미세관내 NH₃의 압력강하 및 비등열전달

최광일(Kwang-Il Choi),M. Rifaldi,오종택(Jong-Taek Oh),Kiyoshi Saito,정종수(Jong Soo Jeong) 대한설비공학회 2010 대한설비공학회 학술발표대회논문집 Vol.2010 No.11

An experimental study for convective boiling heat transfer was conducted with NH₃ in horizontal smooth minichannels. The test section is made of stainless steel tubes with inner diameters of 1.5 ㎜ and 3.0 ㎜, the length 2000 ㎜ each. The minichannels are uniformly heated by engaging an electric current directly to the test section; all components are well insulated to prevent heat losses. Pressure drop and local heat transfer coefficients were obtained for a heat flux range of 10-80 ㎾/㎡, a mass flux range of 50-800 ㎏/㎡s, saturation temperatures of 0, 5, and 10℃ and quality ranges of up to 1.0. Nucleate boiling heat transfer contribution was predominant, especially at low quality region. The effects of mass flux, heat flux, and inner tube diameter, on pressure drop and heat transfer coefficient are presented. The experimental results are compared against several existing correlations. A new boiling heat transfer coefficient correlation for NH3 was developed with 9.79% MD and -3.57% AD.

Boiling Heat Transfer of Propane in Horizontal Smooth Minichannels

Kwang-Il Choi,A.S. Pamitran,Jong-Taek Oh 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.5

This study examined two-phase flow boiling pressure drop and heat transfer for propane in horizontal minichannels. The pressure drop and local heat transfer coefficients were obtained for heat fluxes ranging from 5-20 ㎾/㎡, mass fluxes ranging from 50-400 ㎏/㎡s, saturation temperatures of 10, 5, and 0℃, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 1.5 ㎜ and 3.0 ㎜, and lengths of 1000 ㎜ and 2000 ㎜, respectively. The present study showed the effect of mass flux, heat flux, inner tube diameter, and saturation temperature on pressure drop and heat transfer coefficient. The experimental results were compared against several existing pressure drop and heat transfer coefficient prediction methods. Modifications of pressure drop and boiling heat transfer coefficient correlations for propane in minichannels were developed.

미세관에서 냉매의 상변화 압력강하 상관식 평가

윤정훈(Jung Hoon Yun),정지환(Ji Hwan Jeong) 대한설비공학회 2015 대한설비공학회 학술발표대회논문집 Vol.2015 No.11

Two-phase cooling strategy is efficient method to deliver the heat. Many cooling applications have been using this to meet the demands for high thermal performance and compact designs. Small flow channels are used in compact heat exchangers. Thermal system design of such equipment requires understanding about two-phase pressure drop to improve the performance of the system. There are various approach method to predict the two-phase frictional pressure drop and that can be classified into four types. There are HEM, SFM, direct empirical correlation and flow pattern specific correlation. In order to evaluate the accuracy of existing models and correlations, a broad range of experimental data is collected. The database consist of 933 and 1304 frictional pressure drop data for condensing and boiling flow, respectively, from 14 sources. Among the viscosity models, Cicchitti et al’s viscosity model provides the best predictions. In general, Muller-Steinhagen and Heck’s correlation provide good predictions for both conventional channels and minichannels.

Evaporating Heat Transfer Characteristics of R-134a in a Horizontal Smooth Channel

A. S. Pamitran,Kwang-Il Choi,Jong-Taek Oh,Hoo-Kyu Oh 대한설비공학회 2006 International Journal Of Air-Conditioning and Refr Vol.14 No.4

Convective boiling heat transfer coefficients were measured in a horizontal minichannel with R-l34a. The test section was made of stainless steel tube with an inner diameter of 3.0 ㎜ and a length of 2 m. It was uniformly heated by applying electric current directly to the tube. Local heat transfer coefficients were obtained for heat fluxes from 10 to 40 ㎾/㎡, mass fluxes from 200 to 600㎏/㎡s, qualities up to 1.0, and the inlet saturation temperature of 10℃. The experimental results were mapped on Wojtan et al.'s(7) and Wang et al.'s(8) flow pattern maps. The nucleate boiling was predominant at low vapor quality whereas the convective boiling was predominant at high vapor quality. Laminar flow appeared in the flow with minichannel. The experimental results were compared with six existing two-phase heat transfer coefficient correlations. A new boiling heat transfer coefficient correlation based on the superposition model for refrigerants was developed with mean and average deviations of 10.39% and -3.66%, respectively.

