Numerical investigations of simultaneously developing flow in wavy microchannels under pulsating inlet flow condition

The simultaneously developing unsteady laminar fluid flow and heat transfer inside a two dimensional wavy microchannel, due to sinusoidally varying velocity component at inlet has been numerically investigated. The flow was both thermally and hydrodynamically developing while the channel walls were kept at a uniform temperature. The transient solution of two-dimensional Navier-Stokes equation was obtained using the SIMPLE algorithm with the momentum interpolation technique of Rhie and Chow. The simulation was performed in the laminar regime for Prandtl number 7 and Reynolds number ranging from 0.1 to 100. Based on the comparison with steady flow in wavy channel it was found that imposed sinusoidal velocity at inlet can provide improved heat transfer performance at different amplitude (0.2, 0.5, 0.8) and frequency (1, 5, 10).     

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.     

Numerical study of convective flow with condensation of a pure fluid in capillary regime

A stationary 2-phase flow model with condensation in the capillary regime, based on a separate flow approach was developed. One of the specificities of the model is that it takes into account the coupling between a cylindrical interface (region with a thin film of liquid) and a hemispherical interface (main meniscus at the end of the condensation region). A specific algorithm was developed for numerical resolution to overcome the difficulty related to the presence of a free boundary condition. Analysis of the liquid–vapour interface profiles and the various local parameters allowed us to establish the heat and mass transfer laws for the particular type of regime studied. We analysed the dominant effects of this type of flow, which are characterised by dimensionless numbers Ca (capillary number) and Bo (boiling number), representing the competition between the capillary, viscous and phase-change effects. The effects due to the difference in density between the two phases and to the Reynolds number were also studied. We show that the mean heat transfer coefficients are driven by the profile of the interface. Hence, in certain situations, even when the liquid film becomes thinner on average an unexpected lowering of the efficiency of heat transfer is obtained. These effects are closely related to the coupling between the thin liquid film region and the main meniscus.     

Numerical simulation of roughness effects on flow and heat transfer in microchannels at slip flow regime

A numerical simulation for studying fluid flow and heat transfer characteristics in microchannels at slip flow regime with consideration of slip and temperature jump is studied. The wall roughness is simulated in two cases with periodically distributed triangular microelements and random shaped micro peaks distributed on the wall surfaces. Various Knudsen numbers have used to investigate the effects of rarefaction. The numerical results have also checked with available theoretical and experimental relations and good agreements has achieved. It has been found that rarefaction has more significant effect on flow field in microchannels with higher relative roughness. The negative influence of roughness on fluid flow and heat transfer found to be the friction factor increment and Nusselt number reduction. In addition high influence of roughness distribution and shape has been shown by a comparison of Poiseuille and Nusselt numbers for tow different cases.     

An experimental study of flow friction and heat transfer in wavy microchannels with rectangular cross section

Experimental investigation has been conducted on the flow friction and heat transfer in sinusoidal microchannels with rectangular cross sections. The microchannels considered consist of ten identical wavy units with average width of about 205 μm, depth of 404 μm, wavelength of 2.5 mm and wavy amplitude of 0–259 μm. Each test piece is made of copper and contains 60–62 wavy microchannels in parallel. Deionized water is employed as the working fluid and the Reynolds numbers considered range from about 300 to 800. The experimental results, mainly the overall Nusselt number and friction factor, for wavy microchannels are compared with those of straight baseline channels with the same cross section and footprint length. It is found that the heat transfer performance of the present wavy microchannels is much better than that of straight baseline microchannels; at the same time the pressure drop penalty of the present wavy microchannels can be much smaller than the heat transfer enhancement. Conjugate simulation based on the classical continuum approach is also carried out for similar experimental conditions, the numerical results agree reasonably well with experimental data.     

Visualization study of steam condensation in triangular microchannels

A visualization experiment is conducted to investigate the condensation of steam in a series of triangular silicon microchannels. The results indicate that droplet, annular, injection and slug-bubbly flow are the dominant flow patterns in these triangular silicon microchannels. With increased mass flow rate, or an increase in the hydraulic diameter under the same Reynolds number, the location at which the injection occurred is observed to move towards the channel outlet. The frequency of the injection increases, i.e. the flow of condensation instability is higher with increased inlet vapor Reynolds number, condensate Weber number and the prolongation of the injection location, or with a decrease in the hydraulic diameter of the channel. In addition, the wall temperature of the channel decreases along the condensation stream. The total pressure drop, the average condensation heat transfer coefficient and the average Nusselt number are observed to be larger with increased inlet vapor Reynolds number. Moreover, it is found that the condensation heat transfer is enhanced by a reduction in the channel scale.     

