微细圆管及扁平管内液-液Taylor流动特性的数值研究

微细通道内Taylor流动广泛应用于能源化工领域,为分析其相界面及阻力特性,利用相对坐标系的方法,研究了竖直圆管及扁平管内的液-液Taylor流动,讨论了通道宽高比、Reynolds数（Re）及分散相体积分数对液膜厚度和两相压降的影响。结果表明：圆管内液滴头部和尾部可以膨胀至近似球形,而扁平管内壁面的限制作用较强,液滴呈现扁平状。随Reynolds数增大,两相界面逐渐收缩,液膜厚度逐渐上升。圆管内液膜厚度比较均匀,扁平管内液膜在通道顶部较薄,而圆弧部分较厚。两相压降随Re和宽高比的增大而增大,随分散相体积分数的增大而降低。相比连续相和分散相压降,界面压降所占的比重最高,并依据模拟结果,提出了圆管及扁平管内液-液Taylor流动的压降预测公式。     

凹穴型微通道液-液两相流动特性

为了研究凹穴型结构对微通道内液-液两相流动特性的影响,利用高速摄像机,结合边缘检测算法(Canny),获得两相流型、液滴长度和膨胀面积等参数。结果表明:基于无量纲液滴长度参数,凹穴微通道内流型主要为膨胀流、过渡流及滴状流。随两相体积流量的增大,液滴长度逐渐减小,流型由膨胀流逐渐转换为过渡流,当液滴长度小于通道宽度时,凹穴结构对液滴形状的影响可以忽略。凹穴型通道内的液滴长度呈周期性变化,长度变化振幅为平直段的3～5.6倍。无量纲液滴膨胀面积随离散相与连续相流量之比的增大逐渐增大,随连续相毛细管数增大而逐渐降低。     

大深宽比双T形微通道内液-液两相流可视化研究

通过高速摄像对水力直径0.176mm、深宽比2.4的双T形矩形微通道内的液-液两相流动进行了可视化实验研究。改变连续相（硅油）和分散相（水）的流量比,记录分析了微通道不同部位油-水两相流的流型和流型发展演化情况。实验结果表明,在微通道上游T形部位的油-水两相流型主要包括滴状流、弹状流、波平行流和平行流;在微通道的中间部位,绘制了基于水和硅油量纲为1韦伯数的流型图,并将其与相关文献进行了比较。同时,发现微通道内液塞及液滴的长度（量纲为1）与油/水流量比之间存在线性关系,液塞/液滴速度比两相混合物表观速度大,建立了能够准确描述液塞/液滴运动速度的实验拟合公式。最后,研究了液滴在微通道下游T形部位的行为,观察到断裂和不断裂两种模式并进行了分析,给出了划分断裂与否的流型图。     

微通道液-液两相流动特性实验研究

利用高速摄像机与Canny算法,以硅油为离散相,含0. 5%SDS的蒸馏水为连续相,研究了凹穴型微通道内液-液两相流动特性。结果表明,直通道内观察弹状流、过渡流、滴状流3种流型。随着毛细数的增大,液滴形成机理由挤压机制向剪切机制转变,液滴速度逐渐增大,液滴长度逐渐减小。随着连续相流量的增加,液滴形成时间逐渐减小,且挤压机制生成液滴的时间大于剪切机制。凹穴结构减弱了壁面对液滴的限制,液滴速度降低,T型交汇处压差降低,相同工况下的液滴尺寸大于对冲T型微通道的液滴尺寸。     

微小通道内气液两相流数值模拟

基于CFD流体动力学计算软件FLUENT模拟了微小通道中气液两相流流动与换热,分析了截面方管与截面圆管内不同含气率下两相流的换热特性;对比微小通道内气液两相流的温度及含气率变化,分析得出同等条件下方管的换热特性更好。     

T型微通道内液-液两相流流型研究

论文建立了T型微通道内液-液两相流动理论模型并采用格子Boltzmann方法进行了数值模拟,研究了T型微通道内液-液两相流型形成机制及其影响因素。采用已有文献中的实验数据验证了模型的合理性。研究结果表明,T型微通道液-液两相流流型主要由两相入口流量比和离散相We数决定,通过调整入口流量比和离散相We数,可获得弹状流、离散液滴、平行流等经典流型。离散相的相对长度受连续相毛细数Ca影响明显。当连续相毛细数较大,离散相形成液滴,离散相相对长度随连续相毛细数Ca呈单调减少趋势。     

微通道内液-液两相流研究进展

从流型、传质和应用3个方面,介绍了近年来微通道内液-液两相流的研究进展。液-液两相流型研究内容主要有两部分,即流型的观察、流型谱图的绘制以及考察多种因素对流型的影响,但是具有普适性的流型谱图和流型转变线仍未提出。液-液两相传质研究方法包括实验研究和数值模拟两种,主要研究在液滴流、弹状流和平行流3种稳定流型下的传质过程;且相对于定性研究,定量的传质研究较少。对于微通道内液-液两相流应用研究,主要体现在萃取、材料合成、生物结晶等方面。此外,对今后微通道内液-液两相的流型、传质和应用研究进行了展望,指出需从实验与模拟计算相结合以及拓展微通道内液-液两相流的应用研究两个方面进行深入研究,推进其工业化进程。     

