• 한국신재생에너지학회:학술대회논문집
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• 2007.06a
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• pp.672-675
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• 2007

Experimental data are presented which describe heat losses of cavity type receiver in wind tunnel. Experiments are conducted at various conditions such as the heater temperature in cavity changes from 300, 400, and 500 oC, wind speed in tunnel from 2 to 8 m/s, and four different tilt angle of 30, 50, 70, 90o. The power consumption including temperature, voltage and current for each experimental conditions are measured and stored in data logger at everyone second interval. The experimental results show that heat losses increase with increasing wind speed and with tilt angle. However, heat losses for the tilt angle of 70 and 90o is almost same at each heater temperature. In addition, the effects of natural convection in combined convection heat losses vary in according to the tilt angle.

• 한국신재생에너지학회:학술대회논문집
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• 2007.06a
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• pp.680-683
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• 2007

In dish concentrating system, natural convection heat loss occurs in cavity receiver. Heat loss mechanisms of conduction, convection, and radiation can reduce the system efficiency. To obtain the high efficiency, the receiver is to absorb the maximum of solar energy and transfer to the working fluid with maximum of heat losses. The convection heat loss is an important factor to determine the system performance. Numerical analysis of the convection heat loss of receiver was carried out for varing inclinaton angle from 0$^{\cdot}$ to 70$^{\cdot}$ with temperature range from 400$^{\cdot}C$ to 600$^{\cdot}C$ using the commercial software package, Fluent 6.0. The result of numerical analysis was comparable with convection heat loss model of solar receiver.

• Solar Energy
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• v.8 no.2
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• pp.3-11
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• 1988

To suppress the natural convection within enclosure spacing it has been shown theoretically and experimentally that the introduction of cell walls will effectively raise the critical Rayleigh number by providing more shear surfaces within the fluid. For a solar collector, a useful solar thermal converter requires effective control of heat losses. It has been reported that the natural convection can be suppressed and the heat performances of the solar collector increased by placing thin, poorly conducting material honeycomb between the absorber plate and the coverglass. The heat performances were measured and compared directly throughout the simultaneous installation of two solar collectors, one with honeycomb structures fabricated from thin poly carbonate sheet and the other without honeycomb structures. Various tilt angles of 30, 45 and 60 deg. from the horizontal and the honeycomb sizes ($W{\times}H$) of $10{\times}10,\;10{\times}20$ and $10{\times}40mm$ were utilized in the present investigation. It is found that the larger the tilt angle are, the greater the heat losses are, and that the smaller the honeycomb size is, the larger suppression effect of heat losses are. Especially, at tilt angles of 30 degree, the heat use ratio of solar collector with the honeycomb sizes of $10{\times}10mm$ improved approximately 29.5% more than that without honeycomb structures.

• 한국전산유체공학회:학술대회논문집
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• 2008.03b
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• pp.107-108
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• 2008

The temperature rise of GIS (Gas Insulated Switchgear) busbar system is a vital factor that affects its performance. In this paper, a two-dimensional model is presented by commercial code CFX11 for the evaluation of natural convection in the busbar system. In the model, SF6 (Sulfur Hexafluoride) is used to insulate the high voltage device and improves the heat transfer rate. The power losses of a busbar calculated by the magnetic field analysis are used as the input data to predict the temperature rise by the nature convection analysis. The heat-transfer coefficients on the boundaries are analytically calculated by applying the Nusselt number considering material property and model geometry for the natural convection. The temperatures of the tank and conductors from CFX simulation and the experiment were compared. The results show a good agreement. In the future, we will calculate the 3-D model and try to reduce the temperature by adjusting some dimensional parameters.

• Journal of Mechanical Science and Technology
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• v.14 no.12
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• pp.1403-1411
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• 2000

Heat losses from the receivers for a dish-type solar energy collecting system constructed at Korea Institute of Energy Research are analyzed. The Stine and McDonald's model is used to estimate the convection loss. The Net Radiation method and the Monte-Carlo method are used to calculate the radiation heat transfer rate from the inside surface of the receiver to the surroundings. Two different receivers are suggested here and the performances of the receivers are estimated and compared with each other based on the prediction of the amount of heat losses from the receivers. The effects of the receiver shape and the radiation properties of the surface on the thermal performance are investigated. The performance of Receiver I is better than that of Receiver II, and the amount of solar irradiation that is not captured by the captured by the receiver after being reflected by the concentrator becomes significant if the temperature of the working fluid is low.

