| Microfluidic devices with microchannels as the basic unit can precisely control fluid dynamics parameters and significantly improve heat and mass transfer so that it has been widely used in microelectronic device heat dissipation,mixing and separations and chemical reactions.Two-phase Taylor flow is a simple and effective method to produce vortices by introducing gas or immiscible liquid and has received wide attention in recent years.However,the flow structure and transfer behavior of Taylor flow in the microchannel are much more complicated than single-phase flow and are affected by many factors.Therefore,it is of great significance to systematically and deeply study the flow and heat transfer mechanism of Taylor flow and develop the corresponding heat transfer enhancement technology.This paper systematically analyzes the flow and heat transfer characteristics of gas/liquid-liquid two-phase Taylor flows in micro/mini-channels using a combination of numerical simulations and experimental studies.Two new structural microchannels were designed to modulate Taylor flow behavior and enhance microchannel heat transfer.Two-dimensional numerical simulations of the frictional pressure drop and heat transfer characteristics of the gas-liquid Taylor flow in the microchannel were carried out using the fixed frame of reference method(FFR).The simulation results show that the Taylor flow pressure drop decreases periodically along the flow direction.Compared with the homogeneous and separated flow models,the flow-pattern-related model proposed by Kreutzer has better prediction results.The internal circulation flow inside the liquid slug causes a sharp increase in the shear stress in the X direction,and the shear stress in the Y direction is no longer zero.The heat transfer process in the slug was analyzed in detail,and it was found that the heat transfer process in the slug is closely related to the internal recirculation flow field.The recirculation flow promotes rapid heating of the fluid in the center of the channel,thus enhancing the heat transfer capacity.The average Nu number of Taylor flow increases by about 1.8 times compared to pure water flow.The local Mu number distribution along the axis may be divided into four parts.The local Nu number rises sharply in the transition region between the two ends of the bubble with high circulation intensity.The Nu number increased with the mixture velocity and decreased with increasing void fraction.At low mixing velocity,the temperature field distribution is no longer symmetrical.The T-shaped diverging junction structure is proposed to enhance the channel’s Taylor flow heat transfer process.The Taylor flow dynamics behavior and heat transfer characteristics in microchannels are systematically studied by combining experiments and three-dimensional conjugate heat transfer numerical calculations.The results show that the diverging junction can significantly increase the bubble generation frequency and shorten the bubble and slug length.On the one hand,the continuous fluid is separated into more parts,reducing axial mixing.On the other hand,the circulation flow time in a single slug is shortened so that the liquid contacts the wall more frequently,and the heat transfer rate is significantly increased.The overall thermal performance of Taylor flow in square microchannels was systematically evaluated,and the comprehensive performance of the microchannel is the highest when the homogeneous void fraction is equal to 0.5.The heat transfer under Taylor flow in microchannels was quantified by the Graetz number.A correlation for predicting the heat transfer coefficient of Taylor flow in a three-sided heated rectangular channel is proposed,and the average prediction error of the correlation is 11.77%.The simulation results show that the diverging junction can shorten the circulation flow time,significantly reduce wall temperature and heat the fluid above the channel more quickly.When the slug length is long,the velocity distribution is similar to the classical Poiseulle flow pattern.As the decrease of the slug length,the secondary vortexes appear,and the smaller the slug length,the larger the area occupied by secondary vortices.The "leakage flow" in the square channel reduces the temperature at the corners of the channel,but the vortex formed at the end of the bubble prevents the "leakage flow" from flowing forward,resulting in its limited influence on heat transfer.New branch microchannels were designed to split single-channel Taylor flow into multichannel Taylor flow.The bubble splitting process,bubble velocity,flow pattern distribution,daughter bubble merge and heat transfer characteristics of Taylor flow in the branch channel were analyzed by flow and convective heat transfer experiments.The splitting process of the bubble in the branch channel was divided into the squeezing stage,the neck break stage and the Pinch-off stage.For the two-branch channel,only at high flow velocities will there be a significant velocity difference between the two sub-channels,resulting in a staggered distribution of Taylor bubbles.Finally,the sub-bubbles do not necessarily merge at the end of the channel.The bubble velocity of the middle sub-channel in the three-branch channel is significantly higher than that of the other two sub-channels so that the bubbles that merge at the end of the branch channel do not necessarily come from the same mother bubble.Branch channels can effectively improve the heat transfer capacity of microchannels under Taylor flow.Compared with the pure water flow and conventional Taylor flow,the average heat transfer coefficient in the two-branch channel is increased by 157%and 48.1%,respectively,and it is found that the void fraction on the heat transfer performance enhancement of the branch channel is weak.Liquid-liquid Taylor flow also plays an important role in the microfluidic system.In this paper,flow patterns,plug hydrodynamics and heat transfer characteristics of liquid-liquid twophase flow in a small circular channel were studied systematically based on the experiment and numerical simulation.Five distinct flow patterns were observed,including droplet flow,slug flow,throat-annular flow,annular flow and stratified flow,and the interaction between different internal forces in each flow pattern was analyzed with the flow pattern map.Under the liquidliquid Taylor flow pattern,three dimensionless parameters were defined to characterize the plug shape.It is found that as the Ca increases,the curvature radius of the plug head becomes smaller.In contrast,the curvature radius of the tail cap gets progressively larger.And the transformational boundary of the curvature of the plug tail cap becomes 0 is given.It is pointed out that the dtail*/dfront*satisfies a linear relationship with the Ca number,and the plug shape can be predicted when the Ca number is known.A new plug length prediction correlation was proposed,and the predicting results agree well with the literature data.The Nu number of liquidliquid Taylor flow increases with increasing Re number and decreases with the increase of dimensionless plug length.Optimizing the plug length can increase the Nu number by almost 26%.The numerical simulation results show that liquid-liquid Taylor flow with a small Pe number has an obvious diffusion effect,so the temperature field no longer has the obvious characteristics brought by internal circulation.A heat transfer model under liquid-liquid Tayor flow was established,and it was found that there is obvious heat transfer at the interface between the two ends of the plug. |
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