• The Transactions of the Korean Institute of Electrical Engineers
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• v.39 no.9
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• pp.903-911
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• 1990

When load areas on a feeder are deenergized due to faults and scheduled outage, operators need to identify neighboring feeders, try to restore customers and minimize out-of-service areas. These cases include knowledge of system states and various constraints such as voltage drop. This paper concerns the load transfer in fault restoration and scheduled outage. Also, the operating constraints such as line current capacity, relay trip current, transformer capacity, voltage drop and line loss are considered. This expert system can propose the optimal load transfer method by analyzing the system state and considering the constraints.

• Journal of Advanced Marine Engineering and Technology
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• v.28 no.1
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• pp.65-74
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• 2004

The heat transfer coefficient and pressure drop of $CO_2$(R-744) during gas cooling Process of carbon dioxide in a horizontal tube were investigated experimentally and theoretically. The experiments were conducted without oil in the refrigerant loop. The main components of the refrigerant loop consist of a receiver. a variable-speed pump. a mass flowmeter, an evaporator. and a gas cooler(test section). The main components of the water loop consist of a variable-speed Pump. an constant temperature bath. and a flowmeter. The gas cooler is a counterflow heat exchanger with refrigerant flowing in the inner tube and water flowing in the annulus The test section consists of smooth, horizontal stainless steel tube of 9.53 mm outer diameter and 7.75 mm inner diameter. The length of test section is 6 m. The refrigerant mass fluxes were 200 ~ 300 kg/($m^2{\cdot}s$) and the inlet pressure of the gas cooler varied from 7.5 MPa to 8.5 MPa. The main results were summarized as follows : The predicted correlation can evaluated the R-744 exit temperature from the gas cooler within ${\pm}10%$ for most of the experimental data, given only the inlet conditions. The predicted gas cooley capacity using log mean temperature difference showed relatively food agreement with gas cooler capacity within ${\pm}5%$. The pressure drop predicted by Blasius estimated the pressure drop on the $CO_2$ side within ${\pm}4.3%$. The predicted heat transfer coefficients using Gnielinski's correlation evaluated the heat transfer coefficients on the $CO_2$ side well within the range of experimental error. The predicted heat transfer coefficients using Gao and Honda's correlation estimated the heat transfer coefficients on the coolant side well within ${\pm}10\;%$. Therefore. The predicted equation's usefulness is demonstrated by analyzing data obtained in experiments.

• Proceedings of the KSME Conference
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• 2001.11b
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• pp.191-198
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• 2001

A numerical analysis of the heat and mass transfer and pressure drop characteristics in modular shell and tube bundle heat exchanger was carried out. Finite Concept Method based on FVM and $k-\varepsilon$ turbulent model were used for this analysis. Condensation heat transfer enhanced total heat transfer rate $4\sim8%$ higher than that of dry heat exchanger. With increasing humid air inlet velocity, temperature and relative humidity, and with decreasing heat exchanger aspect ratio and cooling water velocity, total heat and mass transfer rate could be increased. Cooling water inlet velocity had little effect on total heat transfer.

• Journal of the Korean Society of Industry Convergence
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• v.4 no.4
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• pp.395-401
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• 2001

The characteristics of heat transfer and pressure drop through tubes has been investigated experimentally for a compound heat transfer enhancement. The test tubes were spirally indented tubes with wire coil inserts which had a various combinations of pitch and helix angles. Pure water was used as working fluids for the experiments, Heat transfer coefficients and friction factors of the test tubes were evaluated from the values of measured temperatures, flow rates and pressure drops. An performance evaluation was performed to find an optimal combination of spirally indented tubes with wire coil inserts. When the helix angle of wire coil insert are $71^{\circ}-72^{\circ}$, the best heat transfer enhancement was shown. The friction factor was 9 - 13 times higher than those in smooth tubes, and the heat transfer was enhanced a maximum of 500%.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.10 no.3
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• pp.271-281
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• 1998

In this study, condensation heat transfer experiments were conducted with two small diameter(ø7.5, ø4.0) tubes. Comparison with existing in-tube condensation heat transfer correlations indicated that the correlations overpredict the present data. For example, Akers correlation overpredicts the data upto 104%. The condensation heat transfer coefficient of the ø4.0 I.D. tube was smaller than that of the ø7.5 I.D tube; at the mass velocity of 300kg/$m^2$s, the difference was 12%. The pressure drop data of the small diameter tubes ware highly(two to six times) overpredicted by the Lockhart-Martinelli correlation. Subcooled forced convection heat transfer test confirmed that Gnielinski's single phase heat transfer correlation predicted the data reasonably well.

• Journal of Advanced Marine Engineering and Technology
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• v.24 no.2
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• pp.31-37
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• 2000

This study investigated heat transfer characteristics of refrigeration system using new type aluminium heat transfer tube for evaporator of refrigeration and air-conditioning comparing with bare tube. From the result of heat transfer experiment form one phase flow using cooled and hot water, about 20% heat transfer performance is superior in case of same quantity of flow and about 4% heat transfer performance if superior in case of same velocity comparing with bare tube. Casing of two phase flow, heat transfer performance of new type aluminum heat transfer tube shows about 50% superior heat transfer performance comparing with bare tube in the same evaporating pressure when using heat transfer tube as evaporator and shows about 47% increase when expressing performance coefficient as the rate of refrigerating capacity and compressing work. However, it can be known that pressure drop in the heat transfer tube is taken higher value of about 18% in case of new type aluminum heat transfer tube. From the above result, new type aluminum heat transfer tube is excellent comparing with bare heat transfer tube using the existing heat exchanger for refrigerator.

