사각 마이크로채널 내의 2 상 유동 압력강하 상관식의 검증 및 개발

최치웅(Chi-woong Choi),유동인(Dong In Yu),김무환(Moo Hwan Kim) 대한기계학회 2010 大韓機械學會論文集B Vol.34 No.1

2 상 유동 압력강하에 대한 모델은 균질유동모델과 분리유동모델 두 가지가 있다. 많은 선행 연구자들은 마이크로채널에서의 2 상 유동 압력강하에 대한 상관식을 제시하였고, 대부분은 분리유동모델에 해당하는 Lockhart-Martinelli<SUP>(27)</SUP>의 수정된 상관식에 기초하고 있다. 본 연구에서는 사각 마이크로채널에서의 압력강하에 대한 연구를 위해서 액상의 물과 기상의 질소를 사용하여 사각 마이크로채널에서의 실험을 수행하였다. 2 상 마찰 압력강하는 2 상 유동양식에 큰 연관성을 가지고 있는 결과를 확인할 수 있었다. 6 가지의 2 상 점성 모델을 포함한 균질유동 모델 (Owen<SUP>(21)</SUP>’s, MacAdams<SUP>(22)</SUP>’s, Cicchitti et al.<SUP>(23)</SUP>’s, Dukler et al.,<SUP>(24)</SUP> Beattie and Whalley,<SUP>(25)</SUP> Lin et al.<SUP>(26)</SUP>)과 6 가지의 분리유동 모델 (Lockhart and Martinelli,<SUP>(27)</SUP> Chisholm,<SUP>(31)</SUP> Zhang et al.,<SUP>(15)</SUP> Lee and Lee,<SUP>(5)</SUP> Moriyama and Inue,<SUP>(4)</SUP> Qu and Mudawar<SUP>(8)</SUP>)에 대한 평가를 실험결과와 비교를 통해 수행하였다. 가장 우수한 2 상 점성 모델은 Beattie and Whalley 의 모델이었고, 가장 우수한 분리유동 모델은 Qu and Mudawar 의 상관식이였다. 균질유동모델과 분리유동모델 모두에 대해서 2 상 유동양식에 종속성을 나타내었다. 그러므로, 본 연구에서는 2 상 유동 양식에 기초한 새로운 상관식을 균질유동모델과 분리유동모델에 대해 각각을 제시하였다. There are two kinds of models in two-phase pressured drop; homogeneous flow model and separated flow model. Many previous researchers have developed correlations for two-phase pressure drop in a microchannel. Most correlations were modified Lockhart and Martinelli’s correlation, which was based on the separated flow model. In this study, experiments for adiabatic liquid water and nitrogen gas flow in rectangular microchannels were conducted to investigate two-phase pressure drop in the rectangular microchannels. Two-phase frictional pressure drop in the rectangular microchannels is highly related with flow regime. Homogeneous model with six two-phase viscosity models: Owen<SUP>(21)</SUP>’s, MacAdams<SUP>(22)</SUP>’s, Cicchitti et al.<SUP>(23)</SUP>’s, Dukler et al.<SUP>(24)</SUP>’s, Beattie and Whalley<SUP>(25)</SUP>’s, Lin et al.<SUP>(26)</SUP>’s models and six separated flow models: Lockhart and Martinelli<SUP>(27)</SUP>’s, Chisholm<SUP>(31)</SUP>’s, Zhang et al.<SUP>(15)</SUP>’s, Lee and Lee<SUP>(5)</SUP>’s, Moriyama and Inue<SUP>(4)</SUP>’s, Qu and Mudawar<SUP>(8)</SUP>’s models were assessed with our experimental data. The best two-phase viscosity model is Beattie and Whalley’s model. The best separated flow model is Qu and Mudawar’s correlation. Flow regime dependency in both homogeneous and separated flow models was observed. Therefore, new flow pattern based correlations for both homogeneous and separated flow models were individually proposed.

사각 유리 마이크로채널에서의 기포에 의한 압력 강하에 관한 연구

최치웅(Chi Woong Choi),유동인(DongIn Yu),김무환(MooHwan Kim) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.5

Major two-phase flow regime in a microchannel is an elongated bubble. This flow pattern is similar to long slug flow in a macroscale tube. In this study, therefore, mechanical analysis was conducted based on flow pattern. Experiments with water and nitrogen gas two-phase flow in glass rectangular microchannels were conducted. Two microchannels with different aspect ratio were used and their hydraulic diameters were approximately 503 and 332 ㎛. Ranges of superficial velocities of water and nitrogen gas are 0.06 - 0.6 ㎧ and 0.06 - 1 ㎧, respectively. In addition, visualization of two-phase flow was conducted using high-speed camera and long distance microscope. And pressure drop by a single bubble motion was evaluated using a unit cell model. The pressure drop in single bubble was proportional to Capillary number and inverse proportional to hydraulic diameter. Finally, empirical correlation for single elongated bubble was proposed.

