• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.14 no.10
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• pp.833-843
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• 2002

The present experimental and numerical investigations are performed for the characteristics of transitional flow in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary and inner one rotating. The pressure losses and skin- friction coefficients have been measured for the fully developed flow of water and glycerine-water solution (44%) with the inner cylinder rotating at speed of 0∼600 nm, respectively. The transitional flow has been examined by the measurement of pressure losses to reveal the relation of the Reynolds and Rossby numbers with the skin-friction coefficients. The occurrence of transition has been checked by the gradient changes of pressure losses and skin-friction coefficients with respect to the Reynolds numbers. The increasing rate of skin-friction coefficient due to the rotation is uniform for laminar flow regime, whereas it is suddenly reduced for transitional flow regime and, then, it is gradually declined for turbulent flow regime.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 1998.06a
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• pp.423-426
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• 1998

Piezoresistive pressure sensors have fabricated using electrochemical etch-stop technique. Si diaphragm having thickness of n-epi. layer was fabricated and used to detect pressure range from 0 to 1 kg/$\textrm{cm}^2$. Piezoresistors were diffused 3${\times}$10$\^$18/ cm$\^$-3/ and placed at diaphragm edge for maximum pressure detection. The characteristics of electrochemical etch-stop in TMAH/lPA/pyrazine solution were also discussed. I-V curves of n and p-type Si in TMAH/lPA/pyrazine solution were obtained. Etching rate is highest at optimum etching condition, TMAH 25wt.%/IPA 17vo1.%/pyrazine 0.1/100m1, thus the elapsed time of etch-stop was reduced.

• 한국전산유체공학회:학술대회논문집
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• 2002.05a
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• pp.146-151
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• 2002

Characteristics of flow rate control has been studied for a cavitating venturi adopted in a liquid rocket propellant feed system. Numerical simulation has been peformed to give about $10\%$ discrepancy of mass flow rate to the experimental data for cavitating flow regime. Mass flow rate is confirmed to be saturated for pressure difference higher than $3\times10^5$pa when the upstream pressure is fixed to $22.8\times10^5$pa and the downstream pressure is varied. The evaporation amount depends substantially to non-condensable gas concentration. However the mass flow rate characteristic is relatively insensitive to the mass fraction of non-condensable gas. So it is reduced by only $2\%$ when the non- condensable gas concentration is increased from 1.5PPM to 150PPM. From the previous comparison the expansions of the non-condensable gas and the evaporation of liquid are verified to have same effect to pressure recovery.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.5
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• pp.709-717
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• 2000

Cyclic variability has long been recognized as limiting the range of operating conditions of spark ignition engines, in particular, under lean and highly diluted operation conditions. The cyclic combustion variations can be characterized by the pressure parameters, combustion parameters, and flame front parameters. The coefficient of variation in indicated mean effective pressure ($COV_{IMEP}$) defines the cyclic variability in indicated work per cycle, and it has been found that vehicle driveability problems usually result when $COV_{IMEP}$ exceeds about 10%. For analysis of the cyclic variability in SI engines at idling, the results show that cyclic variability by the $COV_{IMEP}$ or the coefficient of variation in maximum pressure can be explained and may be consequently reduced by the help of the optimum spark timings.

• Transactions of the Korean Society of Machine Tool Engineers
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• v.16 no.6
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• pp.21-28
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• 2007

Injection molding process factors such as molding temperature, injection pressure, flow rate and flow velocity, must be controlled properly in filling and packing phases in the injection molding process. In this study, effects of these factors on the injection molding were investigated through the flow analysis for birefringence and refractive index for pickup lens. This paper presents the birefringence and refractive index reduced with increasing the holding pressure and also the holding pressure time. And there are interaction with birefringence and fill time in the injection process. The optimal conditions through DOE are validated by using injection molding analysis.

• International Journal of Air-Conditioning and Refrigeration
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• v.13 no.3
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• pp.128-137
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• 2005

In this study, dry and wet surface pressure drop and heat transfer characteristics of heat exchangers having plain fins were investigated. Nine samples having different fin pitches and rows were tested. The wet surface heat transfer coefficient was reduced from experimental data using enthalpy-potential method. The wet surface heat transfer coefficients were approximately equal to the dry surface values except for one row configuration. For one row configuration, the wet surface heat transfer coefficients were approximately $30\%$ lower than the dry surface values. For the pressure drop, the wet surface yielded approximately $30\%$ higher values compared with the dry surface counterpart. Data were compared with existing correlations.

• The KSFM Journal of Fluid Machinery
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• v.14 no.4
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• pp.57-63
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• 2011

Regenerative blower is suitable for hydrogen recirculation in fuel cell vehicle due to its capability of high pressure rise in single stage. Numerical models were applied to investigate inner gap leakage flow characteristics. A leakage flow in the inner gap is dominantly affected by pressure gradient. Therefore a blower with concentric channel type was suggested as one of modified models for reducing the inner gap pressure gradient. Also numerical results such as pressure rise, efficiency, leakage flow rate and torque were compared between modified and reference models. The performance of concentric channel type was improved as a result of reduced leakage flow.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2001.11a
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• pp.85-88
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• 2001

The NdBaCuO superconductor samples were Zone-melted in low oxygen partial pressure (1% O$_2$+99% Ar). The Zone-melting temperature was decreased about 120$^{\circ}C$ from 1060$^{\circ}C$ the zone-melting temperature in air. Thus the loss of liquid phase (BaCuO$_2$ and CuO) was reduced during the zone-melting process. The content of non-superconducting phase Nd422 in zone-melted NdBaCuO samples was clearly decreased, so was the substitution of Nd for Ba. The superconductivity of zone-melted Nd$\sub$1+x/Ba$\sub$2-x/Cu$_3$O$\sub$y/ prepared under low oxygen partial pressure was distinctively improved.

• Journal of Electrical Engineering and Technology
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• v.11 no.4
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• pp.969-973
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• 2016

Interruption tests were conducted using the same circuit breaker for an initial pressure of SF₆ 0.5 MPa (gauge pressure) and CO₂ mixture 1.0 MPa, 0.8 MPa, and 0.6 MPa. The pressure-rises in the compression and thermal expansion chambers were measured for verifying the computational results using a simplified synthetic test facility. Further, the possibility of the CO₂ mixture substituting SF₆ gas was confirmed. Moreover, in view of the thermal recovery capability, it has also been confirmed that the pressure of the CO₂ mixture can be reduced almost to the same value as that of the SF₆ gas by optimizing the design parameters of the interrupter.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.16 no.3
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• pp.218-229
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• 2004

In this study, dry and wet surface pressure drop and heat transfer characteristics of heat exchangers having plain fins were investigated. Nine samples having different fin pitches and rows were tested. The wet surface heat transfer coefficient was reduced from experimental data using enthalpy-potential method. The wet surface heat transfer coefficients were approximately equal to the dry surface values except for one row configuration. For one row configuration, the wet surface heat transfer coefficients were approximately 30% lower than the dry surface values. For the pressure drop, the wet surface yielded approximately 30% higher values compared with the dry surface counterpart. Data were compared with existing correlations.