Numerical simulation for steady annular condensation flow in triangular microchannels

A steady one-dimensional model for annular condensation flow in triangular microchannels is developed. The curvature radius distribution of the condensate stream along the channel has been determined numerically. The results indicate that the curvature radius of the liquid phase would increase rapidly at the beginning, and then as the condensation process progresses along the length of the microchannels, the radius increase would proceed more slowly. At the end of the condensation flow, the radius increases rapidly again. A smaller contact angle and heat flux or a larger hydraulic diameter and steam pressure will all result in a longer condensation length.     

Fluid flow in micro-channels

We consider the problem of liquid and gas flow in micro-channels under conditions of a small Knudsen and Mach numbers, that correspond to continuum model. Data from the literature on pressure drop in circular, rectangle, triangular and trapezoidal micro-channels with hydrodynamic diameter ranging from 1.01 μm to 4010 μm are analyzed. The Reynolds number at transition from laminar to turbulent flow is considered. Attention is paid to comparison between predictions of the conventional theory and experimental data, obtained during the last decade, as well as to discussion of possible sources of unexpected effects which were revealed by a number of previous investigations.     

A Prediction Model for Condensation on Single Horizontal Rectangular Fin Tube

INTRODUCTI0NItiswellknownthatusingintegralfinnedtubet0replacesm0othtubemayenhancecondensati0nheattransfer.Predictingcondensationheattransfercoeffi-cientofthefinnedtubeisnecessaryforapplicationandf0ranalyzingtheeffectsofworkingcondition,work-ingfluidpropertiesandfingeometry.Therefore,re-searchersalwayspayattentiontoit.Thefirstm0delwasbuiltbyBeattyandKatz[11.Itisagravity-drivingm0delthatneglectstheinfluence0fsurfacetensi0n.Althoughitmaywellpredictc0ndensationoflowsur-facetensionfluidsonlow…     

Transition from annular flow to plug/slug flow in condensation of steam in microchannels

A visualization study has been conducted to investigate the transition from annular flow to plug/slug flow in the condensation of steam in two different sets of parallel microchannels, having hydraulic diameters of 90 μm and 136 μm, respectively. The steam in the parallel microchannels was cooled on the bottom by forced convection of water and by natural convection of air from the top. It is found that the location, where the transition from annular flow to plug/slug flow takes place, depends on mass flux and cooling rate of steam. The effects of mass flux and cooling rate on the occurrence frequency of the injection flow in a single microchannel, having a hydraulic diameter of 120 μm and 128 μm, respectively, are investigated. It is found that two different shapes of injection flow occur in the smooth annular flow in microchannels: injection flow with unsteady vapor ligament occurring at low mass flux (or high cooling rate) and injection flow with steady vapor ligament occurring at high mass flux (or low cooling rate). It is also found that increase of steam mass flux, decrease of cooling rate, or decrease of the microchannel diameter tends to enhance instability of the condensate film on the wall, resulting in occurrence of the injection flow further toward the outlet with an increase in occurrence frequency.     

Numerical investigation of fluid flow and heat transfer under high heat flux using rectangular micro-channels

A 3D-conjugate numerical investigation was conducted to predict heat transfer characteristics in a rectangular cross-sectional micro-channel employing simultaneously developing single-phase flows. The numerical code was validated by comparison with previous experimental and numerical results for the same micro-channel dimensions and classical correlations based on conventional sized channels. High heat fluxes up to 130 W/cm² were applied to investigate micro-channel thermal characteristics. The entire computational domain was discretized using a 120 × 160 × 100 grid for the micro-channel with an aspect ratio of (α = 4.56) and examined for Reynolds numbers in the laminar range (Re 500–2000) using FLUENT. De-ionized water served as the cooling fluid while the micro-channel substrate used was made of copper. Validation results were found to be in good agreement with previous experimental and numerical data [1] with an average deviation of less than 4.2%. As the applied heat flux increased, an increase in heat transfer coefficient values was observed. Also, the Reynolds number required for transition from single-phase fluid to two-phase was found to increase. A correlation is proposed for the results of average Nusselt numbers for the heat transfer characteristics in micro-channels with simultaneously developing, single-phase flows.     

A void fraction model for annular flow in horizontal tubes

An important feature of detailed system simulation models for unitary air conditioners is the calculation of charge inventory. Void fraction determination in the two-phase regions of the heat exchangers is the primary challenge associated with charge inventory calculations. Annular flow is one of the predominant flow regimes encountered in horizontal heat exchangers. Analytical annular flow models typically fail to accurately represent void fraction. Thus, many of the available void fraction models are empirically based. To improve the prediction capabilities of void fraction models, a mechanistic void fraction model has been developed for annular flow in horizontal tubes. The present model considers the effect of momentum eddy diffusivity damping at the liquid-vapor interface. Two approaches are presented for determining the wall shear stress. The modeling results are compared to predictions from various void fraction models found in the literature. The present model is found to work well at moderate mass fluxes.     

