• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.10
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• pp.2678-2689
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• 1995

In this study, a new hot-wire anemometer which has greater sensitivity than that of a constant temperature anemometer (CTA) was proposed. In contrast to CTA, the wire working resistance of the new anemometer increases with flow velocity, that is, the operating mode of the wire becomes variable temperature. The variable temperature anemometer(VTA) was made by substituting a voltage controlled variable resistor such as photoconductive cell or transistor for one of the resistors in the bridge. By positively feeding back the bridge top signal to the input side of these electronic components, the wire overheat ratio could be increased with velocity automatically. Static response analyses of the VTA, constant voltage anemometer (CVA) and CTA were made in detail and calibration experiments were performed to validate the proposed operating principle. The wire operating resistance of the CVA decreases with velocity and this leads to lower sensitivity than that of a CTA. But the sensitivity of the newly proposed VTA is superior to that of a CTA, since the wire overheat ratio increases with velocity. Consequently, it is found that the major factor that is responsible for large sensitivity of a VTA is not the working resistance itself but the change of the wire working resistance with velocity.

• Proceedings of the KSME Conference
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• 2000.11b
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• pp.680-685
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• 2000

Investigation on ignition and flame propagation of pyrolyzing fuel in a cylindrical enclosure is accomplished. The pyrolyzing fuel of cylindrical shape is located in an outer cylinder sustained at high-temperature. Due to gravity, the buoyancy motion is inevitably incurred in the enclosure and this affects the flame initiation and propagation behavior. The radiative heat transfer plays an important role since a high temperature difference is involved in the problem. Numerical studies have been performed over overheat ratio, and vertical fuel eccentricity. The location of flame onset is affected by the vertical eccentricity of inner pyrolyzing fuel as well as thermal conditions applied.

• 한국전산유체공학회:학술대회논문집
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• 2005.04a
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• pp.75-78
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• 2005

A numerical investigation has been performed to discuss the radiation-affected steady-laminar natural convection in an enclosure under a large temperature difference. Due to inherent nature of this study, the Boussinesq approximation is no longer valid. Therefore the radiating fluid in an enclosure is treated as a ideal gas. To examine the effects of thermal radiation on thermo-fluid dynamic behaviors in complex geometries, two incomplete partitions are introduced. Based on the results of this study, the dispositions of incomplete partitions with radiatively participating medium are found to incur a distinct difference in fluid-dynamic as well as thermal behavior.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.1
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• pp.133-142
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• 2001

Investigation on ignition and flame propagation of pyrolyzing fuel in a cylindrical enclosure is accomplished. The pyrolyzing fuel of cylindrical shape is located in an outer cylinder sustained at high-temperature. Due to gravity, the buoyancy motion is inevitably incurred in the enclosure and this affects the flame initiation and propagation behavior. The radiative heat transfer plays an important role since a high temperature difference is involved in the problem. Therefore in all cases presented here, the intrinsic radiation effects are considered. Numerical studies have been performed over various governing parameters, such as Grashof number, overheat ratio, and vertical fuel eccentricity. Depending on the Grashof number, the flame behavior is found to be totally different: a separated visible flame appears as the Grashof number reaches 10(sup)7. The location of flame onset is also affected by the vertical eccentricity of inner pyrolyzing fuel as well as thermal conditions applied.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.12
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• pp.1796-1804
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• 1998

A numerical investigation has been performed to discuss the radiation-affected steady-laminar natural convection induced by a hot inner cylinder under a large temperature difference in the annuli filled with a gray gas. To examine the effects of thermal radiation on thermo-fluid dynamic behaviors in the eccentric geometry, the generalized body-fitted coordinate system is introduced while the finite volume method (FVM) is used for solving the radiative transport equation. After validating the numerical results for the case without radiation, the detailed radiation effect has been discussed. Based on the results of this study, when there exists a large temperature difference between two cylinders, the existence of radiatively participating medium is found to incur a distinct difference in fluid dynamic as well as thermal behavior.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.20 no.6
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• pp.685-691
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• 2011

The Bearing preload units are used for stable rotational movements of high speed spindles. The feeding mechanism of the preload unit is important to prevent overheat of bearings and to keep constant bearing load under thermal deformation of spindle unit. In this study, ball slide guide and ball bush as feeding mechanism of preload unit are selected. The maximum static friction force, radial stiffness and damping ratio of ball slide guide with ball load, ball number and ball size are widely investigated. In conclusion, the surface of ball slide guide must be heat treated to reduce static friction force. The number and size of ball are increased to control sensitive bearing preload force.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.9
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• pp.1201-1208
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• 2001

Variable temperature anemometer(VTA) has greater sensitivity than a conventional constant temperature anemometer(CTA). In order to design a reliable VTA system, however, an elaborate photoconductive cell stabilizing circuit which plays a key role in setting wire-overheat ratio should be firstly developed. In this study, a stabilizing circuit which adopts proportional-integral analog controller was proposed and thoroughly tested for its accuracy and reproducibility. In contrast to the available circuit suggested by Takagi, the present circuit has characteristic that the resistance of a photoconductive cell increases with the increase of input voltage, which makes the current circuit very suitable for the design of VTA. Finally, VTA adopting stabilizing circuit was made and the enhanced sensitivity of the VTA was validated experimentally by comparing the calibration curves of VTA and CTA.

• KEPCO Journal on Electric Power and Energy
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• v.8 no.2
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• pp.119-126
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• 2022

A growing number of gas-insulated transformers in underground power substations in urban areas are approaching 20 years of operation, the time when failures begin to occur. It is thus essential to prevent failure through accurate condition diagnosis of the given facility. Various solid insulation materials exist inside of the transformers, and the generated decomposition gas may differ for each gas-insulated equipment. In this study, a simulation system was designed to analyze the deterioration characteristics of SF₆ decomposition gas and insulation materials under the conditions of partial discharge and thermal fault for diagnosis of gas-insulated transformers. Degradation characteristics of the insulation materials was determined using an automatic viscometer and FT-IR. The analysis results showed that the pattern of decomposition gas generation under partial discharge and thermal fault was different. In particular, acetaldehyde was detected under a thermal fault in all types of insulation, but not under partial discharge or an arc condition. In addition, in the case of insulation materials, deterioration of the insulation itself rapidly progressed as the experimental temperature increased. It was confirmed that it was possible to diagnose the internal discharge or thermal fault occurrence of the transformer through the ratio and type of decomposition gas generated in the gas-insulated transformer.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.12
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• pp.2407-2414
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• 1992

Constant temperature hot wire air flow meter for automobiles requires temperature compensation system because hot wire output signal is sensitive to ambient temperature variations as well as fluid velocity. The objectives of the present study are to design an air flow meter circuit which is capable of compensating the hot wire output signal for ambient temperature variations and to investigate the mechanism of such temperature compensation. This circuit is composed of platinum hot wire, platinum resistor, two variable resistors, a constant resistor and a DC-amplifier. In particular, by simply replacing a constant resistor in one of the bridge arms of the conventional circuit with platinum resistor and a variable resistor for the purpose of temperature compensation, the deviation of output signal with respect to ambient temperature variations between 27deg. C 70deg. C could be reduced to less than 2.5% for mass flow rate and to less than 5% for velocity respectively. The mechanism of temperature compensation against ambient temperature variations was explained by means of measuring the heat transfer coefficient with hot wire temperature variations and analyzing and analyzing conventional empirical equations qualitatively.