• 열처리공학회지
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• 제36권6호
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• pp.402-411
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• 2023

In order to examine the effect of cementite precipitated on the steel surface on the carburizing rate, the carburizing process was carried out at various boost times to measure the mass gain and carbon flux, phase analysis and carbon concentration analysis were performed on the surface of the carburized specimen. In the case of the only boost type, the longer the boost time, the more the mass gain by the diffused carbon follows the parabolic law and tends to increase. In particular, as the boost time increased, the depth of cementite precipitation and the average size of cementite on the steel surface increased. At a boost time of 7 min, the fraction of cementite precipitated on the surface is 7.32 vol.%, and the carburizing rate of carbon into the surface (surface-carbon flux) is about 17.4% compared to the calculated value because the area of the chemical (catalyst) where the carburization reaction takes place is reduced. The measured carbon concentration profile of the carburized specimen tended to be generally lower than the carbon concentration calculated by the model without considering precipitated cementite. On the other hand, in the pulse type, the mass gain by the diffused carbon increased according to the boost time following a linear law. At a boost time of 7 min, the fraction of cementite precipitated on the surface was 3.62 vol.%, and the surface-carbon flux decreased by about 4.1% compared to the calculated value. As a result, a model for predicting the actual carbon flux was presented by applying the carburization resistace coefficient derived from the surface cementite fraction as a variable.

• 한국해안·해양공학회논문집
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• 제24권1호
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• pp.16-25
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• 2012

경기만 염하수로 입구인 영종대교 남단에서 평수기와 홍수기에 13시간동안 연속적으로 층별 유속관측을 수행하였다. 홍수기와 평수기에 한강과 임진강으로의 평균 담수유입량은 각각 약 $8000m^3/s$, $200m^3/s$로 40배의 큰 차이를 보이는데 이러한 변화가 잔차류의 공간 분포와 총 유출입량에 어떠한 차이를 보이는지 분석하였다. 수집된 자료의 분석을 위하여 조석주기 동안의 평균이 필요하며, 반복 관측된 자료의 연직위치와 수평격자위치를 일치시킬 필요성이 있다. 따라서 본 연구에서 공간적 sigma 격자 체계로 변환시켰다. 변형된 sigma 좌표체계는 z-level상의 원시자료와 비교하였을 때 5%이내의 오차로 자료분석에 무리가 없는 것으로 판단되었다. 분석결과 평수기 단면 잔차류는 수로의 수심에 따라 패턴이 다른 수평적 2층 흐름 구조를 보였으며, 홍수기의 경우 표층에서는 낙조하고, 저층에서는 창조하는 수직적 2층 흐름구조를 보였다. 이는 담수의 유입량에 따라 단면에서 공간적 잔차류 흐름구조가 크게 변동되는 특성이 나타났다. 총 수송량은 평수기와 홍수기에 각각 $359m^3/s$, $261m^3/s$ 로 약 $100m^3/s$의 차이를 보였다. 홍수기 많은 양의 담수 유입이 발생하였지만, 총 수송량과 적은 상관도를 보인 것은 염하수로 남단 해역에서 Stokes drift의 크기가 강하게 나타나기 때문으로 보여지며, 총량은 강화도와 영종도 사이의 조간대 지역으로 이동하는 것으로 짐작된다.

• 에너지공학
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• 제8권2호
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• pp.298-308
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• 1999

과냉각수조로 분사되는 증기의 직접접촉 응축특성을 알아보기 위하여 다섯 개의 수평 노즐에 대해 증기 질량유량과 수조 온도를 여러 가지로 변화시키면서 실험적 연구를 수행하였다. 증기응축현상을 육안관찰과 고속 비디오 카메라를 사용한 방법으로 분석한 결과, 안정된 증기제트인 경우 증기 질량속과 수조온도가 변화함에 따라 타원형 및 원추형 증기제트 형상이 나타나는 것을 관찰하였다. 증기제트 팽창비, 증기제트 길이 및 응축연전달계수를 구하였고, 증기 질량속, 수조온도 및 노즐 내경이 미치는 영향을 분석하였다. 증기제트 길이와 응축열전달계수를 증기 질량속 및 응축추진 포텐셜의 함수로 나타낸 상관식을 구하였다. 증기제트 내부와 주위 수조온도 분포를 구했으며 증기 질량속, 수조온도 및 노즐 내경이 미치는 영향을 분석하였다. 처깅, 천이처깅, 응축진동, 안정응축, 방울응축진동 및 간헐진동응축 등 여섯 가지 영역으로 구분된 응축 영역도를 작성하였다. 그 외, 수조 벽면에서의 동압을 측정하였고, 증기 질량속과 수조온도에 따라 변화하는 증기 응축모드와 동압과는 밀접한 연관성이 있음을 확인하였다.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2011년도 제42회 하계학술대회
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• pp.1043-1044
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• 2011

This paper discusses the electromagnetic and mechanical design considerations to improve the design accuracy and power to mass ratio of multi-stack axial flux permanent magnet synchronous generator (AFPMSG). Design accuracy of multi-stack AFPMSG for direct drive wind turbine application is improved by considering magnetic flux leakages and fringing effect. FEM structural analysis is utilized to increase power to mass ratio of three-stack AFPMSG by reducing the rotor yoke thickness considering magnetic and centrifugal forces and Von Mises stress distribution.

