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열교환기의 헤더 내 이상유동의 분배에 관한 수치적 연구
최치웅(Chiwoong Choi),이장호(Jangho Lee),김무환(Moo Hwan Kim),조남수(Namsoo Cho),이장석(Jangsuk Lee),박진구(Jinkoo Park) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.5
Two phase flow distribution is numerically analyzed in the evaporator header having multi-channels. In the numerical calculation, two types of two-phase model such as homogeneous model and VOF model are employed. To have the better design of the header, several different geometries are numerically tested in the view point of the flow distribution into channels. The different geometries include the inlet tube position into the header and the width of header. In this study, the mal-distribution number, Md, is newly defined to estimate the averaged level of the flow distribution in the whole passes of the evaporator. As results, two phase flow in the header can be visualized using post-processing of numerical results. Furthermore, the optimum position of the inlet tube into the header and the width of header can be proposed for the better distribution of refrigerant(R-134a) flow.
새로운 사각 유리 마이크로채널의 제작 방법을 이용한 강제비등에서 표면 적심 영향에 대한 연구
최치웅(Chiwoong Choi),김무환(MooHwan Kim) 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.5
The understanding of boiling phenomena in microchannel is not achieved yet and some disagreements of results still exist. There are many effective parameters in micro-system: for example, channel shape, roughness, material and so on. The one of the possibilities is wettability in surface of microchannel due to interfacial force has more effective role in microscale. In our study, the new fabrication technique of glass rectangular microchannel is developed. And self-aligned bonding technique for glass to glass is developed. The glass was used for hydrophilic surface and SAM (self-assembled monolayer) is treated with OTS (octadecyltrichlorosilane) on bare glass to obtain hydrophobic surface. The experiment of boiling heat transfer in microchannel using two different wettability surfaces was conducted. The flow visualization is carried out using long-distance microscape with high-speed camera. The flow regimes for different wettability surface are obviously different. And new flow regimes are observed in hydrophobic surface, which is similar to multi-droplets. The two-phase pressure drop is increased and heat transfer enhanced in hydrophobic surface. This result is partially different from previous results for wettability effect. The quantitative extension of experiment is needed to understand wettability effect on flow boiling in microchannel.
소수성 미세관 내의 비윤활 기포들 사이의 액체 슬러그에 대한 수치적 연구
최치웅(ChiWoong Choi),박수청(SooChung Park),김무환(MooHwan Kim) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.11
Usually, bubbles in capillary was surrounded by thin liquid film, however, bubbles in a hydrophobic capillary tube has no liquid film. The bubble motion in the hydrophobic capillary was not well understood. In this study, numerical analysis of the liquid slug flow between bubbles in hydrophobic capillaries was conducted using a commercial CFD solver, FLUENT 6.5. The diameters were 0.5, 1.0, 1.5 mm. liquid slug velocities were from 0.01 ? 0.1 m/s. Lengths of the liquid slug were 3D, 5D, 7D and 9D. Flow circulation was observed in liquid slug between bubbles. The length of region satisfying theory of single phase pressure drop, which called effective liquid slug length, is smaller than visualized region owing to radial flow components in the bubble nose and tail regions. And this region is reduced as increasing diameter and as increasing velocity. Finally, the effective length of the liquid slug was correlated with diameter, length of liquid slug and liquid slug velocity.
마이크로 채널을 포함한 마이크로 구조물의 복합 열전달에서 전도 영향에 대한 수치적 연구
최치웅(ChiWoong Choi),김수재(Soojae Kim),김무환(MooHwan Kim) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.11
In this study, three dimensional numerical analysis of conjugate heat transfer in single rectangular microchannel with different conductivities was carried out using FLUENT 6.3 with UDF (user defined function) of natural convection. Conductivities used in this study were 8, 0.8, 0.08, and 0.008. Heat flux was 200 ㎾/㎥ and mass fluxes were 50 ? 400 ㎏/㎡s. Heat loss was evaluated and heat loss evaluation method was verified. The heat loss evaluation method was applicable in microchannel with lower conductivity. However, higher conductive microchannel has different heat distribution. Therefore, the heat loss evaluation method was modified with different conductivity. The heat flow distribution was analyzed and effective area coefficient was proposed to represent heat flow in rectangular microchannel. As the conductivity of microchannel decreased, the heat flow through another walls except heating wall increased.
