• 수산해양교육연구
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• 제5권2호
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• pp.119-127
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• 1993

This work studies for heat transfer and pressure drop performance of integral inner and outer fin tubes, designed to enhance the heat transfer performance of smooth tubes for in recipro and turbo refrigerator or high performance compact heat exchangers. Eight different inner spiral fin copper tubes with integral fin at outside surfaces were employed to improve boiling heat transfer coeffcient. For comparison, tests were made using a plain tube having the inside diameter and an outside diameter equal to that at the root of the fins for the finned tubes. Pool boiling heat transfer is investigated experimentally and theoretically on single tube arrangement. The refrigerant evaporates at a saturation state of 1 bar on the outside tube surface heated by hot water. The refrigerant R11 ($CFCl_3$) was used at a pressure of $P_s=1bar$ as a convenient test fluid with a boiling temperature of $T_s=23.6^{\circ}C$. The observed heat transfer enhancement of boiling for finned tubes significantly exceeded that to be expected on grounds of increased area. The maximum Vapor - side enhancement(i.e., vapor - side heat transfer coefficient of finned tube/vapor - side coefficient for plain tube) was found to be around 4 at 1299fpm - 30grooves tube.

• 한국가시화정보학회지
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• 제21권1호
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• pp.57-66
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• 2023

In this study, we investigate the effects of a single roughness element on Venturi cavitation. The single roughness element of hemispherical shape is installed at the throat inlet of a Venturi tube. Since the wake behind the roughness element induces an additional pressure drop, cavitation inception occurs at a higher Cavitation number for the Venturi model with the single roughness element than for the Venturi model with no roughness. Cavitation bubbles form along the wake of the roughness element and lengthen in the streamwise direction as the Cavitation number decreases, forming a longitudinal cavitation. With a further decrease in the Cavitation number, the longitudinal cavitation bubble merges with the sheet cavitation initiated from the exit edge of the Venturi tube throat, followed by the shedding of cloud cavitation. The merging of the longitudinal cavitation and sheet cavitation is accompanied by a sudden decrease in the discharge coefficient and an increase in the pressure loss coefficient as it chokes the flow inside the Venturi tube.

• 한국마린엔지니어링학회:학술대회논문집
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• 한국마린엔지니어링학회 2005년도 후기학술대회논문집
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• pp.41-42
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• 2005

The experimental apparatus has been set-up as a conventional vapor compression type heat pump system. The test section is a horizontal double pipe heat exchanger. A tube diameter of 12.70 mm, 9.52 mm, 6.35 mm with 1.78 mm,1.52 mm,1.4 mm wall thickness each is used for this investigation. The local evaporating heat transfer coefficients of hydrocarbon refrigerants were superior to that of R-22. and the maximum increasing rate of heat transfer coefficient was found in R-1270. The average evaporating heat transfer coefficient increased with the increase of the mass velocity and it showed the higher values in hydrocarbon refrigerants than R-22. The highest evaporating heat transfer coefficient of all refrigerants was shown in a tube diameter of 6.35 mm with same mass flux.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집B
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• pp.517-522
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• 2001

The exit edges of a diffuser are smoothly rounded, and a wall is located perpendicularly to a diffuser exit. The fluid is discharged towards the radial direction of a diffuser after impinging against a wall from a diffuser. In this flow path, pressure loss coefficients have been calculated by the variables of Reynolds number at a diffuser inlet, distance between a diffuser exit and a wall, and turbulence models. As a result, it was calculated that $h/D_0$ ratio between $0.35\sim0.4$ has the minimum pressure loss coefficient regardless of Reynolds number and turbulence models. It was also found that in case of the flow with relatively high Reynolds number at a diffuser inlet, the pressure loss coefficients by RNG $k-\varepsilon$ model have a tendency to be near to those by standard $k-\varepsilon$ model at small ratio of $h/D_0$, but to those by RSM at large ratio.

• 한국산업융합학회 논문집
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• 제25권1호
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• pp.131-139
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• 2022

The exchange of goods over the sea is a situation in which the amount of trade between countries is gradually increasing. In order to maintain the optimal operating condition, the ship maintains stability and optimal operating conditions by inserting or withdrawing ballast water from the ballast tank according to the loading condition of cargo capacity is also increasing. Control valves play an important role in controlling fluid flow in these pipes. When the flow rate is controlled using a control valve, problems such as cavitation, flashing, and suffocating flow may occur due to high differential pressure, and in particular, damage to valves and pipes due to cavitation is a major problem. Therefore, in this study, the cavitation phenomenon is reduced by installing orifices at the front and rear ends of the high differential pressure control butterfly valve to reduce the sudden pressure drop at the limiting part of the butterfly valve step by step. The flow coefficient according to the shape of the orifice, the degree of cavitation occurrence, and the correlation were analyzed using a CFD(Cumputational Fluid Dynamics), and an optimal orifice design for reducing cavitation is derived.