有/无多孔介质的矩形T型管道内蒸汽 直接接触凝结流型的实验研究

采用高速摄影仪,对填充和未填充球形多孔介质的矩形T型管道内蒸汽直接接触凝结流型进行了可视化实验研究。研究发现未填充多孔介质条件下存在五种典型凝结流型:间歇凝结、振荡射流、稳态射流、尾部振荡射流和发散射流;填充多孔介质条件下则存在三种典型凝结流型:振荡射流、弹状稳定射流和雾状稳定射流。研究表明:多孔介质对凝结流型及汽羽形状有显著影响;由于多孔介质内部的黏滞阻力和惯性阻力,填充多孔介质的凝结流型类型减少;随着蒸汽流量的增加,凝结流型较快的进入到稳定状态,削弱了蒸汽直接接触凝结汽羽的振荡。     

An improved numerical model for annular two-phase flow with liquid entrainment

The development of a transient numerical model to predict the characteristics of annular two-phase flow with entrainment is presented and discussed. The mathematical formulation of this model is based on finite volume mass, momentum and energy balances. Several constitutive relations are employed to describe the fluid-gas and entrainment interactions. Numerical results indicate that this model present a significant improvement over other existing models in predicting the annular flow characteristics.     

Visualization on flow patterns during condensation of R410A in a vertical rectangular channel

The visualization experiments on HFC R410A condensation in a vertical rectangular channel （14.34mm hydraulic diameter, 160mm length） were investigated. The flow patterns and heat transfer coefficients of condensation in the inlet region were presented in this paper. Better heat transfer performance can be obtained in the inlet region, and flow regime transition in other regions of the channel was also observed. Condensation experiments were carried out at different mass fluxes （ from 1.6 kg/h to 5.2 kg/h） and at saturation temperature 28~ C. It was found that the flow patterns were mainly dominated by gravity at low mass fluxes. The effects of interfacial shear stress on condensate fluctuation are significant for the film condensation at higher mass flux in vertical flow, and con- sequently, the condensation heat transfer coefficient increases with the mass flux in the experimental conditions, The drop formation and growth process of condensation were also observed at considerably low refrigerant vapor flow rate.     

竖直矩形狭缝通道内环状流沸腾换热分析模型

对竖直矩形狭缝通道内有液滴卷吸环状流阶段流动沸腾进行分析。以液膜紊流的动量方程和能量方程为基础,加上相应的边界条件和使控制方程组封闭的经验关系式,建立了环状流的教学模型并进行数值计算,得到了矩形狭缝通道内的液膜厚度分布、沸腾传热系数等结果;将模型预测的换热系数同实验关系式作比较,最大相对误差为17．8％。     

A four-zone model for saturated flow boiling in a microchannel of rectangular cross-section

A four-zone flow boiling model is presented to describe saturated flow boiling heat transfer mechanisms in a microchannel of rectangular cross-section. The boiling process in the microchannel is assumed to be a cyclic passage of four zones: (i) liquid-slug zone, (ii) elongated bubble zone, (iii) partially-dryout zone, and (iv) fully-dryout zone. The existence of the partially-dryout zone in this model is proposed to take into consideration of corner effects on boiling heat transfer in the microchannel. To verify this new model, an experimental study was carried out to investigate flow boiling heat transfer of water in a microchannel having a rectangular cross-section with a hydraulic diameter of 137 μm (202 μm in width and 104 μm in depth) with a length of 30 mm under three-side heating condition. The data for bubble nucleation frequency was correlated in terms of the Boiling number, which was used to determine the heat transfer coefficient. It is found that the present four-zone flow boiling model successfully predicts trends of boiling heat transfer data in a microchannel with a rectangular cross-section, having a sharp peak at low vapor quality depending on the mass flow rate. The predictions of flow boiling heat transfer coefficient in the microchannel are found in good agreement with experimental data with a MAE of 13.9%.     

