Flow Instability (FI) for subcooled flow boiling through a narrow rectangular channel under transversely uniform and non-uniform heat flux

Al-Yahia, Omar S.,Kim, Taewoo,Jo, Daeseong Elsevier 2018 INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER - Vol.125 No.-

<P><B>Abstract</B></P> <P>Transversely non-uniform heating enforces high local heat fluxes at a few areas on the heated surface and low local heat fluxes at others. Differences in the heat flux distribution may generate differences in the boiling behavior when compared with that under uniform heating. Thus, the present experiments investigate the influence of transverse heat flux distribution on the flow instability through a narrow rectangular channel (2.35 mm × 54.0 mm × 566 mm). A wide range of experimental operation conditions, such as inlet temperatures (35–65 °C), thermal power (500–6250 W), and mass flow rates (0.03–0.13 kg/s), are applied to the upward flow channel. The working fluid corresponds to demineralized water under atmospheric pressure. The channel is heated from a side, and the other side is a transparent polycarbonate window from which the bubble behavior is visualized by using a high-speed camera. The heating surface is composed of aluminum with a width of 50 mm and a length of 300 mm. The experiments are performed by using two different procedures to achieve the flow instability, namely (1) constant mass flow rate with power increases and (2) constant power with mass flow rate decreases. The results show that the flow instability occurs at similar thermal power and similar mass flow rates for both uniform and non-uniform heating conditions. However, the pressure drop and wall temperature curves exhibit differences in the trends between the two heating conditions, especially after the fluctuation in the inlet pressure. In the uniform case, bubbles are generated uniformly on the whole transverse direction of the heated surface. In the non-uniform case, more bubbles are generated at the higher local heat flux, which disturbs the velocity profile in the transverse direction. The differences in bubble generation in the transverse direction leads to differences in the flow instability in a narrow rectangular channel.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Heat flux distribution has no effect under single-phase flow. </LI> <LI> Heat flux distribution has a high influence on the first bubble generation. </LI> <LI> Pressure drop trend is different between uniformly and non-uniformly heating conditions. </LI> <LI> Inlet pressure fluctuates after significant bubble generation. </LI> <LI> Void fraction oscillation is affected by the heat flux distribution. </LI> </UL> </P>

냉난방 수배관 시스템의 실험적 유량분배 모의 방법론

김진섭(Jinsub Kim),정지환(Ji Hwan Jeong) 대한설비공학회 2020 대한설비공학회 학술발표대회논문집 Vol.2020 No.6

The Hydronic System efficiency can improve by controlling the flow in the system. Therefore, it is necessary to quantitatively study the flow resistance of value considering effective parameters such as pipe length, type of valve, fan coil unit, and so on the flow distribution. In the present study, the flow distribution characteristics are experimentally analyzed according to the flow resistance, and the hydronic System flow distribution is simulated in which water was supplied to fan coil unit through a header. The flow resistance generated in the valve, pipe length, fan coil unit, and so on in each loop was represented through the valve flow coefficient. Moreover, the flow rate distributed in each branch pipe was measured. Variation of the flow rate based on the flow resistance was measured by changing the valve flow coefficient. As a result of the experiments, the distribution condition and flow coefficient range of control valve were derived.

알루미늄 다채널 평판관 증발기 내 냉매분배

김내현,이응렬,박태균,한성필 대한설비공학회 2006 설비공학 논문집 Vol.18 No.10

The R-134a flow distribution is experimentally studied for a heat exchanger composed of round headers and 10 flat tubes. The effects of tube protrusion depth as well as mass flux, and quality are investigated, and the results are compared with the previous air- water results. The flow at the header inlet is stratified. For the downward flow configuration, the liquid distribution improves as the protrusion depth or the mass flux increases, or the quality decreases. For the upward configuration, the liquid distribution improves as the mass flux or quality decreases. The protrusion depth has minimal effect. For the downward configuration. the effect of quality on liquid distribution is significantly affected by the flow regime at the header inlet. For the stratified inlet flow, the liquid is forced to rear part of the header as the quality decreases. However, for the annular inlet flow, the liquid was forced to the frontal part of the header as the quality decreased. For the upward flow, the effect of the mass flux or quality on liquid distribution of the stratified inlet flow is opposite to that of the annular inlet flow. The high gas velocity of the annular flow may be responsible for the trend. Generally, the liquid distribution of the stratified inlet flow is better than that of the annular inlet flow. Possible explanation is provided from the flow visualization results.