微通道内液液两相流压力降的测量和关联

微通道内液液两相流的压力降对系统内部热量和质量传递具有重要影响。针对环己烷-含0.3%SDS(十二烷基硫酸钠)蒸馏水液液两相系统,利用高速摄像仪对2种不同深宽比的矩形截面直管微通道内的液液两相流进行了实时观测和记录,用压差变送器测定了其在弹状流型下的压力降。微通道尺寸(深度×宽度)分别为400μm×600μm,400μm×800μm。结果表明:弹状流型下的压力降随系统各相流率、毛细数、雷诺数、连续相黏度的增加而增加,随两相速度比值的增加而减小,且当毛细数Ca>0.015或雷诺数Re>20时,压力降随着通道截面深宽比的增加而增加。基于实验结果,修正了均相流模型,提出了新的压力降预测关联式,模型计算结果与实验值吻合良好。     

正弦型微通道内液-液两相流型及流动特性实验研究

采用实验的方法对不混溶的液液两相流体在不同入口结构下的正弦微通道(直通道正弦、波峰正弦和波中正弦)内液滴的流动特性进行了分析。硅油作为离散相,含有0.5% SDS的蒸馏水作为连续相,观测到弹状流、滴状流和射状流。分析了两相流动参数及不同的微通道入口结构对流型和液滴长度的影响。流型受微通道入口结构影响较大,波峰正弦微通道能够生成最大范围的稳定的流型。液滴长度随离散相体积流量和离散相与连续相体积流量之比的增大而增大,随连续相的体积流量和毛细数的增大而降低。微通道入口结构对液滴长度有影响,直通道的正弦微通道内液滴长度最短,更有利于液滴的形成。三种通道生成的液滴中,最大的液滴尺寸是最小的液滴尺寸的1.15~1.39倍,但正弦流动段对液滴速度几乎没有影响。     

微通道内气液(液液)二相流的实验研究进展

综述了微通道内气液(液液)二相流的流型特征.微通道内气液二相流常见的流型为泡状流、弹状流、环状流和翻腾流;液液二相流常见的流型为液滴流、塞状流、平行流及环状流.分析了不同操作条件对气液(液液)二相流行为的影响.介绍了微通道内气液(液液)二相流流型判别谱图,对常用的弹状流、液滴流和塞状流进行了重点介绍.指出了微通道内气液...     

定热通量加热下微三角形槽道中滑移流动的换热特性

微三角形截面通道是现代工程实际应用中常涉及到的流动通道.针对微三角形槽道利用正交函数法求解了滑移流区内带温度跳跃边界条件的能量方程,对不可压缩气体在微三角形槽道内充分发展层流滑移流动的换热特性进行了理论分析,获得了轴向定热通量加热、周向均匀壁面温度条件下微三角形槽道内的温度分布和换热特性的分析解.计算结果表明：正交函数法适用于微三角形槽道内滑移流动换热特性的分析计算;在滑移流区,微三角形槽道内的平均Nusselt数随Knudsen数的增加而减小,其随高宽比变化的分布曲线随Knudsen数的增加而平行下移,Nusselt数比随Knudsen数的变化关系基本不受高宽比的影响.     

Effect of wall temperature modulation on the heat transfer characteristics of droplet-train flow inside a rectangular microchannel

The numerical studies of water–oil two-phase slug flow inside a two-dimensional vertical microchannel subjected to modulated wall temperature boundary conditions have been discussed in the present paper. Many researchers have contributed their efforts in exploring the characteristics of Taylor flows inside microchannel under constant wall heat flux or isothermal wall conditions. However, there is no study available in the literature which discusses the impact of modulated thermal wall boundary conditions on the heat transfer behavior of slug flows inside microchannels. Hence, to bridge this gap, an effort has been made to understand the heat transfer characteristics of the flow under sinusoidal wall temperature conditions. Initially, a single phase flow and heat transfer study was performed in microchannels, and the results of the fully developed velocity profile and heat transfer rate were validated with benchmark analytical results. Then an optimal selection of the combination of sinusoidal thermal wall boundary conditions has been made for the two-phase slug flow study. Later, the effects of amplitude(0 b ε b 0.03) and frequency(0 b ω b 750π rad·s~(-1)) of the sinusoidal wall temperature profile on the heat transfer have been studied using the optimal combination of the wall boundary conditions. The results of the numerical study using modulated temperature conditions on channel walls showed a significant improvement in the heat transfer over liquid-only flow by approximately 50% as well as over two-phase flow without wall temperature modulation. The non-dimensional temperature contours obtained for different cases of temperature modulation clearly explain the root cause of such improvement in the heat transfer. Besides,the results based on the hydrodynamics of the flow have also been reported in terms of variation of droplet shapes and film thickness. The influence of Capillary number on the film thickness as well as heat transfer rates has also been discussed. In addition, the measured film thickness has also been compared with that calculated using standard empirical and analytical models available in the literature. The heat transfer rate obtained from the numerical study for the case of unmodulated wall temperature was found to be in a close match with a phenomenological model to evaluate slug flow heat transfer having a mean absolute deviation of 7.56%.     

