• KIEE International Transactions on Power Engineering
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• v.4A no.1
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• pp.1-5
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• 2004

The model of a power system with load dynamics is studied by investigating qualitative changes in its behavior as the reactive power demand at a load bus is increased. The load is created using induction motors parallel with the constant power and constant impedance load. As the load increases, the system experiences various bifurcations such as sub critical and supercritical Hopf, period-doubling and saddle-node bifurcation. The latter may lead the system to voltage collapse. A nonlinear controller is used to control the subcritical Hopf bifurcation and hence mitigate voltage collapse. It is applied to the KEPCO (Korean Electric Power Company) system to demonstrate its validity.

• Communications of the Korean Mathematical Society
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• v.16 no.1
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• pp.137-152
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• 2001

Steady two-dimensional flows due to an applied pressure distribution in water of finite depth are considered. Gravity is included in the dynamic boundary condition. Gravity is included in the dynamic boundary condition. The problem is solved numerically by using the boundary integral equation technique. It is shown that, for both supercritical and subcritical flows, solutions depend on three parameters: (i) the Froude number, (ii) the magnitude of applied pressure distribution, and (iii) the span length of pressure distribution. For supercritical flows, there exist up to two solutions corresponding to the same value of Froude number for positive pressures and a unique solution for negative pressures. For subcritical flows, there are solutions with waves behind the applied pressure distribution. As the Froude number decreases, these waves when the Froude numbers approach the critical values.

• Advances in Energy Research
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• v.8 no.4
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• pp.265-273
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• 2022

In this study, a product gas yield and carbon conversion were measured during the coal pyrolysis. The pyrolysis process occurred under two different atmospheres such as subcritical (45 bar, 10℃) and supercritical CO₂ condition (80 bar, 35℃). Under the same pressure (80 bar), the atmosphere temperature increased from 35℃ to 45℃ to further examine temperature effect on the pyrolysis at supercritical CO₂ condition. For all three cases, a power input supplied to heating wire placed below coal bed was controlled to make coal bed temperature constant. The phase change of CO₂ atmosphere and subsequent pyrolysis behaviors of coal bed were observed using high-resolution camcorder. The pressure and temperature in the reactor were controlled by a CO₂ pump and heater. Then, the coal bed was heated by wire heater to proceed the pyrolysis under supercritical CO₂ condition.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.1
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• pp.131-139
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• 1999

Experimental investigations on flame spread in droplet arrays have been conducted under supercritical ambient pressures of fuel droplet. Flame spread rates are measured for n-Decane droplet of diameters of 0.75 and 1.0mm, using high speed images of OH chemiluminescence up to 3.0MPa. The pattern of flame spread is categorized into two: a continuous mode and an intermittent one. There exists a limit droplet spacing, above which flame spread does not occur. Flame spread rate with the decrease of droplet spacing increases and then decreases after takin& a maximum. It is also seen that there exists a limit ambient pressure, above which flame spread does not occur. Flame spread rate decreases monotonically with the increase of ambient pressure. Exceptionally, In the case of a small droplet spacing, flame spread with the increase of ambient pressure is extended to supercritical pressures of fuel droplet. This is caused by enhanced vaporization with the increase of ambient pressure. Consequently, in flame spread with droplet droplet spacing, the relative position of flame to droplet spacing plays an important role. The monotonic decrease with ambient pressure is mainly related to the reduction of flame radius in subcritical pressures and the extension to supercritical pressures of flame spread is caused by the reduction of ignition time of unburnt droplet due to the enhanced vaporization at supercritical pressures.

• Clean Technology
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• v.13 no.1 s.36
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• pp.46-53
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• 2007

We depolymerized PET in supercritical methanol and observed the yield of DMT at various reaction conditions. At subcritical state below $240^{\circ}C$, the yield of DMT was very low, about only 50%. It increased dramatically to 80% at supercritical state above $260^{\circ}C$, thereafter the increasing rate was reduced significantly. Similarly, at subcritical state of 6.89 MPa, the DMT yield was only 50%, but it increased abruptly to 85% at supercritical state of 10.34 MPa, yielding no further increase above the pressure. Within 10 minutes after the beginning of the reaction, the DMT yield reached 80%, indicating that the significant portion of the reaction has proceeded, and then, the yield increased slowly. The methanol/PET ratio of 8 showed the maximum DMT yield. We found the optimum depolymerization condition fur PET methanolysis is temperature $300^{\circ}C$, pressure 10.34 MPa, reaction time 40 minutes, and methanol/PET ratio of 8.

