• Journal of computational fluids engineering
• /
• v.11 no.1 s.32
• /
• pp.36-42
• /
• 2006

The existing numerical approximations of convection flux, especially the spatial higher-order difference schemes, in unstructured cell-centered finite volume methods are examined in detail with each other and evaluated with respect to the accuracy through their application to a 2-D benchmark problem. Six higher-order schemes are examined, which include two second-order upwind schemes, two central difference schemes and two hybrid schemes. It is found that the 2nd-order upwind scheme by Mathur and Murthy(1997) and the central difference scheme by Demirdzic and Muzaferija(1995) have more accurate prediction performance than the other higher-order schemes used in unstructured cell-centered finite volume methods.

• Journal of Power Electronics
• /
• v.16 no.3
• /
• pp.994-1003
• /
• 2016

Two major obstacles in the utilization of electrical vehicles are their price and range. The collaboration of direct torque control (DTC) with induction motor (IM) is preferred for its low cost, easy implementation, and parameter independency. However, in terms of edges, the method has drawbacks, such as variable switching frequency and undesired current harmonic distortion. These drawbacks result in acoustic noise, reduced efficiency, and electromagnetic interference. A feed-forward approach for stator-flux-oriented DTC with space vector pulse-width modulation is presented in in this paper. The outcome of the proposed method is low current harmonic distortion with fixed switching frequency while preserving the torque performance and simple application feature of basic DTC. The method is applicable to existing and forthcoming IM drive systems via software adaptation. The validity of the proposed method is confirmed by simulation and experimental results.

• The Bulletin of The Korean Astronomical Society
• /
• v.37 no.2
• /
• pp.134.1-134.1
• /
• 2012

The radiation belt structure can be approximately reproduced by a form of diffusion equation, which takes into account the radial diffusion process as well as those in pitch angle and energy. The solution of the equation depends on several factors including initial and boundary conditions, diffusion coefficients, and plasmapause location. In this paper, we have attempted to determine a set of approximate functions for the energetic electron fluxes near the outer edge of the outer belt in terms of solar wind variable. We used the electron flux data from SST onboard the THEMIS spacecraft and determined its correlation with solar wind conditions in a systematic way. The functions were determined separately for different energy channels from ~30 keV up to 719 keV. Our determination of these functions allows us to predict the radial boundary condition for the electron flux, which can be implemented in a forecast model.

• Proceedings of the Korean Society of Propulsion Engineers Conference
• /
• 2004.03a
• /
• pp.713-721
• /
• 2004

Variable air intake and variable exhaust nozzle of hypersonic engines are designed and tested in this study. Dimensions for variable geometry air intake, ram combustor and variable geometry exhaust nozzle are defined based on the requirements of a pre-cooled turbojet engine. Hypersonic Ramjet Engine is designed as a scaled test bed for each component. Actuation forces of moving parts for variable intake and variable nozzle are reduced by balancing the other force in the opposite direction. A demonstrator engine which includes variable intake and variable nozzle is designed and the components are fabricated. Composite material with silicone carbide is applied for high temperature parts under oxidation environment such as leading edge of the variable intake and combustor liner. Internal cooling structure is adopted for both moving and static parts of the variable nozzle. Pressure recovery and mass capture ratio of the variable intake at Mach 5 is obtained by a hypersonic wind tunnel test. Flow characteristics of the variable nozzle are obtained by a low temperature flow test. Wall temperature and heat flux of the nozzle at Mach 3 is obtained by a firing test. As results, the intake and the nozzle are proved to be used at designed pressure and temperature environment.

• Transactions of the Korean Society of Automotive Engineers
• /
• v.12 no.3
• /
• pp.75-82
• /
• 2004

The allowable exhaust standard has been intensified as a part of the countermeasure to decrease air pollution in the world. As the cars with an alternative fuel starts to get into the spotlight, the cars with low emission has been introduced and exhaust gas regulation forced in this country. These days, LPG vehicles, which infrastructure of fuel was already built up, and CNG vehicles are recognized for alternative fuel cars in this country. In this study, the constant volume combustion chamber was manufactured and used for experiments to obtain the ignition characteristics of LPG fuel and the optimal ignition energy. The experiment measured the combustion characteristics, in regard to the change of combustion variable, and the change of ignition energy. During the combustion of fuel, the maximum temperature inside the combustion chamber is higher when the initial pressure is higher. The burning velocity also seems to have the same characteristic as the temperature. However, the heat flux did not change much with the theoretical correct mixture but the various initial temperature of the combustion chamber. The heat flux got faster and ignition energy bigger as the dwell time of the ignition system expanded. When the dwell time get longer, the ignition energy also increased then fixed. The ignition energy increased as the initial pressure inside the combustion chamber higher. The heat flux got faster as the dwell time expanded.

