• Proceedings of the KSME Conference
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• 2000.11a
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• pp.736-742
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• 2000

Mass removed from crystalline silicon samples during high power single-pulse laser ablation was studied by measuring the resulting crater morphology with a white light interferometric microscope. The volume and depth of the craters show a strong nonlinear change as the laser irradiance increases across a threshold value, that is, approximately $2.2{\times}10^{10}\;W/cm^2$. Time-resolved shadowgraph images of the ablation plume show the ejection of large particulates from the sample for laser irradiance above the threshold, with a time delay of about 300-400 nsec. The thickness of superheated liquid layer near the critical temperature was numerically estimated, considering the transformation of liquid metal into liquid dielectric near the critical state (i.e., induced transparency). The estimated thickness of the superheated layer at a delay time of 200 nsec agreed with the measured crater depths, suggesting that induced transparency promotes the formation of a deep superheated liquid layer which leads to an explosive boiling responsible for the sudden increase of crater volume and depth.

• Proceedings of the KSME Conference
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• 2001.06e
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• pp.63-68
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• 2001

An experimental study was carried out to identify the various regimes of natural convective boiling and to determine the Critical Heat Flux(CHF) on a 70mm square surface which is inclined at $180^{\circ}$(upward), $90^{\circ}, \;45^{\circ}$. The heater block made of copper with cartridge heaters is submerged in a water tank with windows for visualization. As the heat flux increases from $100kW/m^2$ to $1.1MW/m^2$, the heat transfer regime migrates from the nucleate boiling to film boiling and results in a rapid heat up of the heater block. An explosive vapor generation on the heated surface, whose size and frequency are characterized by the heat flux, is visualized by using a digital camcorder with $512{\times}512$ pixel size at 30fps.

• Nuclear Engineering and Technology
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• v.41 no.9
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• pp.1205-1214
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• 2009

The critical heat flux (CHF) levels that occurred due to exponential heat inputs for varying periods to a 1.0-mm diameter horizontal cylinder immersed in various liquids were measured to develop an extended database on the effect of high subcoolings for quasi-steady-state and transient maximum heat fluxes. Two main mechanisms of CHF were found. One mechanism is due to the time lag of the hydrodynamic instability (HI) which starts at steady-state CHF upon fully developed nucleate boiling, and the other mechanism is due to the explosive process of heterogeneous spontaneous nucleation (HSN) which occurs at a certain HSN superheat in originally flooded cavities on the cylinder surface. Steady-state CHFs were divided into three regions for lower, intermediate and higher subcooling at pressures resulting from HI, transition and HSN, respectively. HSN consistently occurred in the transient boiling CHF conditions that correspond to a short period. It was also found that the transient boiling CHFs gradually increased, then rapidly decreased and finally increased again as the period became shorter.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.9
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• pp.1237-1244
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• 2001

An experimental study was carried out to identify the various regimes of natural convective pool boiling and to determine the boiling heat transfer curve and Critical Heat Flux(CHF) on a vertical square surface having a 70mm width and a 70mm height. The heater made of copper block with embedded cartridge heaters is submerged in a water tank at atmospheric pressure. As the heat flux increases from 100kW/㎡ to 1.2MW/㎡, the heat transfer regime migrates from the nucleate boiling to the film boiling. The boiling heat transfer data are fitted by Rohsenow type correlation. An explosive vapor generation on the heated surface, whose size and frequency are characterized by the heat flux, is visualized using a high speed digital imaging system.

• Journal of Biomedical Engineering Research
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• v.10 no.2
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• pp.94-96
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• 1989

Explosive evaporative removal process of biological tissue by absorption of a CW laser has been simulated by using gelatin and a multimode Nd:YAG laser. Because the point of maximun temperature of laser-irradiated gelatin exists below the surface due to surface cooling, evaporation at the boiling temperature is made explosively from below the surface. The important parameters of this process are the conduction loss to laser power absorption (defined as the conduction-to-laser power parameter, Nk), the convection heat transfer at the surface to conduction loss (defined as Bi), dimensionless extinction coefficient (defined as Br.), and dimensionless irradiation time (defined as Fo). Dependence of Fo on Nk and Bi has been observed by experiment, and the results have been compared with the numerical results obtained by solving a 2-dimensional conduction equation. Fo and explosion depth (from the surface to the point of maximun temperature) are increased when Nk and Bi are increased.To find out the minimum laser power for explosive evaporative removal process, steady state analysis has been also made. The limit of Nk to induce evaporative removal, which is proportional to the inverse of the laser power, has been obtained.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.16 no.11
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• pp.1051-1059
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• 2004

Two phase thermosyphone loop for electronics cooling are designed and manufactured to test its performance under the partial load and low environment temperature conditions. The thermosyphone device has six evaporators connected parallel for the purpose of cooling six power amplifier units (PAU) independently. The heater modules for simulating PAUs are adhered with thermal pad to the evaporator plates to reduce the contact resistance. There are unbalanced distributions of liquid refrigerant in the differently heated evaporators due to the vapor pressure difference. To reduce the vapor pressure differences caused by partial heating, two evaporators are connected each other using the copper tube. The pressure regulation tube successfully reduces these unbalances and it is good candidates for a field distributed systems. Under the low environment temperature operating condition, such as $-30^{\circ}C$, there may be unexpected subcooling in condenser. It leads the very low saturation pressure, and under this condition there exists explosive boiling in evaporator. The abrupt pressure rise due to the explosive boiling inhibits the supplement of liquid refrigerant to the evaporator for continuous cooling. Finally the cooling cycle will be broken. For the normal circulation of refrigerant there may be an optimum cooling air flow rate in condenser to adjust the given heat load.