소수성 미니채널 내 물-에탄올 혼합 액적 슬러그 거동에 따른 접촉선에서의 압력강하에 대한 실험적 연구

서경덕,Narayan Pandurang Sapkal,유동인 한국기계기술학회 2019 한국기계기술학회지 Vol.21 No.4

At a two phase flow, according to gas and liquid phase flow rates, various flow regimes are developed such as bubbly, slug/plug, churn, annular, droplet flow and so on. At a two phase flow in small scaled channels, among various flow regimes, the intermittent flows such as bubbly, slug/plug flow are developed in the broad regions of two phase flow pattern map. In particular, the flow regimes are influenced by surface wettability. In a case of slug flow in hydrophobic small scaled channels, gas and liquid phases are perfectively separated by interfaces and contact line. The pressure drop of the two-phase flow is largely generated at moving contact line. Therefore, to well design two-phase flow system with small scales, it is important to estimate the pressure drop at moving contact line. In this study, on the basis of previous research, the pressure drop at moving contact line is experimentally measured for a various fluids (0-40% water-ethanol mixtures). And, the previous correlation to estimate pressure drop at moving contact line is verified by experimental data. In an addition, we discuss interfacial broken phenomena of slug flow in a minichannel. (D=1.555mm).

Evaporating Heat Transfer Characteristics of R-l34a in a Horizontal Smooth Channel

Pamitran, A.S.,Choi, Kwang-Il,Oh, Jong-Taek,Oh, Hoo-Kyu The Society of Air-Conditioning and Refrigerating 2006 International Journal Of Air-Conditioning and Refr Vol.14 No.4

Convective boiling heat transfer coefficients were measured in a horizontal minichannel with R-l34a. The test section was made of stainless steel tube with an inner diameter of 3.0 mm and a length of 2m. It was uniformly heated by applying electric current directly to the tube. Local heat transfer coefficients were obtained for heat fluxes from 10 to $40kW/m^2$, mass fluxes from 200 to $600kgT/m^2s$, qualities up to 1.0, and the inlet saturation temperature of $10^{\circ}C$. The experimental results were mapped on Wojtan et $al.'s^(7)$ and Wang et $al.'s^(8)$ flow pattern maps. The nucleate boiling was predominant at low vapor quality whereas the convective boiling was predominant at high vapor quality. Laminar flow appeared in the flow with minichannel. The experimental results were compared with six existing two-phase heat transfer coefficient correlations. A new boiling heat transfer coefficient correlation based on the superposition model for refrigerants was developed with mean and average deviations of 10.39% and -3.66%, respectively.

방사광 X-선 영상법을 이용한 마이크로/미니채널 내 물-에탄올 액적 슬러그 계면 깨짐 현상에 대한 가시화 연구

김도연(DoYeon Kim),백진기(JinGy Baek),한도희(DoHui Han),김보경(BoGyeong Kim),유동인(DongIn Yu) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.11

In various multiphase system, development of recent technologies are often limited by interfacial breakup phenomena. Interfacial phenomena with dynamic conditions has investigated intensively by numerous researchers. However, previous researchs have typically been conducted on dynamic contact angle. In our previous research, it was investigated the interfacial break-up phenomena of a water-ethanol mixture slug flow in a hydrophobic minichannel. However, due to the limitation of the visualization techniques based on light ray, it was difficult to visualize clearly interfacial behavior and shapes. Therefore, in this study, we introduce the synchrotron X-ray imaging with high spatial and temporal resolutions. We confirm the previous flow patterns (steady, loss, separation) in previous research via synchrotron X-ray imaging. On the basis of visualization data, we observe the gas creates/destroys the liquid-gas interface continuously due to the speed differences with the high-speed flow conditions.