竖直矩形窄缝两相同向分相流动沸腾传热模型

首次对竖直矩形窄缝内的汽液分相流动区提出一维两相同向分相流动沸腾传热模型 ,并进行了数值计算 ,得到不同质量流速下液膜厚度变化和沸腾传热系数等结果。沸腾传热系数的模型预测值初步与已有实验关联式进行了比较 ,两者基本吻合 ,偏差在± 1 4% ;从而证实了液膜导热是竖直矩形窄缝内汽液分相流动区沸腾传热的主导机理。     

Effects of choking on flow and heat transfer in micro-channels

The gas flows through micro-channels are encountered in many engineering applications such as the cooling devices of electronic chips, semiconductors, micro-electro-mechanical systems (MEMS), etc. Many works have been performed to investigate the flow and heat transfer characteristics generally occurring in the micro channels. According to these investigations, the majority of heat was transferred in the entrance region of the channel, due to high strain rate of the developing flow. These findings are valid only for unchoked micro channel flows. Once the gas flow is choked, the major flow features may be changed but no detailed works have been carried out to date. In these regards, the choked flow characteristics should be known to investigate the heat transfer phenomena in the micro channel flows. In the present study, numerical simulations have been used to provide detailed flow and heat transfer characteristics of micro-channel gas flows. The main objectives of the present effort are to understand the evolution of choking inside micro-channels with isothermally-heated-walls and to elucidate the regions of high heat transfer. The results obtained show that for choked flow conditions, high heat transfer is generated at both the entrance and the exit of the micro-channel. The exit effects like increased strain rate, high temperature gradient and the thinning of the boundary layer cause a rapid increase in heat transfer at the exit of the micro-channel. The location where the flow is choked is practically important in determining the heat transfer phenomena at the vicinity of the channel exit.     

Reflux condensation in narrow rectangular channels with perforated fins

Reflux condensation is an industrial process that aims to reduce the content of the less volatile component or to eliminate the non-condensable phase of a vapour mixture, by the means of separation. Separation consists in condensing the less volatile phase and to recover the condensate while simultaneously, the non-condensable species are recuperated at the top of the system. Compact plate-fin heat exchangers can be used in gas separation processes. The aim of this study is to test the process of reflux condensation of an air–steam mixture in the channels of a plate fin heat exchanger with a hydraulic diameter of 1.63 mm. The experimental study shows that reflux condensation occurs in specific parts of the heat exchanger, the other parts remaining dry.Moist air condensation is modelled by the film theory and the results show that the model is well adapted to simulating the heat and mass transfer.     

Research on a new heat transfer model for liquid film convective evaporation in annular flow

Based on the phenomenon of turbulence restraint in liquid‐vapor interface, an analytical model is proposed for annular flow with a velocity distribution. The liquid‐vapor interface affecting district mixing length model was amended, and a new liquid film convective evaporation heat transfer model at the annular flow was developed. Compared with the experimental data, the results show that the new model is better than the model based on full tube flow velocity distribution. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 524–530, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10051     

Stability of annular flow

A simple transient model using an unsteady state continuity equation and a quasi steady momentum equation is used to analyse the stability of co-current and counter-current annular flows. It is shown that unstable steady state solutions may take place in both cases. Application of the theory is demonstrated for the problem of flooding and flow reversal.     

Visualization study of steam condensation in wide rectangular silicon microchannels

A visualization study is conducted to investigate condensation flow in wide rectangular silicon microchannels with the hydraulic diameter of 90.6 μm and width/depth ratio of 9.668. Droplet-annular compound flow, injection flow, and vapor slug-bubbly flow are observed along the channel, which differ from that in other cross-sectional shape microchannels. In the droplet-annular compound flow region, the vertical walls (short side) of the channel are completely covered by the condensate, while droplet condensation still exists on the horizontal wall (long side) of the channel. The location of the injection flow will be postponed with the increasing inlet vapor Reynolds number. The injection frequency will increase with the increasing inlet vapor Reynolds number and condensate Weber number. More specifically, the frequency in the wide rectangular microchannels is lower than that in triangular microchannels having the same hydraulic diameter. It is confirmed that the cross-sectional shape of the microchannel plays a significant role on the instability of condensation flow. In addition, the correlation of Nusselt number is also presented.     

三角形粗糙元的微通道内流动换热的模拟分析

应用蒙特卡洛直接模拟(direct simulation Mont Carlo,DSMC)方法数值分析具有三角粗糙元表面平行平板微通道内气体二维流动与换热.模拟表明:微通道内粗糙元对流动与换热有明显的扰动;粗糙微通道内的壁面速度滑移小于光滑微通道,并随粗糙元变大,速度出现更为严重的跳跃,甚至出现漩涡,增加了通道内的压力损失;随粗糙元变大,气体在壁面处滞留时间变长,增加了单位质量气体与壁面之间的换热.     

Steady evaporating flow in rectangular microchannels

Analytical and numerical solutions are presented for steady evaporating flow in open microchannels having a rectangular cross section and a uniform depth. The flow, driven by the axial gradient of capillary pressure, generally consists of an entry region where the meniscus is attached to the top corners of the channel followed by a jump-like transition to a corner-flow region in which the meniscus progressively recedes into the bottom corners of the channel. Illustrative numerical solutions are used to guide the derivation of an easily applied analytical approximation for the maximum sustainable heat flux or capillary limit.