水在梯形截面微通道内流动阻力特性的数值研究

对梯形截面微通道内去离子水的不可压缩充分发展的流动建立数学模型,采用二维SIMPLE算法求解通道横截面上的流向速度分布.对特定几何尺寸的通道内水的充分发展流动在实验雷诺数范围内进行数值模拟,并计算充分发展段的阻力系数,将模拟所得阻力系数随雷诺数的变化关系与有关实验结果进行了对比,得到了较好的一致性.从而表明:N-S方程仍然适用于微通道内去离子水的层流流动的数值模拟.     

Numerical simulation of two-fluid electroosmotic flow in microchannels

This paper presents a numerical scheme for stratified two-liquid electroosmotic flows. The simulation results highlight that using the electroosmotic effects can control the interface location of a pressure-driven two-liquid flow. A finite volume method is used to solve the coupled electric potential equation and Navier–Stokes equation together.The validity of the numerical scheme is evaluated by comparing its predictions with the results of the analytical solutions in the fully developed regions. The liquid–liquid interface developments due to the favorably and adversely applied electric field are examined.     

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.     

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.     

滑移区气体-颗粒流动与传热特性研究

针对气体-颗粒微尺度流动与传热过程开展数值模拟研究,所构建模型中气体处理为可压缩、变物性流体,并在颗粒表面采用速度滑移和温度跳跃边界条件以考虑气体稀薄效应。在数值模拟基础上,研究分析稀薄效应对颗粒与其周围气体流动与换热的影响程度,并进一步提出新的阻力系数与传热努谢尔特数关联式。研究结果表明,气体稀薄效应将减小颗粒阻力系数,同时抑制颗粒与其周围气体的传热过程。     

Numerical study for laminar wavy motions of liquid film flow on vertical wall

Film flows are classified into non-wavy laminar, wavy laminar and turbulence along the Reynolds number or the flow stability. Since the wavy motions of the film flows are so intricate and nonlinear, the studies have largely been dependent upon the experimental way. The numerical approaches have been limited on the non-wavy flow regime. To track the free surface position, various numerical techniques such as the VOF (Volume of Fluids), the MAC (Marker and Cell) and the moving grid have been adopted. However those were for a more accurate estimation of the average film thickness and not for capturing the wavy motion. Because the wavy motion highly affects the heat transfer in the film flow, the profound concern for the wavy motion is significant. In this study, the wavy motions of the laminar wavy film flow with the Reynolds number 200–1000 are successfully found by the VOF and PLIC (Piecewise Linear Interface Calculation) method. The numerical results, including the average film thickness, and the wave’s amplitude, frequency and velocity, are compared with the experimental results.     

Numerical simulation of slip flow through rhombus microchannels

Microgeometry fluid dynamics has gotten a lot interest due to the arrival of Micro-Electro-Mechanical systems (MEMS). When the mean free path of a gas and characteristic length of the channel are in the same order, continuum assumption is no longer valid. In this situation velocity slip and temperature jump occur in the duct walls. Fully developed numerical analysis for characteristic laminar slip flow and heat transfer in rhombus microchannels are performed with slip velocity, and temperature-jump boundary condition at walls. The impacts of Reynolds number (0.1 < Re < 40), velocity slip, and temperature-jump on Poiseuille number, and Nusselt number for different aspect ratio (0.15 < A < 1.0), and Knudsen number are studied in detail. The contours of non-dimensional velocity for some cases are examined as well. The results show that aspect ratio and Knudsen number have important impact on Poiseuille number, and Nusselt number in rhombus microchannels. Reynolds number has considerable influence on Nusselt number at low Reynolds number, but its influence on Poiseuille number is not very important at the studied range.     

Numerical study of hydrodynamic and heat transfer of nanofluid flow in microchannels containing micromixer

In this study heat transfer and fluid flow of Al₂O₃/water nanofluid in two dimensional parallel plate microchannel without and with micromixers have been investigated for nanoparticle volume fractions of ϕ = 0, ϕ = 4%  and base fluid Reynolds numbers of Re_f = 5, 20, 50. One baffle on the bottom wall and another on the top wall work as a micromixer and heat transfer enhancement device. A single-phase finite difference FORTRAN code using Projection method has been written to solve governing equations with constant wall temperature boundary condition. The effect of various parameters such as nanoparticle volume fraction, base fluid Reynolds number, baffle distance, height and order of arrangement have been studied. Results showed that the presence of baffles and also increasing the Re number and nanoparticle volume fraction increase the local and averaged heat transfer and friction coefficients. Also, the effect of nanoparticle volume fraction on heat transfer coefficient is more than the friction coefficient in most of the cases. It was found that the main mechanism of enhancing heat transfer or mixing is the recirculation zones that are created behind the baffles. The size of these zones increases with Reynolds number and baffle height. The fluid pushing toward the wall by the opposed wall baffle and reattaching of separated flow are the locations of local maximum heat transfer and friction coefficients.     

