• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.7 no.1
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• pp.73-80
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• 1995

The object of the present study is to develop a high performance evaporator for automotive air conditioner. The experiment has been conducted on evaporative heat transfer coefficient inside a plate type heat exchanger with a sharp 180-degree turn flow. The test plates have different formed surface, cross-ribbed channel and elliptical-ribbed channel. Also experimental study has been performed to determine optimal design in elliptical-ribbed plate heat exchanger with different turn clearance. In addition to the above experiments, refrigerant behavior and surface temperature distribution in the plate heat exchanger were observed using color thermoviewer(infrared thermometer). In this experiment, working fluid was used R-12 and test conditions were as follows : (1) saturation pressure of $2.116kg/cm^2$, (2) mass fluxes of 40 to $70kg/m^2s$, (3) heat fluxes of 4,500 to $7,300W/m^2$, (4) inlet quality of 0.1 to 0.7. The results indicated that the evaporative heat transfer coefficient of an elliptical-ribbed plate heat exchanger was higher than that of cross-ribbed plate heat exchanger. Also optimal turn clearance in an elliptical-ribbed plate heat exchanger was determined.

• Journal of the Semiconductor & Display Technology
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• v.7 no.1
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• pp.29-34
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• 2008

In this paper, the DE(double erosion) cathode for the reactive magnetron sputtering system is developed for high deposition rate and high target utilization efficiency. The utilization efficiency of the developed DE cathode is 22% higher than that of normal SE(single erosion) cathode. Sputtering process for the nickel oxide thin films with the DE cathode is performed under the following conditions; power with $1kW{\sim}3kW$, pressure with 4mtorr and 8mtorr, oxygen flow ratio with $0%{\sim}80%$. As a result, the hysteresis phenomenon of discharge voltage in 4mtorr is lower than that in 8mtorr and the hysteresis phenomenon of discharge voltage is getting lower as the applied power is getting higher. The structure of cross section and surface roughness of the thin films are observed by FE-SEM and AFM. The structure of cross section of the thin films is columnar and the average surface roughness under oxygen flow ratio of 0%, 52.5% and 65.0% are $2.08{\AA}$, $2.20{\AA}$ and $0.854{\AA}$, respectively.

• Journal of Hydrogen and New Energy
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• v.23 no.5
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• pp.493-502
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• 2012

Syngas laminar burning velocity measurements were carried out at atmospheric pressure and ambient temperature using the Bunsen flame configuration with nozzle burner as a fundamental study on flame stability of syngas fuel. Representative syngas mixture compositions ($H_2$:CO) such as 25:75%, 50:50% and 75:25% and equivalence ratios from 0.5 to 1.4 have been conducted. Average laminar burning velocities have been determined by the stabilized nozzle burner flames using the angle method, radical surface area method and compared with the data obtained from the other literatures. And the results of each experimental methodologies in the various composition ratios and equivalence ratios were coincided with the result of numerical simulation. Especially, it was confirmed that there was necessary to choice a more accurate measurement methodology even the same static flame method for the various composition ratios of syngas fuel including hydrogen. Also, it was reconfirmed that the laminar burning velocities gradually increased with the increasing of hydrogen content in a fuel mixture.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.6
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• pp.3698-3707
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• 2015

In this study, wet surface heat transfer and friction characteristics of non-symmetric slit-finned heat exchangers are experimentally investigated. Louver-finned heat exchangers are also tested for comparison purpose. The effect of fin pitch on j and f factor is negligible. Louver fin samples yield higher j and f factors than slit fin samples. For one row, j and f factors of louver fin are 27% and 31% higher than those of slit fin. For two row, j and f factors of louver fin are 15% and 30% higher. Both j and f factor decrease as the number of tube row increases. For one row, average j/f ratios of slit fin samples are 3.4% larger than those of louver fin samples. For two row, average j/f ratios of slit fin samples are 11.5% larger. A new correlation was developed using the present data.

• Tribology and Lubricants
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• v.30 no.6
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• pp.330-336
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• 2014

Sintered metallic brake pads and low alloy heat resistance steel disks are applied to mechanical brake systems in high energy moving machines that are associated with recently developed 200km/h trains. This has led to the speed-up of conventional urban rapid transit. In this study, we use a lab-scale dynamometer to investigate the effects of the composition of friction materials on the tribological characteristics of sintered metallic brake pads and low alloy heat resistance steel under dry sliding conditions. We conduct test under a continuous pressure of 5.5 MPa at various speeds. To determine the optimal composition of friction materials for 200 km/h train, we test and the evaluate frictional characteristics such as friction coefficients, friction stability, wear rate, and the temperature of friction material, which depend on the relative composition of the Cu-Sn and Fe components. The results clearly demonstrate that the average friction coefficient is lower for all speed conditions, when a large quantity of iron power is added. The specimen of 25 wt% iron powder that was added decreased the wear of the friction materials and the roughness of the disc surface. However when 35 wt% iron powder was added, the disc roughness and the wear rate of friction materials increased By increasing the amount of iron powder, the surface roughness, and temperature of the friction materials increased, so the average friction coefficients decreased. An oxidation layer of $Fe_2O_3$ was formed on both friction surfaces.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.17 no.6
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• pp.569-580
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• 2005