• Nuclear Engineering and Technology
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• v.49 no.1
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• pp.103-112
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• 2017

Application of the supercritical condition in reactor core cooling needs to be properly justified based on the extreme level of parameters involved. Therefore, a numerical study is presented to compare the thermalhydraulic performance of supercritical and single-phase natural circulation loops under low-to-intermediate power levels. Carbon dioxide and water are selected as respective working fluids, operating under an identical set of conditions. Accordingly, a three-dimensional computational model was developed, and solved with an appropriate turbulence model and equations of state. Large asymmetry in velocity and temperature profiles was observed in a single cross section due to local buoyancy effect, which is more prominent for supercritical fluids. Mass flow rate in a supercritical loop increases with power until a maximum is reached, which subsequently corresponds to a rapid deterioration in heat transfer coefficient. That can be identified as the limit of operation for such loops to avoid a high temperature, and therefore, the use of a supercritical loop is suggested only until the appearance of such maxima. Flow-induced heat transfer deterioration can be delayed by increasing system pressure or lowering sink temperature. Bulk temperature level throughout the loop with water as working fluid is higher than supercritical carbon dioxide. This is until the heat transfer deterioration, and hence the use of a single-phase loop is prescribed beyond that limit.

• Journal of ILASS-Korea
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• v.18 no.3
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• pp.140-145
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• 2013

Sub/supercritical spray experiments were conducted, and cryogenic nitrogen and gaseous argon were selected for simulants. liquid nitrogen and gaseous argon were injected in subcritical case, and supercritical nitrogen and near-critical gaseous argon were injected in near-critical/supercritical cases. shadowgraph method was used to visualize spray, and analyzed about the breakup length. The breakup length was measured from numbers of Instantaneous shadowgraph Images from each case, and they were compared with momentum flux ratios and density ratios. It was observed that the relation between breakup length and momentum flux ratio was fitted into former experiment results. and the reasonable constant was suggested about the relation between breakup length and density ratio.

• Environmental Engineering Research
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• v.11 no.6
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• pp.311-317
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• 2006

Feasibility of destroying synthetic and actual leachate containing humic acids and ammonia compounds by supercritical water oxidation (SCWO) was evaluated. In this study, destruction efficiencies of humic acids and ammonia respectively were investigated at various reaction temperatures and residence times under pressure a supercritical pressure (280 atm). To lower reaction temperature, chemical oxidants were used. The experiment was carried out in a cylindrical batch reactor made of Hastelloy C-276 that can withstand high temperature and pressure. Concentrations of humic acids and ammonia were measured using a $COD_{Cr}$ method and an ammonia selective electrode, respectively. The optimal destructive condition of humic acids in the presence of stoichiometric oxygen(air) was 3 min at $380^{\circ}C$, but the temperature could be lowered to subcritical region ($360^{\circ}C$) along with $H_2O_2$ as an oxidant. For ammonia, the optimal destructive condition with air was 5 min at $660^{\circ}C$, but it was possible to operate the process for 3 minutes at $550^{\circ}C$ or 2 min at $600^{\circ}C$ along with $H_2O_2$ as an oxidant. At 2 min and $550^{\circ}C$ along with $H_2O_2$ as an oxidant, humic and ammonia compounds in the actual leachate were easily destructed and the effluent quality met the Korea Standard Leachate Quality.

• Journal of the Korean Society of Propulsion Engineers
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• v.25 no.4
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• pp.53-58
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• 2021

Based on surrogate model, a hydrocarbon mixture was analyzed by visualizing the impinging break up mechanism in subcritical and supercritical conditions. Decane and methylcyclohexane with different critical pressures and temperatures were selected as experimental fluids. The impinging injector was installed inside the chamber, and the spray was visualized through a speed camera in subcritical and supercritical conditions. The injection condition of the mixture and chamber was kept constant at Pr(P/P_c) = 1, and Tr(T/T_c) was increased from 0.48 to 1.02. As Tr increased, the spray angle increased, and the sheet length decreased as the properties of the mixture reached each critical point. In addition, when the mixture approached the near critical point, it was shown that the change in density gradient was largely observed out of the impinging break up mechanism.

• Transactions of the Korean hydrogen and new energy society
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• v.22 no.3
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• pp.402-408
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• 2011

Organic Rankine cycles (ORC) can be used to produce power from heat at different temperature levels available as geothermal heat, as biogenic heat from biomass, as solar or as waste heat. In ORC working fluids with relatively low critical temperatures and pressures can be compressed directly to their supercritical pressures and heated before expansion so as to obtain a better thermal match with their heat sources. In this work thermal performance of ORC with and without an internal heat exchanger is comparatively investigated in the range of subcritical and transcritical cycles. R134a is considered as working fluid and special attention is paid to the effect of turbine inlet pressure on the characteristics of the system. Results show that operation with supercritical cycles can provide better performance than subcritical cycles and the internal heat exchanger can improve the thermal efficiency when the temperature of heat source becomes higher.