• 대한기계학회논문집
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• 제8권1호
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• pp.18-25
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• 1984

The transition boiling of a liquid droplet on a heated flat surface was studied utilizing Kotake's model with the effects of viscosity of a thin vapor layer between the droplet and the hot plate taken into account. This problem was analyzed considering the process of the droplet evaporation which resulted in hydrodynamic instability at the liquid-vapor interface. The results of the study are as follows; (1)The effect of the viscosity in the vapor layer at the interface appears as a dimensionless number N, namely .sigma. .delta.$_{0}$ /.rho.nu.$^{2}$ (2)The time required for evaporation at the transitional region increases with the temperature difference ratio .DELTA. T$_{r}$. The rate of increase of the total evaporation time becomes larger as increasing of N$_{m}$(N number at maximum heat flux) increases.s.

• 한국추진공학회지
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• 제3권1호
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• pp.41-47
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• 1999

고체 추진제의 연소율(burning rate)은 연소의 동적 거동과 추진제의 성능을 판단할 수 있는 중요한 변수이다. 특히 AP계의 고체추진제 표면에서는 발열반응인 분해반응(decomposition) 이외에도 기체로 증발되는 증발반응(evaporation or sublimation)이 존재하므로 이를 고려한 연소 반응율의 해석은 매우 중요한 의미를 갖는다. 본 연구에서는 분해반응과 증발반응이 존재하며 외부로부터 고체추진제 표면으로 입사하는 복사열전달이 있는 경우, 응축영역에서 에너지 방정식과 화학 종 보존식을 사용하여 정상상태의 연소반응율에 관한 이론 해석을 수행하였다.

• 대한기계학회논문집B
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• 제39권9호
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• pp.761-772
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• 2015

가정용 에어컨이나 히트 펌프에 외경 7.0 mm 마이크로핀 튜브가 널리 사용되고 있다. 한편 에어컨이나 히트펌프의 부분 부하 운전시 질량유속은 수십 $kg/m^2s$에 불과하다. 하지만 7.0 mm 튜브에 대한 기존 연구들은 질량유속 $100kg/m^2s$ 이상에서 수행되었다. 본 연구에서는 낮은 질량유속 ($50kg/m^2s$에서 $250kg/m^2s$)에서 외경 7.0 mm 마이크로핀 튜브 내 R-410A 증발 열전달 실험을 수행하였다. 실험 중 포화온도는 $8^{\circ}C$, 열유속 $4.0kW/m^2$으로 유지하였다. 비교를 위해 외경 7.0mm 평활관에 대한 실험도 수행하였다. 실험결과 마이크로핀 튜브의 전열촉진비는 질량유속이 감소할수록 증가하다 $150kg/m^2s$를 기점으로 감소함을 보였다. 이는 마이크로 핀튜브 내 유동이 환상류에서 성층류로 변화하기 때문이다. 실험 범위에서 마이크로핀 튜브의 마찰손실과 평활관의 마찰손실은 거의 같게 나타났다. 실험데이터를 기존 상관식의 예측치와 비교하였다.

• Journal of Mechanical Science and Technology
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• 제19권12호
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• pp.2296-2302
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• 2005

This study is focused on the comparison of heat transfer performance of two thermosyphons having 60 straight and helical internal grooves. Distilled water has been used as working fluid. Liquid fill charge ratio defined by the ratio of working fluid volume to total internal volume of thermosyphon, the inclination angle and operating temperature were used as experimental parameters. The heat flux and heat transfer coefficient are estimated from experimental results. The conclusions of this study may be summarized as follows; Liquid fill charge ratio, inclination angle and geometric shape of grooves were very important factors for the operation of thermosyphon. The optimum liquid fill charge ratio for the best heat flux were $30\%$. The heat transfer performance of helically grooved tube was higher than that of straight grooved tube in low inclination angle (less than $30^{\circ}$), but the results were opposite in high inclination angle (more than $30^{\circ}$). As far as optimum inclination angle concerns, range of $25^{\circ}\~30^{\circ}$ for a helically grooved tube and about $40^{\circ}$ for a straight grooved tube are suggested angles for the best results.

