학회:학술대회논문집
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• 2006.12a
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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.

• Journal of the Korean Society of Visualization
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• v.21 no.1
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• pp.67-73
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• 2023

In the study, pool boing critical heat flux (CHF) was experimentally investigated depending on the length of heaters. A smooth silicon oxide surfaces are used as the boiling surfaces. As the results of pool boiling experiments based on distilled water in ambient pressure condition, the CHF decreased as the length of the heater increased. By the high speed imaging, it was shown that the number of vapor columns increased as the length of the heater increased. Comparing the number of vapor columns and the CHF according to the heater length, the change in the CHF according to the heater length was analyzed based on the hydrodynamic instability.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.12
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• pp.953-963
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• 2015

There is an increasing need to understand the thermal-hydraulic phenomena, including the critical heat flux (CHF), in narrow rectangular channels and consider these in system design. The CHF mechanism under a saturated flow boiling condition involves the depletion of the liquid film of an annular flow. To predict this type of CHF, the previous representative liquid film dryout models (LFD models) were studied, and their shortcomings were reviewed, including the assumption that void fraction or quality is constant at the boundary condition for the onset of annular flow (OAF). A new LFD model was proposed based on the recent constitutive correlations for the droplet deposition rate and entrainment rate. In addition, this LFD model was applied to predict the CHF in vertical narrow rectangular channels that were uniformly heated. The predicted CHF showed good agreement with 284 pieces of experimental data, with a mean absolute error of 18. 1 % and root mean square error of 22.9 %.

• Nuclear Engineering and Technology
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• v.47 no.4
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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.

• Nuclear Engineering and Technology
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• v.44 no.4
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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.

• Korean Journal of Plant Resources
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• v.20 no.4
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• pp.325-330
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• 2007

This study was conducted to investigate the effect of the mixed extract of P. ginseng C.A. Mey. and C. sinensis K. (Gin-CHF) on the infarction area of hippocampus in the mice with Alzheimer's disease induced by ${\beta}-amyloid({\beta}A)$. The Gin-CHF extract reduced the infarction area of hippocampus, and controlled the injury of brain tissue in the mice with Alzheimer's disease induced by ${\beta}A$. The Gin-CHF extract reduced the Tau protein, GFAP protein, and presenilin1/presenilin2 protein (immunohistochemistry) of hippocampus in the mice with Alzheimer's disease induced by ${\beta}A$. These results suggest that the Gin-CHF extract may be effective for the prevention and treatment of Alzheimer's disease. Investigation into the clinical use of the Gin-CHF extract for Alzheimer's disease is suggested for further research.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 1999.10a
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• pp.59-59
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• 1999

In this study, planer inductively coupled $Cl_2$ based plasmas were used to etch InGaN and the effects of plasma conditions on the InGaN etch properties have been characterized using quadrupole mass spectrometry(QMS) and optical emission spectroscopy(OES). As process conditions used to study the effects of plasma characteristics on the InGaN etch properties, $Cl_2$ was used as the main etch gas and $CHF_3,{\;}CH_4$, and Ar were used as additive gases. Operational pressure was varied from SmTorr to 3OmTorr, inductive power and bias voltage were varied from 400W to 800W and -50V to -250V, respectively while the substrate temperature was fixed at 50 centigrade. For the $Cl_2$ plasmas, selective etching of GaN to InGaN was obtained regardless of plasma conditions. The small addition of $CHF_3$ or Ar to $Cl_2$ and the decrease of pressure generally increased InGaN etch rates. The selective etching of InGaN to GaN could be obtained by the reduction of pressure to l5mTorr in $CI_2/IO%CHF_3{\;}or{\;}CI_2/IO%Ar$ plasma. The enhancement of InGaN etch rates was related to the ion bombardment for $CI_2/Ar$ plasmas and the formation of $CH_x$ radicals for $CI_2/CHF_3(CH_4)$ plasmas.

• 한국전산유체공학회:학술대회논문집
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• 2004.03a
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• pp.220-223
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• 2004

The initial conditions such as the film thickness and the void fraction at the onset of annular flow are required for the analytical dryout model. The Disturbance Wave Instability model(DWI model) is one of the model describing the Annular-to-Intermittent Flow regime Transition(AIFT). The experimental CHF conditions for the uniformly heated tube were compared with the predictions by the modified Levy model, for which the initial conditions at AIFT were estimated by the DWI model. For the flow through long tubes with small inlet subcooling, the effect of AIFT model on the dryout prediction was little. However, the use of DWI model gave better prediction of CHF in a short tube.

• Journal of Mechanical Science and Technology
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• v.15 no.7
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• pp.921-930
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• 2001

A new theoretical critical heat flux (CHF) model was developed for the forced convective flow boiling at high pressure, high mass velocity, and low quality. The present model for an intermittent vapor blanket was basically derived from the sublayer dryout theory without including any empirical constant. The vapor blanket velocity was estimated by an axial force balance, and the thickness of vapor blanket was determined by a radial force balance for the Marangoni force and lift force. Based on the comparison of the predicted CHF with the experimental data taken from previous studies, the present CHF model showed satisfactory results with reasonable accuracy.