평판관 열교환기 내 공기-물 2상류 분지

김내현,박태균,한성필,신태룡 대한설비공학회 2006 설비공학 논문집 Vol.18 No.9

The air and water flow distribution are experimentally studied for a heat exchanger composed of round headers and 10 flat tubes. The effects of tube protrusion depth as well as mass flux, and quality are investigated, and the results are compared with the previous 30 channel results. The flow at the header inlet is annular. For the downward flow configuration, the water flow distribution is significantly affected by the tube protrusion depth. For flush-mounted geometry, significant portion of the water flows through frontal part of the header. As the protrusion depth increases, more water is forced to the rear part of the header. The effect of mass flux or quality is qualitatively the same as that of the protrusion depth. Increase of the mass flux or quality forces the water to rear part of the header. For the upward flow configuration, different from the downward configuration, significant portion of the water flows through the rear part of the header. The effect of the protrusion depth is the same as that of the downward flow. As the protrusion depth increases, more water is forced to the rear part of the header. However, the effect of mass flux or quality is opposite to the downward flow case. As the mass flux or quality increases, more water flows through the frontal part of the header. Compared with the previous thirty channel configuration, the present ten channel configuration yields better flow distribution. Possible explanation is provided from the flow visualization results.

Flow between eccentric cylinders: a shear-extensional controllable flow

Guoqiang Tian,Mengmeng Wang,Xiaolin Wang,Gang Jin 한국유변학회 2016 Korea-Australia rheology journal Vol.28 No.2

In this work the non-Newtonian fluid between eccentric cylinders is simulated with finite element method. The flow in the annular gap between the eccentric rotating cylinders was found to be a shear-extensional controllable flow. The influence of rotating speed, eccentricity as well as the radius ratio on the extensional flow in the vicinity of the minimum gap between the inner and outer cylinder was quantitatively investigated. It was found that both the strengths of shear flow and extensional flow could be adjusted by changing the rotating speed. In respect to extensional flow, it was also observed that the eccentricity and radius ratio exert significant influences on the ratio of extensional flow. And it should be noted that the ratio of extensional flow in the mix flow could be increased when increasing the eccentricity and the ratio of shear flow in the mix flow could be increased when increasing the radius ratio.

Flow Characteristics of Pulp Suspension in a 90° Bend

Masaru Sumida,Kentaro Imamura 한국유체기계학회 2020 International journal of fluid machinery and syste Vol.13 No.2

This study investigates the flow characteristics of pulp suspensions flowing through a 90° bend, which is used in the production system of an actual papermaking machine, e.g., for transporting the pulp suspensions from its stock reservoir to the header. Experiments were conducted on pulp suspensions with a fiber concentration Cs of 0.3 and 0.6 wt% and a bend with a diameter of 22 mm and a curvature radius ratio of 4.0. Flow visualization and measurements of the distributions of time-averaged fiber concentration Ca and axial velocity U were performed with a light section method and the particle image velocimetry (PIV) method, respectively, at representative bulk velocities and at various longitudinal stations. The influence of the flow rate on the changes of their distributions in the streamwise direction was examined. The flow characteristics of the pulp suspension depend on the flow pattern in the upstream straight tube and are greatly different from those of the single-phase water flow. For a low flow rate, the flocculated pulp fibers move without getting disentangled in the bend and the distribution of Ca shows a shape bias towards the inner wall side. As the flow rate is increased, Ca becomes larger in the outer part of the bend, and it changes into a rather flat distribution in the downstream tangent.

Refrigerant distribution in an evaporator having a horizontal header and horizontal mini-channel tubes

김철환,NAE-HYUN KIM 대한기계학회 2021 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.35 No.9

In this study, we investigated the R-1234yf distribution from a header into minichannel tubes located in a horizontal plane, which may be of importance for cooling of an electric vehicle battery. Especially, the focus was concentrated on the effect of number of branch tubes on the two-phase distribution. The tests were conducted for the mass flux from 49 to 147 kg/m 2 s and inlet quality from 0.2 to 0.8. The effect of heat exchanger inclination was also investigated. The results showed that the flow distribution worsened as the number of branch tube increased. Between the normal and the parallel inlet configuration, the normal inlet yielded a better distribution, probably due to the smaller number of branch tubes. The flow distribution was mainly governed by the balance of relating forces-momentum and gravitational. Thus, a poor distribution was obtained for a tilted case, where the momentum and the gravitational forces were assisting. The data showed that a vapor quality was always beneficial to the distribution. However, the effect of mass flux was case-dependent. For the horizontal case, the mass flux pushed the flow downstream, and worsened the distribution. For the tilted case, the mass flux assisted or opposed the gravity, yielding a better distribution.