• Journal of the Korean Solar Energy Society
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• v.21 no.4
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• pp.63-71
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• 2001

This work has been carried out to find the ideal operating conditions for solar vacuum tube collectors which are widely used at present. Various types of solar collectors including a flat plate one were experimentally tested and examined to determine their thermal efficiencies and operating characteristics. Generally, solar vacuum tubes can be classified into two groups according to their design features. Of these, one is characterized by the insertion of a metallic device(such as a finned heat pipe) in an evacuated glass tube for the collection and transportation of solar energy. The other utilizes double glass tubes where the smaller one is contained inside the bigger one and soldered to each other after the small gap between them is evacuated. Both of these solar collectors are designed to minimize convection heat losses by removing the air which is in direct contact with the absorber surface. The performance of the former type can be readily analyzed by applying the relevant correlations developed for flat plate solar collectors. This has been demonstrated in the present study for the case of a solar collector where a heat pipe is inserted in an evacuated tube.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.266-271
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• 2003

The numerical study was carried out to effectively cool Induction motor applied to a washing machine. The outer rotor made of steel periodically spins up and down. The stator consists of the thin layered iron plates and copper coil. The effective cooling system is necessary to solve the reliability problem caused by the electric losses at the coil and the iron plate. Because the heat transfer rate of the natural convection in partially open space is generally low, thus it is necessary to enhance the heat transfer using rotating perforated plate. The flow phenomena around the motor are very complex due to the motor geometry and the outer rotor motion. The mixed convection takes place due to the slow rotation speed. The three dimensional flow simulation was performed using rotating reference frame technique and Boussinesq approximation but the radiation effect was neglected. It was found that the angle and direction of the cooling blades play an important role in the stator temperature.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.9
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• pp.1166-1174
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• 2000

This work presents a two-dimensional quasi-steady state model to study the fluid flow and heat transfer in high-power density welding process of thin AISI-304 stainless steel plates. The enthalpy method and the finite volume method were used for a numerical analysis of the mushy region phase change as well as the heat flow at the weld pool and the heat-affected zone. The results show that the mushy region distributed around the weld pool becomes wider downstream and the surface heat losses by convection and radiation can be significant factors in welding process especially when a welding speed is relatively low.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.12
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• pp.1383-1389
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• 2011

The current study focuses on the consistent analysis of heat transfer in multichannel volumetric solar receivers used for concentrating solar power. Changes in the properties of the absorbing material and channel dimensions are considered in an optical model based on the Monte Carlo ray-tracing method and in a one-dimensional heat transfer model that includes conduction, convection, and radiation. The optical model results show that most of the solar radiation energy is absorbed within a very small channel length of around 15 mm because of the large length-to-radius ratio. Classification of radiation losses reveals that at low absorptivity, increased reflection losses cause reduction of the receiver efficiency, notwithstanding the decrease in the emission loss. As the average temperature increases because of the large channel radius or small mass flow rate, both emission and reflection losses increase but the effect of emission losses prevails.

• The Magazine of the Society of Air-Conditioning and Refrigerating Engineers of Korea
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• v.11 no.2
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• pp.1-9
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• 1982

Friction losses and heat transfer characteristics were studied experimentally for fully developed tubulent flow in tubes with twisted tape swirl generators. Data were obtained for pitch- to-diameter ratios from 8.10 to 25.71 with water under forced convection cool ins conditions. The experimental results of this and that of previous swirl flow investigation by Smithberg and Landis were compared. Nusselt number and friction data are combined in a constant pumping power comparision for swirl and straight flow, which indicates that improvement of 2.0 times in the best performance can be obtained with twisted tape-insert tube. A twisted tape-insert tube is recommended in the case of low temperature difference in heat-exchanger.