• Journal of Energy Engineering
• /
• v.5 no.1
• /
• pp.19-27
• /
• 1996

Self-pressurization of cylindrical container of cryogen is numerically analyzed. The container is axi-symmetric and heated from side wall with constant heat flux. Natural convection by external heat flux is studied numerically using finite difference method. Oxygen, nytrogen and hydrogen are working fluids in this paper. Liquid is considered incompressible fluid and vapor is assumed to behave as gas meeting with virial equation of gas. The Second virial coefficients of gas are obtained from Lennard-jones model. The important variables which have effects on self-pressurization are external heat flux, heat capacity of wall and initial ullage in container. The most important variable of them is external heat flux. The pressure rise calculated from the virial gas model is slightly different from that calculated using Ideal gas model for oxygen.

• Proceedings of the Korean Society of Soil and Groundwater Environment Conference
• /
• 2006.04a
• /
• pp.293-297
• /
• 2006

Submarine groundwater discharge (SGD) and the interface between seawater and freshwater in an unconfined coastal aquifer was evaluated by numerical modeling. A two-dimensional vertical cross section of the aquifer was constructed. Coupled flow and salinity transport modeling were peformed by using a numerical code FEFLOW In this study, we investigated the changes in groundwater flow and salinity transport in coastal aquifer with hydraulic condition such as the magnitude of recharge flux, hydraulic conductivity. Especially, transient simulation considering tidal effect and seasonal change of recharge rate was simulated to compare the difference between quasi-steady state and transient state. Results show that SGD flux is in proportion to the recharge rate and hydraulic conductivity, and the interface between the seawater and the freshwater shows somewhat retreat toward the seaside as recharge flux increases. Considered tidal effect, SGD flux and flow directions are affected by continuous change of the sea level and the interface shows more dispersed pattern affected by velocity variation. The cases which represent variable daily recharge rate instead of annual average value also shows remarkably different result from the quasi-steady case, implying the importance of transient state simulation.

• Transactions of the Korean hydrogen and new energy society
• /
• v.18 no.2
• /
• pp.189-196
• /
• 2007

The evaporation heat transfer coefficient and pressure drop of $CO_2$(R-744) in a horizontal tube was investigated experimentally. The main components of the experimental apparatus are a receiver, a variable-speed pump, a mass flow meter, a pre-heater and an evaporator(test section). The test section consists of a horizontal stainless steel tube of 4.57 mm inner diameter. The experiments were conducted at mass flux of $200{\sim}1000\;kg/m^2s$ saturation temperature of $0{\sim}20^{\circ}C$, and heat flux of $10{\sim}40\;kW/m^2$. The test results showed that the heat transfer coefficient of $CO_2$ has a greater effect on nucleate boiling more than convective boiling. Mass flux of $CO_2$ does not affect nucleate boiling too much. In comparison with test data and existing correlations, All of the existing correlations for the heat transfer coefficient underestimated the experimental data. However lung et al.'s correlation showed a good agreement with the experimental data. The evaporation pressure drop of $CO_2$ increases with increasing mass flux and decreasing saturation temperature. When comparison between the experimental pressure drop and existing correlations. Existing correlations failed to predict the evaporation pressure drop of $CO_2$.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
• /
• v.28 no.11
• /
• pp.112-122
• /
• 2014

This paper proposes a PMSM torque control method considering DC-link voltage variation and friction torque. In general EV/HEV application, two dimensions look-up table(2D-LUT) is used for reference current generation due to its stable and robust torque control performance. Conventionally, this 2D-LUT is established by flux-torque table to overcome the DC-link voltage variation. However, the flux table establishment is more complex than the speed table establishment. Moreover, one flux data reflects several speed conditions in variable DC-link voltage, friction torque cannot be considered by using the flux table. In this paper, speed-torque 2D-LUT is used for current reference generation. With this table, PMSM torque control is well achieved regardless of DC-link voltage variation by the proposed control method. Simulation and experimental results validate improvement of torque control error through friction torque compensation.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.16 no.12
• /
• pp.1134-1139
• /
• 2004

The evaporative heat transfer coefficient of $CO_2$ (R-744) in a horizontal tube was investigated experimentally. The experiments were conducted without oil in a closed refrigerant loop which was driven by a magnetic gear pump. The main components of the refrigerant loop are a receiver, a variable-speed pump, a mass flow meter, a pre-heater and evaporator (test section). The test section consists of a smooth, horizontal stainless steel tube of inner diameter of 7.75 mm. The experiments were conducted at mass flux of 200 to 500 kg/m$^2$s, saturation temperature of -5 to 5$^{\circ}C$, and heat flux of 10 to 40kW/m$^2$. The test results showed the heat transfer of $CO_2$ has a greater effect on nucleate boiling more than convective boiling. Mass flux of $CO_2$ does not affect nucleate boiling too much, and the effect of mass flux on evaporative heat transfer of $CO_2$ is much smaller than that of refrigerant R-22 and R-134a. In comparison with test results and existing correlations, correlations failed to predict the evaporative heat transfer coefficient of $CO_2$, therefore, it is necessary to develope reliable and accurate predictions determining the evaporative heat transfer coefficient of $CO_2$ in a horizontal tube.