Numerical computation of fluid flow and heat transfer in microchannels

Three-dimensional fluid flow and heat transfer phenomena inside heated microchannels is investigated. The steady, laminar flow and heat transfer equations are solved using a finite-volume method. The numerical procedure is validated by comparing the predicted local thermal resistances with available experimental data. The friction factor is also predicted in this study. It was found that the heat input lowers the frictional losses, particularly at lower Reynolds numbers. At lower Reynolds numbers the temperature of the water increases, leading to a decrease in the viscosity and hence smaller frictional losses.     

Numerical investigation of subcooled flow boiling in segmented finned microchannels

Bubble growth behavior and heat transfer characteristics during subcooled flow boiling in segmented finned microchannels have been numerically investigated. Simulations have been performed for a single row of segmented finned microchannel and predicted results are compared with experimental investigations. Onset of nucleation, formation of bubbles, their growth and movements have been investigated for different values of applied heat flux. Mechanism of bubble expansion without clogging resulting in enhanced heat transfer in segmented finned microchannels has been explained. Temperature and pressure fluctuations during subcooled flow boiling condition have been investigated. It is observed that at high heat flux, thin liquid film trapped between the bubble and channel wall is evaporated leading to localized heating effect. Predicted flow patterns are similar to experimental results. However, simulations over predict the bubble growth rate and heat transfer coefficient.     

矩形微通道内流体流动特性的数值研究

对矩形微通道实体模型进行简化处理,并建立微通道内流体流动的数学模型.设定矩形微通道水力直径Dh=120～480 μm,入口雷诺数Re=ll.9～3 817.1,以20℃蒸馏水为流动工质,借助FLUENT分别对不同水力直径的三组矩形微通道内流体流动特性展开数值模拟研究,并将数值模拟结果与理论预测值及其他学者的研究结论进行对比.结果表明:随着微通道水力直径的减小,摩擦阻力系数、速度梯度和压强梯度都呈现增大趋势;在微尺度下,矩形微通道内临界Re提前,而且水力直径越小,临界Re值越小.     

Numerical simulation of thermofluid characteristics of two-phase slug flow in microchannels

A fundamental study of heat transfer characteristics of two-phase slug flow in microchannels is carried out employing the Volume-of-Fluid (VOF) method. Despite of the fact that numerical simulations of two-phase flows in microchannels have been attempted by many investigators, most efforts seem to have failed in correctly capturing the flow physics, especially those pertaining to the slug flow regime characteristics. The presence of a thin liquid film in the order of 10 μm around the bubble is a contributing factor to the above difficulty. Typically, liquid films have a significant effect on the flow field and heat transfer characteristics. In the simulations reported in this paper, the film is successfully captured and a very high local convective heat transfer coefficient is observed in the film region. A strong coupling between the conductive heat transfer in the solid wall and the convective heat transfer in the flow field is observed and characterized. Results showed that unsteady heat transfer through the solid wall in the axial direction is comparable to that in the radial direction. Results also showed that a fully developed condition could be achieved fairly quickly compared to single-phase flows. The fully developed condition is defined based on the Peclet number (Pe) and a dimensionless length of the liquid slug. Local and time-averaged Nusselt numbers for slug flows are reported for the first time. It was found that significant improvements in the heat transfer coefficient could be achieved by short slugs where the Nusselt number was found to be 610% higher than in single-phase flows. The study revealed new findings related to slug flow heat transfer in microchannels with constant wall heat flux.     

Prediction of two-phase condensation in horizontal tubes using probabilistic flow regime maps

A flow regime based condensation model is developed for refrigerants in single, smooth, horizontal tubes utilizing a generalized probabilistic two-phase flow map. Flow map time fraction information is used to provide a physically based weighting of heat transfer models developed for different flow regimes. The developed model is compared with other models in the literature, with experimentally obtained condensation data of R134a in 8.92 mm diameter tubes, and with data found in the literature for 3.14 mm, 7.04 mm, and 9.58 mm tubes with R11, R12, R134a, R22, R410A, and R32/R125 (60/40% by weight) refrigerants and a wide range of mass fluxes and qualities.     

Effects of compressibility and transition to turbulence on flow through microchannels

A detailed experimental study of flow through long microchannels of hydraulic diameter ranging from 60.5 to 211 μm has been carried out. The internal pressure distribution along the length of the channel has been measured to analyze the local flow behaviour. The effects of compressibility and transition to turbulence occurring in the microchannel flow were investigated in detail. In addition, the resulting flow has been analyzed numerically using a commercially available CFD code, FLUENT. It has been shown that there are no special micro-scale effects, including early transition to turbulence at least in the present range of hydraulic diameters after the significant effects of compressibility are accounted appropriately.