• 유체기계공업학회:학술대회논문집
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• 2001.11a
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• pp.325-333
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• 2001

The present study investigates convective heat/mass transfer and flow characteristics inside the cooling passage of the gas-turbine blades. It is important to increase not only the heat transfer rates but also the uniformity of heat transfer in the cooling passage. The square duct has compound-angled ribs with $60^{\circ},\;70^{\circ}$ and $90^{\circ}$ attack angles, which are installed on the test plate surfaces. a naphthalene sublimation technique is employed to determine the detailed local heat transfer coefficients using the heat and mass transfer analogy. The ribs disturb the main flow resulting in the recirculation and secondary flows near the ribbed wall and the vertices near the side-wall. The local heat transfer and the secondary flow in the duct are changed largely according to the rib orientation. Therefore, geometry and arrangement of the ribs are important fur the advantageous cooling performance. The angled ribs increase the heat transfer discrepancy between the wall and center regions because of the interaction of the secondary flows. The average heat/mass transfer coefficient and pressure drop of the ribs with the $60^{\circ}$ $-90^{\circ}$ compound-angle are higher than those with the $60^{\circ}$ attack angle. Also, the thermal efficiency of the compound-angled rib is higher than that with the $60^{\circ}$ attack angle. The uniformity of heat/mass transfer coefficient on the cross ribs may is higher than that on the parallel ribs array.

• Journal of Advanced Marine Engineering and Technology
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• v.30 no.4
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• pp.463-473
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• 2006

The effect of tube diameter on heat and mass transfer characteristics of absorber in absorption chiller/heater using LiBr solution as a working fluid has been investigated by both of numerical and experimental study to develop a high performance and compact absorber. The diameter of the heat exchanger tube inside absorber was changed from 15.88mm to 12.70mm and 9.52mm. In numerical study a model of vapor pressure drop inside tube absorber based on a commercial 20RT absorption chiller/heater was performed. The effect of tube diameter, longitudinal pitch, vapor Reynolds number, longitudinal pitch to diameter ratio on vapor pressure drop across the heat exchanger tube banks inside absorber have been investigated and found that vapor pressure drop decreases as tube diameter increases, longitudinal pitch increases, vapor Reynolds number decreases and longitudinal pitch to diameter ratio increases. In experimental study, a system includes a tube absorber, a generator, solution distribution system and cooling water system was set up. The experimental results shown that the overall heat transfer coefficient, mass transfer coefficient. Nusselt number and Sherwood number increase as solution flow rate increases. In both of study cases, the heat and mass transfer performance increases as tube diameter decreases. Among three different tube diameters the smallest tube diameter 9.52mm has highest heat and mass transfer performance.

• Journal of Advanced Marine Engineering and Technology
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• v.31 no.6
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• pp.707-714
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• 2007

The heat transfer coefficient and Pressure drop during gas cooling process of $CO_2$ (R-744) in inclined helical coil copper tubes were investigated experimentally. The main components of the refrigerant loop are a receiver. a variable-speed pump. a mass flow meter, a pre-heater and a inclined helical coil type gas cooler (test section). The test section consists of a smooth copper tube of 2.45mm inner diameter. The refrigerant mass fluxes were varied from 200 to $600[kg/m^2s]$ and the inlet Pressures of gas cooler were 7.5 to 10.0 [MPa]. The heat transfer coefficients of $CO_2$ in the inclined helical coil tubes increases with the increase of mass flux and gas cooling pressure of $CO_2$. The pressure drop of $CO_2$ in the gas cooler shows a relatively good agreement with those Predicted by Ito's correlation developed for single-phase in a helical coil tube. The local heat transfer coefficient of $CO_2$ agrees well with the correlation by Pitla et al. However, at the region near pseudo-critical temperature. the experiments indicate higher values than the Pitla et al. correlation. Therefore. various experiments in the inclined helical coil tubes have to be conducted and it is necessary to develop the reliable and accurate prediction determining the heat transfer and pressure drop of $CO_2$ in the inclined helical coil tubes.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.5
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• pp.2985-2992
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• 2015

In this study, the characteristics of heat transfer and pressure drop was theoretically analyzed by changing longitudinal pitch, bump phase, location of vortex generator about the staggered tube banks by applying SST (Shear Stress Transport) turbulence model of ANSYS FLUENT v.14. Before carrying out CFD (Computational Fluid Dynamics) analysis, It is presumed that the boundary condition is the tube surface temperature of 363 K, the inlet air temperature of 313 K and the inlet air velocity of 5-10 m/s. The results indicated that the heat transfer coefficient is not affected by the longitudinal pitch and the bump phase of circle type was more appropriate than serrated type in the characteristics of heat transfer and pressure drop. Additionally, in case of vortex generator location, the heat transfer characteristics showed that forward location of tube was more favorable 4.6% than backward location.