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剪切稀化流体内单气泡上浮特性的研究
论文作者:童鞋论文网  论文来源:www.txlunwenw.com  发布时间:2019/9/29 12:00:30  

摘要:气泡运动广泛存在于鼓泡塔和发酵反应器等工业设备中,气泡运动特性会影响设备内流体的持气率以及气泡间的相互作用,进而影响设备内气液介质间的传热和传质效率。目前学术界对剪切稀化流体中气泡运动特性的理解十分有限,为此本文使用数值和实验相结合的方法对剪切稀化流体中气泡的运动特性展开了深入的研究。首先利用数值模拟方法研究了不同壁面效应(气泡中心距壁面的距离)、重力水平(Ga数)、表面张力(Eo数)和剪切稀化效应(流变指数n)下,气泡形状、液相流场、气泡终端速度,以及气泡尾部黏度盲区的分布规律和成因机制;然后利用实验方法对牛顿流体(甘油溶液)和剪切稀化流体(卡波姆溶液)内不同体积气泡的形状和终端速度进行了研究。基于当前的研究,可以总结出如下结论:

(1)从不同壁面效应下气泡上升运动特性的数值研究发现,气泡形状、终端速度和液相流场与气泡中心距壁面的距离密切相关;距离越小,气泡变形以及尾涡形成越易受到抑制,壁面处越易出现引发局部液相表观黏度大幅度下降的高剪切速率区,进而导致气泡终端速度减小;当距离达到10倍气泡直径时,壁面效应对气泡运动特性的影响基本消失。

(2)基于对剪切稀化流体内重力水平和表面张力对气泡上升运动特性影响的数值研究发现,随着表面张力的减小,气泡形状会从椭圆形转变为球帽形,最后变为带裙边球帽状气泡,同时随着重力水平和剪切稀化程度的增大,气泡变形程度增加;随着表面张力的减小,或者重力水平和剪切稀化效应的增大,气泡尾部会出现尾涡,且尾涡尺寸随着表面张力进一步的减小而增大,随重力水平和剪切稀化效应进一步的增大而增大。

(3)基于对剪切稀化流体内重力水平和表面张力对气泡周围液相流场的数值研究发现,气泡周围液相表观黏度呈现梯度分布(距离气泡越近,液相表观黏度下降越剧烈),而在气泡尾部可能会出现黏度盲区;在气泡顶部和尾部会出现两个高剪切速率区域,高剪切速率区域的面积随剪切稀化效应、重力水平和表面张力的增大而增大。

(4)从不同重力水平和表面张力下对气泡终端速度的数值和实验研究发现,气泡终端速度随着重力水平和剪切稀化效应的增大而增大,而随着壁面效应和表面张力的减小而增大;在甘油(牛顿流体)和卡波姆溶液(剪切稀化流体)中,受气泡形状的影响,随着气泡体积的增大,小体积工况下气泡终端速度的增幅比大体积气泡的更大。

(5)基于对剪切稀化流体内壁面效应、重力水平和表面张力对气泡上升运动特性影响的数值研究发现,随着重力水平和剪切稀化效应的增强,壁面效应和表面张力的减弱,黏度盲区更易形成;黏度盲区与气泡底部的距离随重力水平和剪切稀化效应的增大而增大,随表面张力和壁面效应的减小而增大;气泡尾涡将黏度盲区切割成第一和第二黏度盲区,随着尾涡尺寸的增大,尾涡对黏度盲区的切割效应增强,导致黏度盲区的面积随之减小。

Bubble flow widely exists in industrialequipment such as bubble column and fermentation reactor. The dynamics ofbubbles will affect the gas holdup of equipment and the interaction betweenbubbles, and then affecting the heat and mass transfer efficiency. At present,the understanding of the dynamics of bubbles in shear-thinning fluids is as yetimperfectly understand in academia. Therefore, in this paper, numerical andexperimental methods are implemented to study the dynamics of bubble inshear-thinning fluids. Firstly, the bubble shape, flow field, bubble terminalvelocity, and the distribution and formation mechanism of the viscosity blindregion under different wall effects (distance between the bubble center and thewall), gravity level (Ga number), surface tension (Eo number) andshear-thinning effect (rheological index n) were studied by numericalsimulation. And then the shape and terminal velocity of bubble with differentvolume in Newtonian fluid (glycerol solution) and shear-thinning fluid (carbopolsolution) were studied experimentally. Based on current research, someconclusions can be summarized as follows:

(1)According to the numerical study on thedynamics of bubble under different wall effects, it is found that the bubbleshape, terminal velocity and flow field are closely related to the distancebetween the bubble center and the wall. The smaller the distance is, the bubbledeformation and formation of wake vortex are more easily suppressed. At thesame time high shear rate region will more easily to appear which causes asharply decrease of the local apparent viscosity of fluid on the wall, thesewill lead to the bubble terminal velocity to decrease; When the distancereaches 10 times the diameter of the bubble, the effect of wall effect on thedynamics of bubble disappears basically.

(2)Based on the numerical study of theeffects of gravity level and surface tension in shear-thinning fluids on thedynamics of bubble, it is found that with the decrease of surface tension, theshape of bubble will change from ellipse to spherical cap, and finally tospherical cap with skirt. At the same time, with the increase of gravity leveland shear thinning effect, the deformation degree of bubble will increase, andthe wake vortices will appear in the bubble wake. The size of wake vorticesincreases with the decrease of surface tension, and increases with the increaseof gravity level and shear thinning effect.

(3)Based on the numerical study of theeffects of gravity level and surface tension in shear-thinning fluid on theflow field around bubble, it is found that the apparent viscosity of fluidaround bubble presents a gradient distribution (the closer to the bubble, thegreater the decrease of apparent viscosity of liquid is), and the viscosityblind region will appear in the bubble wake; There are two high shear rateregions will appear in the top and wake of bubble. The area of high shear rateregion increases with the increase of shear thinning effect, gravity level andsurface tension.

(4)According to the numerical andexperimental studies of the terminal velocity of bubbles at different gravitylevels and surface tensions, it is found that the bubble terminal velocityincreases with the increase of gravity level and shear thinning effect, butincreases with the decrease of wall effect and surface tension; In glycerol(Newtonian fluid) and corbopol solutions (shear-thinning fluid), the terminalvelocity of bubbles increases more in small volume than in large volume withthe increase of bubble volume due to the influence of bubble shape.

(5)Based on the numerical study of theeffects of wall effect, gravity level and surface tension in shear-thinningfluids on the dynamics of bubble, it is found that with the increase of gravitylevel and shear thinning effect, the wall effect and surface tension decrease,and the viscosity blind region is easier to form.

关键词:剪切稀化流体;VOF方法;气泡上浮特性;黏度盲区;局部流场

Shear thinning fluid;VOF method;Bubbledynamics; Viscosity blind region;Local flow field

上一篇:轻介质旋流沉降分离过程的研究     下一篇:大直径塔式反应器密相装填过程的数值模拟及试验研究
 
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