微通道内气液两相流行为研究进展

综述了微通道内的气液两相流行为及传质特性。在微通道内流型一般分为泡状流、弹状流、环状流和弹状-环状流,没有分层流。气液传质效率比常规尺度中的提高了2～3个数量级。讨论了气泡对气液两相流的影响及其生成、生长和聚并规律。介绍了微通道内气液两相流的计算机模拟结果。从实验、理论和数值模拟3个方面对微通道内气液两相流的研究和应用前景进行了展望。     

宽矩形硅微通道中流动冷凝的流型

对水力直径90.6 μm、宽深比9.668的矩形硅微通道中的流动冷凝过程进行了可视化研究。研究发现,宽矩形硅微通道中的冷凝,沿程主要有珠状-环状复合流、喷射流和弹状-泡状流等流型。在珠状-环状复合流区,冷凝液膜可覆盖通道竖直侧壁,而在通道长边上,仍然为珠状凝结。喷射流位置随着入口蒸气Reynolds数的增大而延后,通道截面形状对流动冷凝不稳定性也存在很大影响。喷射流之后为弹状-泡状流,弹状气泡沿程逐渐缩短,并在表面张力的作用下收缩成圆球形气泡。冷凝通道的平均传热系数将随着入口蒸气Reynolds数的增大而增大。     

Gas-liquid-liquid slug flow and mass transfer in hydrophilic and hydrophobic microreactors

Gas-liquid-liquid three-phase slug flow was generated in both hydrophilic and hydrophobic microreactors with double T-junctions. The bubble-droplet relative movement and the local mass transfer within the continuous slug and the dispersed droplet were investigated. It was found that bubbles moved faster than droplets under low capillary number (Ca), while droplets moved faster upon the increase of Ca due to the increased inertia. For the first time, we observed that the increased viscosity of droplets fastened the droplet movement. The mass transfer in the continuous slug was dominated by convection, leading to nearly constant global mass transfer coefficient (k_La); while that in the dispersed droplet was dominated by diffusion, resulting in k_L decreasing along the channel. Such features are analogical to the corresponding gas-liquid or liquid-liquid two-phase slug flow, but the formation of bubble-droplet clusters caused by relative movement lowered the absolute mass transfer coefficient. These results provide insights for the precise manipulation of gas-liquid-liquid slug flow in microreactors towards process optimization.     

Hydrothermal performance analysis of various surface roughness configurations in trapezoidal microchannels at slip flow regime

The effects of various surface roughness geometrical properties including roughness height (5%, 10%, 15%), number (3, 6), and shape (rectangular and triangular) on the flow and heat transfer of slip-flow in trapezoidal microchannels were investigated. The effects of mentioned parameters on the heat transfer coefficient through the microchannel, average Nusselt number and pressure drop for Reynolds number of 5, 10, 15 and 20 were examined. The obtained results showed that increasing the roughness height and number increases the pressure drop due to higher stagnation effects before and after roughness elements and decreases the Nusselt number due to higher recirculation zones effects than obstruction effects. The most reduction in Nusselt number and the most increment in pressure drop occur at the roughness height of 15%, roughness number of 6 and Reynolds number of 20 by about 10.6% and 52.8% than the smooth microchannel respectively.     

Effects of roughness elements on laminar flow and heat transfer in microchannels

A model of laminar flow and heat transfer in rough microchannels is developed and analyzed numerically to compare the effect of roughness elements on the thermal and hydrodynamic characteristics. In this model, the rough surfaces are configured with triangular, rectangular and semicircular roughness elements, respectively. Here, the effects of the Reynolds number, roughness height, and roughness element spacing on pressure drop and heat transfer in rough microchannels are all investigated and discussed. The results indicate that the global heat transfer performance is improved by the roughness elements at the expense of pressure head when compared to the smooth channel. Differing from the smooth microchannels, both the Poiseuille number and average Nusselt number of rough microchannels are no longer constant with Reynolds numbers and are larger than the classical value. Especially, the difference from the effects of three types of roughness elements is identified. With the increasing roughness height, the flow over surfaces with semicircular and triangular roughness elements induces stronger recirculation and flow separation. This contributes to heat transfer enhancement but also increases the pressure drop. However, the influence of the rectangular roughness element height is weaker than that of semicircular and triangular elements. In addition, the effects of the spacing of roughness elements on the Poiseuille number and average Nusselt number are in decreasing order for semicircular, triangular and rectangular roughness elements, respectively.