• 대한기계학회논문집B
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• 제22권7호
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• pp.1001-1012
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• 1998

This paper reports the analysis of dynamic characteristics of air-cooled condenser. At first, there is an assumption that the superheated vapor flows into the condenser inlet. And in order to consider the effect of pressure change in the dynamic characteristics of the condenser the combined system of condenser and compressor was used. By using the equation of energy balance and the equation of mass balance, the basic equation for describing the dynamic characteristics of condenser can be derived. The transfer function for describing dynamic response of the condenser to flow rate change outlet can be obtained from using linearizations and Laplace transformations of the equation. From this transfer function, analytical investigation which affects the frequency responses of condenser has been made. Through this study, it became possible that the information about the dynamic characteristics of air-cooled condenser is offered. While the average heat transfer coefficient of the refrigerant side necessary for the theoretical calculation of the dynamic characteristics is given by calculation method for the tube length and pressure drop of air-cooled condenser.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 추계학술대회
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• pp.717-722
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• 2003

The aspect ratio is the main parameter which governs the outer flow pattern and nozzle performance. And in this study, some flow characteristics with the variation of nozzle aspect ratios such as mean pressure distributions along the center line of the outer flow, flow coefficients and the diffusion angles have been experimentally investigated. Through the experimental analysis, the higher aspect ratio was known to decrease the jet kinetic energy because of the friction losses at the outer of nozzle. As the result, it is found that the nozzle performance depends mainly on the aspect ratio of nozzle.

• 청정기술
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• 제14권1호
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• pp.53-60
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• 2008

본 연구에서는 내경이 각각 5.80 mm와 10.07 mm인 두 개의 수평 이중관열교환기 내에서의 탄화수소계 냉매의 응축열전달과 압력강하에 대한 실험을 $35.5{\sim}210.4\;kg/m^2\;s$의 질량유속과 응축온도인 $40^{\circ}C$에서 수행하였다. 탄화수소계 냉매 R-290과 R-600a의 응축열전달계수가 프레온계 냉매인 R-22보다 높았다. 응축압력강하는 R-600a > R-290 > R-22 순으로 나타났고, 내경이 5.8 mm인 관 내 보다 10.07 mm인 관 내의 압력강하가 R-600a에서는 $6{\sim}15%$ 정도 높았으며, R-290과 R-22에서는 각각 $9.8{\sim}12.5%$와 $2.1{\sim}4.6%$ 정도 높게 나타났다. 실험으로 구한 응축열전달계수와 종래의 상관관계식을 비교해 본 결과, 모든 관경에 대해 Haraguchi 등의 상관관계식과 가장 좋은 일치를 보였다.

• 대한기계학회논문집B
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• 제36권4호
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• pp.397-404
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• 2012

디젤엔진 SCR 시스템 내에서 $NO_x$를 저감을 위한 베인 타입 스태틱 믹서의 혼합유동특성을 수치적으로 연구하였다. 믹서는 원형관의 입구로부터 유동방향으로 직경의 57배 떨어진 구간에 설치하였다. 베인이 유동 축과 이루는 각과 베인의 크기 그리고 위치의 변화에 따른 유동 및 혼합특성을 고찰하였다. 원형관내에서 믹서를 통과하는 유동의 특성은 UI, 선회비, 그리고 압력계수와 같은 특성화된 성능지수로 나타내었다. 해석결과 성능지수들은 베인 각과 차단비, 베인 위치와 같은 기하학적 변수에 영향을 받음을 확인하였다. 특히, 베인 각, 베인 크기가 커지거나 원형관내의 벽면에 가까이 설치될수록 선회비는 증가하는 것을 확인하였다.

• 한국지열·수열에너지학회논문집
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• 제18권3호
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• pp.7-18
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• 2022

Secondary batteries used in electric vehicles have a potential risk of ignition and explosion. Various safety measures are being taken to prevent these risks. A numerical study was performed using a computational fluid dynamics code on the cases where pressure relief vents that can reduce the blast overpressures of batteries were installed in the through-compression test room, short-circuit drop test room, combustion test room, and immersion test room in facilities rleated to battery used in electric vehicles. This study was conducted using the weight of TNT equivalent to the energy release from the battery, where the the thermal runaway energy was set to 324,000 kJ for the capacity of the lithium-ion battery was 90 kWh and the state of charge (SOC) of the battery of 100%. The explosion energy of TNT (△H_TNT) generally has a range of 4,437 to 4,765 kJ/kg, and a value of 4,500 kJ/kg was thus used in this study. The dimensionless explosion efficiency coefficient was defined as 15% assuming the most unfavorable condition, and the TNT equivalent mass was calculated to be 11 kg. The internal explosion generated in a test room shows the very complex propagation behavior of blast waves. The shock wave generated after the explosion creates reflected shock waves on all inner surfaces. If the internally reflected shock waves are not effectively released to the outside, the overpressures inside are increased or maintained due to the continuous reflection and superposition from the inside for a long time. Blast simulations for internal explosion targeting four test rooms with pressure relief vents installed were herein conducted. It was found that that the maximum blast overpressure of 34.69 bar occurred on the rear wall of the immersion test room, and the smallest blast overpressure was calculated to be 3.58 bar on the side wall of the short-circuit drop test room.