MEMS로 제작된 마이크로 채널에서의 3차원 열전달 해석

최치웅(Chi-woong Choi),허철(Cheol Huh),김동억(Dong-eok Kim),김무환(Moo-Hwan Kim) 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.5

The microchannel heat sink is promising heat dissipation method for high heat flux source. Contrary to conventional circular channel, MEMS based microchannel had rectangular or trapezoidal cross-sectional shape. In our study, we conducted three dimensional conjugate heat transfer calculation for rectangular shape microchannel. First, we simulated that channel was completely drained with known heating power. As a result we obtained calibration line, which indicates heat loss was function of temperature. Second, we simulated single phase heat transfer with various mass flux, 100-400 ㎏/㎡s. In conclusion, the single phase test verified that the present heat loss evaluation method is applicable to micro scale heat transfer devices. Heat fluxes from each side wall shows difference due to non-uniform heating. However those ratios were correlated with supplied total heat. Finally, we proposed effective area correction factor to evaluate appropriate heat flux.

사각 마이크로채널 내의 2 상유동 압력강하와 유동양식에 대한 젖음성의 영향에 대한 연구

최치웅(Chi-woong CHOI),유동인(Dong In Yu),김무환(Moo Hwan Kim) 대한기계학회 2009 大韓機械學會論文集B Vol.33 No.12

Wettability is a critical parameter in micro-scale two-phase system. Several previous results indicate that wettability has influential affect on two-phase flow pattern in a microchannel. However, previous studies conducted using circular microtube, which was made by conventional fabrication techniques. Although most applications for micro thermal hydraulic system has used a rectangular microchannel, data for the rectangular microchannel is totally lack. In this study, a hydrophilic rectangular microchannel was fabricated using a photosensitive glass. And a hydrophobic rectangular microchannel was prepared using silanization of glass surfaces with OTS (octa-dethyl-trichloro-siliane). Experiments of two-phase flow in the hydrophilic and the hydrophobic rectangular microchannels were conducted using water and nitrogen gas. Visualization of twophase flow pattern was carried out using a high-speed camera and a long distance microscope. Visualization results show that the wettability was important for two-phase flow pattern in rectangular microchannel. In addition, two-phase frictional pressure drop was highly related with flow patterns. Finally, Two-phase frictional pressure drop was analyzed with flow patterns.

다른 세장비의 사각 마이크로채널 내의 기포 거동에 관한 연구

최치웅(Chi-woong CHOI),유동인(Dong In Yu),김무환(Moo Hwan Kim) 대한기계학회 2010 大韓機械學會論文集B Vol.34 No.5

다른 세장비에 따른 단일 사각 마이크로 채널 내의 이상유동연구를 수행하였다. 본 연구에서는 대략 넓이가 500 ㎛ 이며 수력직경이 각각 490, 322, 143 ㎛ 인 사각 마이크로채널 내에서의 물-질소 유동에 대한 실험이 수행되었다. 또한, 고속카메라와 장거리 현미경을 통해 이상유동양식을 가시화하였다. 본 연구는 이상유동 중 기포류에 중점을 두었으며 가시화 결과를 통해 기포의 속도, 기포의 길이, 관 내 기포의 개수, 기공률을 산출하였고 단위 셀 모델을 기반으로 늘어진 단일 기포의 압력강하를 해석하였다. 실험을 통해 기포의 속도, 기공률, 단일 기포의 압력강하가 각각 겉보기 속도와 체적건도, 세장비와 연관이 있음을 확인하였으며, 사각 마이크로 채널 내 늘어진 단일 기포의 압력강하에 대한 상관식을 개발하였다. The adiabatic two-phase flow in single rectangular microchannels was studied for different aspect ratios. The working fluids were liquid water and nitrogen gas. The hydraulic diameters of the rectangular microchannels were 490, 322, and 143 ㎛, and the widths of the microchannels were around 500 ㎛. The two-phase flow pattern was visualized using a high-speed camera and a long-distance microscope. This study was focused on bubble flow regimes. From the visualized images, the bubble velocity, bubble length, number of bubbles, and void fraction were evaluated. Further, the pressure drop in a single bubble was evaluated by using a unit cell model. The bubble velocity is proportional to the superficial velocity. Further, the relationship between the void fraction and the volumetric quality is linear. The pressure drop in a single elongated bubble is strongly related to the aspect ratio. Finally, the new correlation about the pressure drop of a single elongated bubble in the rectangular microchannel was proposed.

수치 해석을 이용한 단일 마이크로채널의 단면 가열 조건의 열전달 특성에 관한 연구

최치웅(Chi-woong Choi),허철(Cheol Huh),김동억(Dongeok Kim),김무환(Moo Hwan Kim) 대한기계학회 2007 大韓機械學會論文集B Vol.31 No.12

The microchannel heat sink is promising heat dissipation method for high density electronic devices. The cross-sectional shape of MEMS based microchannel heat sink is limited to triangular, trapezoidal, and rectangular due to their fabrication method. And heat is added to one side surface of heat source. Therefore, those specific conditions make some complexity of heat transfer in microchannel heat sink. Though many previous research of conjugate heat transfer in microchannel was conducted, most of them did not consider heat loss. In this study, numerical investigation of conjugate heat transfer in rectangular microchannel was conducted. The method of heat loss evaluation was verified numerically. Heat distribution was different for each wall of rectangular microchannel due to thermal conductivity and distance from heat source. However, the ratio of heat from each channel wall was correlated. Therefore, the effective area correction factor could be proposed to evaluate accurate heat flux in one side heating condition.