Flow distribution of two-phase fluid through multiple microchannels

Jae Hyun Park,박일석 대한기계학회 2021 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.35 No.6

To enhance the heat transfer efficiency of multiple flow-passage heat exchangers (MFHE) used in various industrial fields, it is very important to evenly distribute the working fluid in each flow passage. The flow distribution of multiphase fluids has been mostly studied experimentally thus far because of its phenomenological complexity. In this study, a direct numerical analysis of the flow distribution of the liquid–vapor two-phase refrigerant in MFHEs was conducted. The two-phase flow distribution through the branched multiple microchannels was numerically simulated by using volume of fluid (VOF) interface tracking method. The proposed numerical procedure was successfully verified through comparison with prior experimental results for a horizontal header with 10 vertical branch tubes. The results showed satisfactory agreement. The intermittent inflow of liquid into the microchannels was revealed to play an important role in enhancing the two-phase flow distribution and preventing flow clogging in the microchannels.

유동 가시화를 이용한 PCHE의 유동분배에 관한 실험적 연구

설한이,손상호,이공훈,김정철 대한설비공학회 2023 설비공학 논문집 Vol.35 No.1

PCHE, Printed Circuit Heat Exchanger, is widely used for industrial applications with increasing demand in extreme environments. As interest in hydrogen for future fuel is increased, the interest in PCHE is also increased. In this study, we focused on the flow distribution in channels of PCHE. Although flow distribution has been employed a lot of times for decades, there are few reliable studies based on experimental results that can be directly applied in actual industrial fields. Here, we introduce experimental results about flow distribution by flow visualization. Effects of flow rates and the verticality are addressed to clarify the difference in distribution. When the flow rate is low, the flow speed difference between channels is not significantly large. If it becomes larger, the difference becomes large. Moreover, when inlet and outlet pipes are installed at a slight angle, back flow is observed. The speed profile of the tilted case in high flow rate is not easily predicted. Although the tilting angle looks very small, the measured speed profile can be completely different. 본 연구에서는 PCHE에 유입되는 유동의 방향 및 유량에 따른 불균등도에의 영향을 알아보기 위해 유동가시화 실험을 진행하였다. 일반적으로 고유량에서는 불균등도가 더욱 증가하였고, 저유량에서는 감소하였다. 또한, 액체가 중앙에서 수직방향으로 유입될 때, 저유량에서 더욱 대칭적인 속도장이 관찰되었다. 유입 방향의 경우, 작은 정도로만 기울어져도, 유동 불균등도가 증가할 뿐만 아니라, 역방향 유동이 발생할 정도로 수직인 경우와 큰 차이를 보였다. 본 연구의 실험 결과는 실제 산업의 PCHE와 비교해 볼 때, 비교적 큰 헤더를 사용한 경우에 해당된다. 따라서 헤더 내부의 chaotic 유동이 더욱더 극대화된 것으로 생각된다. 다만, 실제는 헤더의 종횡비가 더욱 큰 경우가 많고, 채널 내 평균 유속도 본 실험의 약 50 cm/s보다 더 큰 경우도 있기 때문에, 실제 산업에서의 불균등도는 더욱 안 좋을 수도 있다. 따라서 여러 가지 다양한 경우에 대해, 실질적인 유동 분배 문제가 실험적으로 밝혀져야 한다. 또한, 향후에는 최외곽 채널에 대한 분석 이외에도 내부 유동에 대한 분석도 이루어져야 하고, 기체를 포함한 다른 유체에 대해서도 분석이 이루어져야 한다. 그리고, 채널 방향과 수직인 경우 및 입출구 배관 크기의 영향 등에 대해서도 심도있는 연구가 이루어져야 할 것이다.

Estimating Suitable Probability Distribution Function for Multimodal Traffic Distribution Function

Yoo, Sang-Lok,Jeong, Jae-Yong,Yim, Jeong-Bin The Korean Society of Marine Environment and safet 2015 海洋環境安全學會誌 Vol.21 No.3

The purpose of this study is to find suitable probability distribution function of complex distribution data like multimodal. Normal distribution is broadly used to assume probability distribution function. However, complex distribution data like multimodal are very hard to be estimated by using normal distribution function only, and there might be errors when other distribution functions including normal distribution function are used. In this study, we experimented to find fit probability distribution function in multimodal area, by using AIS(Automatic Identification System) observation data gathered in Mokpo port for a year of 2013. By using chi-squared statistic, gaussian mixture model(GMM) is the fittest model rather than other distribution functions, such as extreme value, generalized extreme value, logistic, and normal distribution. GMM was found to the fit model regard to multimodal data of maritime traffic flow distribution. Probability density function for collision probability and traffic flow distribution will be calculated much precisely in the future.