• 설비공학논문집
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• 제18권12호
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• pp.971-976
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• 2006

The objectives of this paper are to study the characteristics of pool boiling heat transfer for enhanced tubes used in the evaporator of turbo chiller and to provide a guideline for optimum design of an evaporator using HFC134a. Three different enhanced tubes are tested at 4 different saturation temperatures. The wall super heated temperature difference ranges from $0.5^{\circ}C\;to\;3.5^{\circ}C$. The refrigerant, HFC134a evaporates on the outside of the tube while the chilled water flows inside the tube. This study provides experimental heat transfer coefficients for evaporation on the enhanced tubes. It is found that the turbo-II tube provides the highest heat transfer coefficient.

• Journal of applied mathematics & informatics
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• 제9권2호
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• pp.731-744
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• 2002

The temperature distribution in human skin and subdermal tissue layer is presented using bioheat transfer equation. The body temperature is determined by the balance between heat produced and heat lost by our body. The time-dependent solutions have been found to be affected by the metabolic heat generation rate, blood mass flow, the rate of evaporation of perspiration and also by the atmospheric temperature. The analytic solutions for different layers have been calculated numerically and are also shown graphically.

• 동력기계공학회지
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• 제17권2호
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• pp.70-77
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• 2013

An experimental study was carried out to understand the heat transfer performance of a miniature loop heat pipes using water-based copper nanoparticles suspensions as the working fluid. The suspensions consisted of deionized water and copper nanoparticles with an average diameter of 80 nm. Effects of the cupper mass concentration and the operation pressure on the average evaporation and condensation heat transfer coefficients, the critical heat flux and the total heat resistance of the mLHPs were investigated and discussed. The pressure frequency also depends upon the evaporator temperature which has been maintained from $60^{\circ}C$ to $90^{\circ}C$. The Investigation shows 60% filling ratio gives the highest inside pressure magnitude of highest number pressure frequency at any of setting of evaporator temperature and 5wt% results the lowest heat flow resistance.

• 대한기계학회논문집B
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• 제25권11호
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• pp.1492-1499
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• 2001

In order to develop the compact and flexible heat exchangers, we made the helically coiled heat exchangers. They can be manufactured with small diameter copper tubes without the need for fins; inner diameter=1.0 mm, straight tube length=1.5 m. The experiments were carried out with the following conditions; evaporation pressure=0.6 MPa, air velocity=0.7 ∼ 1.7 m/s, and working fluid=R-22. Pressure drop and heat transfer coefficient of heat exchangers were experimented according to the air velocity. The results of heat transfer coefficient show a 35% beneficial increase fur these heat exchangers over the other covered fin-tube heat exchangers. A cooling capacity of about 3 kW was obtained with an air velocity of 1.5 m/s. The distribution header has also been designed fur efficient distribution of refrigerant flow.

• 한국분무공학회지
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• 제25권2호
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• pp.68-73
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• 2020

In the present study, experiments on the film boiling of liquid droplets on oxidized copper surface was conducted. The shape of pure water droplets was observed, and the evaporation rate of them was measured during the film boiling evaporation process. The droplet of initial volume 16 ~ 30 µl was applied onto the oxidized copper surface heated up to 300 ~ 500℃, then the shape of the droplet was analyzed during the film boiling evaporation. Experimental results showed that there was good correlation between dimensionless volume and dimensionless time. However, a significant difference in evaporation rate for small and large droplets discussed in previous study was not found.

• 대한설비공학회:학술대회논문집
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• 대한설비공학회 2005년도 동계학술발표대회 논문집
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• pp.335-340
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• 2005

Nonlinear and forced oscillations of supported viscous droplet were focused in the present study. The droplet has a free contact line with solid plate and inviscid fluid. Natural frequencies of a pendant droplet have been investigated experimentally by imposing the acoustic wave while the frequency is being increased at a fixed amplitude. The evaporation was observed at atmosphere pressure. The droplet was recorded throughout the entire evaporation process and transient variations of the volume was measured. The evaporation process of oscillating droplet with thermofoil has been also observed to investigate analyzing the resonance effect on the thermal characteristics of droplet. It is found that a pendant droplet shows the resonant behaviors at each mode similar to the theoretical analysis. During imposing the acoustic wave, the pendant droplet makes a rotating motion in its longitudinal axis which is a new shape oscillation mode. The evaporation rate of a pendant droplet at resonant frequency is significantly enhanced.