• Journal of Biomedical Engineering Research
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• v.16 no.1
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• pp.67-76
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• 1995

Explosive evaporative removal process of biological tissue by absorption of a CW laser has been simulated by using gelatin and a multimode Nd: YAG laser. Because the point of maximun temperature of laser-irradiated gelatin exists below the surface due to surface cooling, evaporation at the boiling temperature is made explosively from below the surface. The important parameters of this process are the conduction loss to laser power absorption (defined as the conduction-to-laser power parameter, Nk), the convection heat transfer at the surface to conduction loss (defined as Bi), dimensionless extinction coefficient (defined as BrJ, and dimensionless irradiation time (defined as Fo). Dependence of Fo on Nk and Bi has been observed by experiment, and the results have been compared with the numerical results obtained by solving a 2-dimensional conduction equation. Fo and explosion depth (from the surface to the point of maximun temperature) are increased when Nk and Bi are increased. To find out the minimum laser power for explosive evaporative removal process, steady state analysis has been also made. The limit of Nk to induce evaporative removal, which is proportional to the inverse of the laser power, has been obtained.

• Journal of the Korean GEO-environmental Society
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• v.21 no.5
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• pp.23-33
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• 2020

As the scale of the economy expands, the number of cases of damage in enclosed spaces such as tunnels is increasing due to the accident of transportation vehicles of dangerous substances such as explosive flammable materials that have increased rapidly. In the case of road tunnels in particular, in the aspect of protection against the long-winding trend and the environment in the downtown area, the number of cases of passing through the downtown area increases, and securing the safety of structures against unexpected extreme disasters such as explosions during tunnel passage is very urgent. For this reason, developed countries are already conducting a review of internal bombardment, but there are almost no evaluation and countermeasures for explosion risk in Korea. Therefore, in this study, in order to evaluate the explosion safety of road tunnels, a boiling liquid explosive explosion (BLEVE), which is considered to have the greatest explosion load among vehicles driving on the road, is set as a reference explosion source, and the equivalent TNT explosion load is used for simulation of the explosion. A method of conversion was presented. In addition, by applying the derived explosion load, dynamic behavior simulation was performed by assuming various variables for the tunnel, and the explosion safety of the tunnel was analyzed.

• Journal of the Korean Society of Safety
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• v.39 no.3
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• pp.14-19
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• 2024

Hydrogen is one of the most popular eco-friendly energy sources for reducing global warming. To use hydrogen as a conventional fuel, liquid hydrogen plants should introduce waste hydrogen treatment processes. A major safety issue of liquid hydrogen plants is choosing the most suitable purge gas to use in case of an accident. A purge gas prevents the formation of explosive mixed gases in the vent header. In general, nitrogen is the main purge gas used in chemical plants. Nitrogen has a freezing point of -210℃, which is higher than the boiling point of hydrogen. Helium, with a freezing point lower than hydrogen, is instead recommended as a purge gas of the vent header during hydrogen liquefaction. However, helium is roughly 100 times more expensive than nitrogen. To address this issue, this study uses simulations to investigate safe conditions for introducing nitrogen as the purge gas during hydrogen liquefaction. The temperature change from the safety valve to the vent header is evaluated when the external temperature of the safety valve discharge pipe is at 5℃, 10℃, and 20℃. Additionally, the most optimal length for a discharge pipe according to pipe diameter is investigated.

• The Journal of the Petrological Society of Korea
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• v.11 no.2
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• pp.74-89
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• 2002

Rock units, relating with the Guamsan caldera, are composed of Guamsan Tuff and rhyolitic intrusions. The Guamsan Tuff consists almost entirely of ash-flow tuffs with some volcanic breccias and fallout tuffs. The volcanic breccia comprises block and ash-flow breccias of near-vent facies and caldera-collapse breccia near the ring fracture. The lower ash-flow tuffs are of an expanded pyroclastic flow phase from the pyroclastic flow-forming eruption with an ash-cloud fall phase of the fallout tuffs on the flow units, but the upper ones are of a non-expanded ash-flow phase from the boiling-over eruption. The rhyolitic intrusions are divided into intracaldera intrusions and ring dikes that are subdivided into inner, intermediate and outer dikes. We compile the volcanic processes along a single cycle of cadela development from the eruptive phases in the Guamsan area. The explosive eruptions began with block and ash-flow phases from collapse of glowing lava dome caused by Pelean eruption, progressed through expanded pyroclastic flow phases and ash-cloud fallout phases during high column collapse of pyroclastic flow-forming eruption from a single central vent. This was followed by non-expanded ash-flow phases due to boiling-over eruption from multiple ring fissure vents. The caldera collapse induced the translation into ring-fissure vents from a single central vent in the earlier eruption. After the boiling-over eruption, there followed an effusive phase in which rhyolitic magma was injected and erupted to be progressively emplaced as small plugs/dikes and ring dikes with many lava domes on the surface. Finally rhyodacitic magma was on emplaced as a series of dikes along the junction of both outer and intermediate dikes on the southwestern side of the caldela.