• 동력기계공학회지
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• 제14권4호
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• pp.11-16
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• 2010

The evaporation heat transfer characteristics of propane(R-290) in horizontal small diameter tubes were investigated experimentally. The test tubes have inner diameters of 1 mm and 4 mm. Local heat transfer coefficients were measured at heat fluxes of 12, $24\;kW/m^2$, mass fluxes of 150, $300\;kg/m^2s$, and evaporation temperature of $15^{\circ}C$. The experimental results showed that the evaporation heat transfer coefficient of R-290 has an effect on heat flux, mass flux, tube diameter, and vapor quality. The evaporation heat transfer of R-290 has an influenced on nucleate boiling at low quality and convective boiling at high quality. The evaporation heat transfer coefficient of R-290 increases with decreasing inner tube diameter. And the evaporation heat transfer coefficient of R-290 is about 1~3 times higher than that of R-134a.

• 설비공학논문집
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• 제13권12호
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• pp.1214-1222
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• 2001

The present study investigated the effect of diameter and length on the absorption performance of a vertical falling film type absorber using $LiBr-H_2$O solution of 60 wt%. The parameters were diameter of absorber (17.2, 23.4, 31.1 mm), length of absorber (771, 1150, 1528 mm), and film Reynolds numbers (50, 70, 90, 110, 130, 150). As the diameter of the absorber was increased, the absorption mass flux, Sherwood number, heat flux, and heat transfer coefficient were increased, in which Sherwood number and heat transfer coefficient were increased up to 13% and 30% respectively. As the length of the absorber was increased, the total absorption rate and heat transfer coefficient were increased by 37% and 35% respectively, while the absorption mass flux was decreased.

• 설비공학논문집
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• 제11권6호
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• pp.835-845
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• 1999

The objective of the present study was to investigate heat and mass transfer characteristics in a vertical falling film type absorber using LiBr-$H_2O$ solution with 6owt%. The experimental apparatus consisted of an absorber with inner diameter of 17.2 mm and length of 1150mm, a generator, an evaporator/condenser, a solution tank, a sampling trap etc. The parameters were solution temperature of 45 and $50^{\circ}C$, coolant temperature of 30 and $35^{\circ}C$, and film Reynolds numbers from 50 to 150. Pressure drop in the absorber increased as solution and coolant temperatures decreased. Pressure drop in the absorber increased up to the film Reynolds number of 90, and then decreased at the further increase of the Reynolds number above 90. The maximum absorption mass flux observed at the film Reynolds number of 90. Absorption mass flukes increased as coolant temperature decreased. Absorption mass fluxes and heat transfer coefficients under subcooled condition were larger than those under superheated condition. Heat transfer coefficients were affected by solution temperature more than coolant temperature. The maximum absorption effectiveness under the subcooled condition was 23% for coolant temperature of $30^{\circ}C$ and 31% for coolant temperature of $35^{\circ}C$ under the present experimental conditions.

• Journal of Korean Society for Atmospheric Environment
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• 제21권E3호
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• pp.75-85
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• 2005

A mathematical sensitivity analysis of the flow-through dynamic flux chamber technique, which has been utilized usually for various trace gas flux measurement from soil and water surface, was performed in an effort to provide physical and mathematical understandings of parameters essential for the NO flux calculation. The mass balance equation including chemical reactions was analytically solved for the soil NO flux under the steady state condition. The equilibrium concentration inside the chamber, $C_{eq}$, was found to be determined mainly by the balance between the soil flux and dilution of the gas concentration inside the chamber by introducing the ambient air. Surface deposition NO occurs inside the chamber when the $C_{eq}$ is greater than the ambient NO concentration ($C_{0}$) introducing to the chamber; NO emission from the soil occurs when the $C_{eq}$ is less than the ambient NO concentration. A sensitivity analysis of the significance of the chemical reactions of NO with the reactive species (i.e. $HO_{2},/CH_{3}O_{2},/O_{3}$) on the NO flux from soils was performed. The result of the analysis suggests that the NO flux calculated in the absence of chemical reactions and wall loss could be in error ranges from 40 to $85\%$ to the total flux.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회B
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• pp.2200-2205
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• 2007

In order to investigate the variation on a heat transfer coefficient during evaporation of $CO_2$, basic experiment on the evaporation heat transfer characteristics in a horizontal micro-channel tube was performed. Hydraulic diameters of micro-channels were 0.68 and 1.46 mm. The experiment apparatus consisted of a test section, a DC power supply, a heater, a chiller, a mass flow meter, a pump and a measurement system. Experiments were conducted for various mass fluxes of 300 to 800 kg/$m^2s$, heat fluxes of 10 to 40 kW/$m^2$ and saturation temperatures of -5 to 5$^{\circ}C$. With the increase heat flux, the evaporation heat transfer coefficient increased. And the significantly change of the heat transfer coefficient was observed at any heat flux and mass flux. As the saturation temperature increased and the hydraulic diameter decreased, the heat transfer coefficient increased.

• 대한기계학회논문집B
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• 제24권2호
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• pp.233-240
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• 2000

An experimental study of condensation heat transfer was performed for pure refrigerants HFC32, HFCI25, and HFC134a, and a ternary refrigerant mixture of HFC32/125/134a (23/25/52wt%). The heat transfer coefficients were measured inside a horizontal smooth tube 5.8 mm I.D. and 8.0 m long. The refrigerant temperature at inlet was 40 $^{\circ}C$, and the mass flux was varied from 150 to 400 $kg/m^2s$. As for the pure refrigerants, the heat transfer coefficient of HFC32/125/l34a decreased as the quality decreased. In addition, the heat transfer coefficient of HFC32/l25/134a was about 20 % lower than HFC 134a at a low mass flux but showed no reduction at a high mass flux. The heat transfer coefficient of ternary refrigerant mixtures was 30% lower on the average than that of the pure refrigerant.