마이크로채널 내의 비등 열전달에 대한 3 구간모델의 비교
최치웅(Chiwoong Choi),허철(Cheol Huh),김무환(Moohwan Kim) 대한기계학회 2006 대한기계학회 춘추학술대회 Vol.2006 No.11
Many researchers conducted flow boiling experiments in minichannel and multi microchannel. Resent results reveal that the elongated bubbles play on important role in flow boiling heat transfer in mini- and microchannels. The elongated bubble is like long slug bubble and grows in length rather than in diameter, due to bubble is confined by the channel. In 2004, Thome et al. proposed three-zone model, which explained the heat transfer mechanism in microchannel flow boiling using liquid zone, elongated bubble zone, and dry vapor zone. In our study, experimental investigations were carried out in single 100㎛ hydraulic diameter single microchannel with water to evaluate the validity of the three-zone model. A comparison the experimental data with three-zone model showed that modification of the three-zone model is required, especially for water and microscale diameter. To improve the model, some suggestions are made.
소수성 표면의 미니채널 내에서의 삼중선에 의한 압력강하의 실험적 연구
유동인(Dongin Yu),최치웅(Choi chiwoong),김무환(Moohwan Kim) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.5
Slug flow regime is more broaden than the others in minichannels, comparing with the flow of the conventional channel. Moreover, the flow pattern is different according to wettability of surface in minichannels. Generally, bubbles at the slug flow regime are lubricated by liquid film in a hydrophilic surface of mini and micro-channel. However, bubbles are not lubricated by liquid film in a hydrophobic surface of mini and micro-channel. Instead, bubbles with triple-lines are developed. And Non-lubricated bubbles made higher pressure drop than lubricated bubbles due to the triple-line moving. To understand the pressure drop of motion of triple-lines of non-lubricated bubble, we studied the dynamic contact angle model and experimented of the pressure drop of triple lines of single droplet. In the experiment, we measured the pressure drop difference between single droplet(Nitrogen)-liquid(DI water) flow and only single Gas flow in the various diameter Teflon tube(1.6, 1.0㎜). Consequently, we could measure the pressure drop of triple lines of single droplet and analyzed the pressure drop relation with channel diameter, droplet length and velocity.
다공성 매질 개념을 이용한 응축기의 응축 열전달에 관한 수치해석
제준호(Junho Je),최치웅(Chiwoong Choi),김무환(Moohwan Kim) 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.5
In this study, the numerical analysis to estimate condensation heat and mass transfer of the condenser was carried out using the PMA (porous medium approach). In the PMA, the details of tube bundle in the condenser are replaced by the porous medium, and the flow resistance term is added in the momentum equation. In this regard, the PMA is quite helpful for the study of tube bundle in the large condenser. The pressure loss through tube bundle can be compensated by viscous and inertial momentum sink terms, which was validated numerically. Value of the pressure drop was compared to that of Butterworth correlation. Three dimensional analysis of condensation for McAllister condenser with the PMA was conducted using Fluent 6.2 and UDFs (use-defined functions). The result of condensation rate was analogous to previous results (experimental and numerical data).
〈학술논문〉 다양한 유체 및 관 직경에 따른 미니 마이크로 채널 내에서의 삼중선에 의한 압력강하의 실험적 연구
유동인(Dongin Yu),최치웅(Chiwoong Choi),김무환(Moohwan Kim) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.11
Slug flow regime is more broaden than the others in mini-microchannels comparing with the flow of the conventional channel. According to wettability of surface in mini-microchannels, the flow pattern is different. Bubbles at the slug flow regime are lubricated by liquid film in a hydrophilic surface of mini-microchannel. However, bubbles are not lubricated by liquid film in a hydrophobic surface of mini-microchannel. Thus, bubbles with triple-lines are developed. And the non-lubricated bubbles caused higher pressure drop than the lubricated bubbles due to the triple-lines moving. In this research, the experiments were conducted to understand the pressure drop of the motion of the triple-lines in the hydrophobic mini-microchannels. The pressure drop of N₂ gas and water liquid slug flow is measured in the various diameter Teflon tubes (1.5, 1.0, 0.5㎜) and superficial velocity region (0.02m/s~0.4m/s). Additionally, to understand effect of the property (surface tension, viscosity) at the pressure drop of triple-lines, various DI water-ethanol mixture (pure, 1, 5, 10%) liquid slug experiment were conducted.
고든형 열유속 센서의 민감도 관련 인자에 대한 수치 해석적 연구
제준호(Junho Je),김형대(Hyungdae Kim),최치웅(Chiwoong Choi),김무환(Moohwan Kim),강요섭(Yoseob Kang) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.11
The sensitivity of Gardon gage type HFS(heat flux sensor) is affected by some factors that are changeable by fabrication. We investigated effects of parameters on HFS using the numerical analysis. Fluent 6.2 was used for the numerical simulation. The sensitivity and thermal characteristics of the sensor corresponding to four parameters such as constantan foil thickness, copper wire diameter, copper wire position, and coolant velocity were analyzed. The results showed that the change of constantan foil thickness caused a dominant effect on sensitivity of the Gardon gage type HFS, however the others made relatively nominal effects.