Cavitating nozzle flows in micro- and minichannels under the effect of turbulence

Morteza Ghorbani,Mehmet Yildiz,Devrim Gozuacik,Ali Kosar 대한기계학회 2016 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.30 No.6

The cavitation phenomenon inside micro- and minichannel configurations was numerically investigated. The simulations for each channel were performed at different upstream pressures varying from 1 to 15 MPa. Two microchannel configurations with inner diameters of 152 and 254 μm and two minichannel configurations with inner diameters of 504 and 762 μm were simulated. To validate the numerical approach, micro-jet impingement from a microchannel with an inner diameter of 152 μm was first simulated at different Reynolds numbers. Then, the mixture model was used to model the multiphase flow inside the channels. The results of this study present major differences in the cavitating flows between the micro- and miniscale channels and show that the pressure profile and vapor phase distribution exhibit different features. The static pressure drops to negative values (tensile stress) in microchannels, while the minimum static pressure in minichannels is found to be equal to vapor saturation pressure, and higher velocity magnitudes especially at the outlet are visible in the microchannels. It is shown that for higher upstream pressures, the cavitating flow extends over the length of the micro/minichannel, thereby increasing the possibility of collapse at the outlet. The effect of energy associated with turbulence was investigated at high Reynolds numbers for both micro/minichannels and its impact was analyzed using wall shear stress, turbulence kinetic energy and mean velocity at various locations of the micro/minichannels.

미니채널내 암모니아 비등열전달에 미치는 관경의 영향

최광일(Kwang-Il Choi),M. Rifaldi,오종택(Jong-Taek Oh),Kiyoshi Saito,정종수(Jong Soo Jeong) 대한설비공학회 2010 대한설비공학회 학술발표대회논문집 Vol.2010 No.6

Convective boiling heat transfer experimental in horizontal minichannels was conducted with NH₃ as the working refrigerant. The test section is made of stainless steel tubes with inner diameters of 3.0 ㎜ and 1.5 ㎜, the lengths and 2000 ㎜ and 1000 ㎜, respectively. The minichannels are uniformly heated by engaging an electric current directly to the single tubes. The local heat transfer coefficients were obtained over a heat flux range of 20 to 80 ㎾/㎡, a mass flux range of 50 to 500 ㎏/㎡s, a saturation temperature range of 0 to 10℃, and quality up to 1.0. Nucleate boiling heat transfer contribution was predominant, especially at low quality region. The effect of inner diameter, mass fluxes, heat fluxes and saturation temperatures are explained.

Thermo-hydrodynamic investigation into the effects of minichannel configuration on the thermal performance of subcooled flow boiling

Igaadi Amal,El Amraoui Rachid,El Mghari Hicham 한국원자력학회 2024 Nuclear Engineering and Technology Vol.56 No.1

The current research focuses on the development of a numerical approach to forecast strongly subcooled flow boiling of FC-72 as the refrigerant in various vertical minichannel shapes for high-heat-flux cooling applications. The simulations are carried out using the Volume of Fluid method with the Lee phase change model, which revealed some inherent flaws in multiphase flows that are primarily due to an insufficient interpretation of shearlift force on bubbles and conjugate heat transfer against the walls. A user-defined function (UDF) is used to provide specific information about this noticeable effect. The influence of shape and the inlet mass fluxes on the flow patterns, heat transfer, and pressure drop characteristics are discussed. The computational results are validated with experimental measurements, where excellent agreements are found that prove the efficiency of the present numerical model. The findings demonstrate that the heat transfer coefficient decreases as the mass flux increases and that the constriction design improves the thermal performance by 24.68% and 10.45% compared to the straight and expansion shapes, respectively. The periodic constriction sections ensure good mixing between the core and near-wall layers. In addition, a slight pressure drop penalty versus the thermal transfer benefits for the two configurations proposed is reported.