응축기의 비응축 가스 배출 타입에 따른 틈 간격의 영향에 대한 수치적 연구

제준호(Junho Je),최치웅(Chi-woong Choi),김수재(Soo Jea Kim),김무환(Moo Hwan Kim) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.5

The numerical study to estimate performance for gap size effect for non-condensable gases ventilation type in condensers was carried out using the porous medium approach (PMA). In the PMA, the details of tube bundle in the condenser are considered as 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 huge condenser. Three dimensional analysis of condensation for condenser with the PMA was conducted using Fluent and user-defined functions (UDFs). Gap size effect on condensation was negligible in pure steam condition. However, gap size effect was dominant in condensation with non-condensable gas and external venting. As gap size was decreased, condensation rate was increased for non-condensable gas and external venting system.

응축기의 비응축 가스 배출 타입에 따른 질량 확산율의 영향에 대한 수치적 연구

제준호(Junho Je),최치웅(Chi-woong Choi),김수재(Soo Jea Kim),김무환(Moo Hwan Kim) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.11

The numerical study to estimate performance for mass diffusivity effect for non-condensable gases ventilation systems in condensers was carried out using the porous medium approach (PMA). In the PMA, the details of tube bundle in the condenser are considered as 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 huge condenser. Three dimensional analysis of condensation for condenser with the PMA was conducted using Fluent and user-defined functions (UDFs). The mass diffusivity effect on condensation was small in internal vent system. However, the mass diffusivity effect was dominant in condensation for external venting system. As the ratio of turbulent viscosity to fluid viscosity was increased, mass fraction of non-condensable gas was decreased. Therefore, total condensation rate was increased for external venting system.

고로 Stave 열전달 해석 및 관리방안

이건호(Kun Ho Lee),최치웅(Chi Woong Choi),김무환(Moo Hwan Kim) 대한기계학회 2005 대한기계학회 춘추학술대회 Vol.2005 No.11

The blast furnace is used for iron making from ore and coke and it needs cooling system for furnace body. Stave which is made by cast iron is a type of blast furnace cooling system and designed according to the heat load of furnace. Wearing of stave surface is continued because of the friction with materials in furnace and accelerated rapidly over certain temperature. In this study, the cooling system of stave are numerically investigated with three parameters; wear thickness and the change of gas temperature. Fluent 6.2 is used for simulation. So, flow and thermal characteristics of cooling water for each wear thickness and gas temperature are obtained. With the results, acceptable method for cooling system management is studied to prolong the life of blast furnace by decreasing of stave wear.

응축기의 비응축 가스 배출 타입에 따른 틈 간격 비율의 영향에 대한 수치적 연구

제준호(Junho Je),김수재(Soo Jea Kim),최치웅(Chi-woong Choi),김무환(Moo Hwan Kim) 대한기계학회 2012 大韓機械學會論文集B Vol.36 No.1

본 논문은 응축기의 비 응축 가스 배출 타입에 따른 틈 간격의 비율이 응축기의 성능에 미치는 영향에 관하여 다공성 매질 개념을 적용한 수치적 연구에 관한 것이다. 다공성 매질의 개념을 이용한 응축기의 성능 해석에서는 응축기기 내부의 다관군을 다공성 매질로 간주하며, 다관군에 의한 압력강하는 상관식으로 반영한다. 상용수치해석 프로그램인 Fluent 와 user-defined functions 를 이용하여 McAllister 응축기에 다공성 매질 개념을 적용하여 3 차원 응축량을 해석하였다. 순수증기의 해석에서는 틈 간격이 응축량에 미치는 영향이 거의 없었다. 그러나 비 응축가스가 포함되어 있으며, 외부 배출의 경우 틈 간격은 응축량에 매우 큰 영향을 미쳤는데, 틈 간격이 줄어듦에 따라 응축량이 매우 증가하는 결과를 얻었다. A numerical analysis was carried out to estimate the effect of the gap size ratio on the performance of condensers under noncondensable gas ventilation using the porous medium approach (PMA). In the PMA, the details of the tube bundle in the condenser are considered to be those of a porous medium, and the flow resistance term is added in the momentum equation. Three-dimensional analysis of the condensation for a McAllister condenser was conducted with the PMA using Fluent and user-defined functions (UDFs). The gap size effect on the condensation was negligible under pure steam conditions. However, the gap size effect was dominant in condensation with noncondensable gas and external venting. As the gap size decreased, the condensation rate increased for noncondensable gas in an external venting system.