The present study investigates the convective heat/mass transfer characteristics in wavy ducts of a primary surface heat exchanger. The effects of duct aspect ratio and flow velocity on the heat/mass transfer are investigated. Local heat/mass transfer coefficients on the corrugated duct sidewall are determined using a naphthalene sublimation technique. The aspect ratios of the wavy duct are 7.3, 4.7 and 1.8 with the corrugation angle of $145\Omega$. The Reynolds numbers, based on the duct hydraulic diameter, vary from 300 to 3,000. The results show that at the low Re(Re $\leq$ 1000) the secondary vortices called Taylor-Gortler vortices perpendicular to the main flow direction are generated due to effect of duct curvature. By these secondary vortices, non-uniform heat/mass transfer coefficients distributions appear. As the aspect ratio decreases, the number of cells formed by secondary vortices are reduced and secondary vortices and comer vortices mix due to decreased aspect ratio at Re$\leq$1000. At Re >1000, the effects of corner vortices become stronger. The average Sh for the aspect ratio of 7.3 and 4.7 are almost same. But at the small aspect ratio of 1.8, the average Sh decreases due to decreased aspect ratio. More pumping power (pressure loss) is required for the larger aspect ratio due to the higher flow instability.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.14 no.5
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• pp.220-225
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• 2004

Four different Fluorine-based gases ($SF_6/,NF_3, PF_5,\; and \; BF_3$) were examined for high rate Inductively Coupled Plasma etching of Si. Etch rates up to ~8$\mu\textrm{m}$/min were achieved with pure $SF_6$ discharges at high source power (1500 W) and pressure (35 mTorr). A direct comparison of the four feedstock gases under the same plasma conditions showed the Si etch rate to increase in the order $BF_3$ < $NF_3$< $PF_5$ < $SF_6$. This is in good correlation with the average bond energies of the gases, except for $NF_3$, which is the least strongly bound. Optical emission spectroscopy showed that the ICP source efficiently dissociated $NF_3$, but the etched Si surface morphologies were significantly worse with this gas than with the other 3 gases.

• Journal of Power System Engineering
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• v.21 no.3
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• pp.102-107
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• 2017

A tungsten inert gas (TIG) welding method was used for the bonding of stainless steel. TIG welding using inert gas (He or Ar gas) is a method to prevent oxidation and nitriding of materials and to combine non-ferrous metals. This method has the advantage of obtaining a smooth weld surface. In this study, the welding characteristics of 304 stainless steel welded by TIG welding method were analyzed by using nondestructive technique. Ultrasonic and Acoustic Emission (AE) was applied to evaluate the micro-damage of TIG welded 304 stainless steel. The velocity and damping coefficient of ultrasonic wave showed a slight difference in HAZ, which is the welding part of stainless steel. The AE parameters of average frequency, rise time and event were analyzed for the dynamic behavior of stainless steel during loading. Optimal AE parameters for evaluating the degree of damage to the specimen have been derived. Fractograph and metal structures of 304 stainless steel using SEM and optical microscope were discussed.

• Journal of Powder Materials
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• v.15 no.4
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• pp.279-284
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• 2008

Dense nanostructured $NbSi_2$ was synthesized by high-frequency induction-heated combustion synthesis (HFIHCS) method within 1 minute in one step from mechanically activated Nb and Si powders. Highly dense $NbSi_2$ with relative density of up to 99% was simultaneously synthesized and consolidated under the combined effects of an induced current and mechanical pressure of 60 MPa. The average grain size and mechanical properties (hardness and fracture toughness) of the compound were investigated.

• 한국기계연구소 소보
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• s.15
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• pp.91-103
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• 1985

In this paper, temperature distribution and thermal stress are investigated considering engine peak pressure and the time average temperature distribution in the piston under running conditions for the diesel engine. The induced stress are calculated by the Finite Element Method(FEM). The results obtained are summerized as follows. 1) The results calculated by the FEM present good agreement with other numerical solution in literature. 2) It is confirmed that maximum compressive stress are induced in the part of outside wall between the piston crown and the pin bush. 3) In the axial direction, the hoop stresses are changed its sigh at the portion of crown near the inner wall side 4)Large gradient of temperature is shown in the piston crown near the side wall in the axial direction, in the part between the piton crown and the pin bush in radical direction 5)in case of stress distribution of piston wall surface in the axial direction, the hoop stress is a little greater than axial stress, and the latter is greater than the radial stress