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球形气泡界面浓度对其界面参数和上浮运动特性影响研究
论文作者:童鞋论文网  论文来源:www.txlunwenw.com  发布时间:2019/9/29 12:06:33  

摘要:泡状流动广泛存在于矿物浮选、热交换和化学反应等诸多工业过程,在这些过程中液相或多或少会受到污染物的污染,从而使气泡界面也随之受到污染。当气泡受污染后,其水动力学特性的变化会改变泡状流的流动结构、以及气-液交界面的传热和传质性能,并由此对实际工业过程产生影响,因此对受污染气泡的水动力学特性进行研究具有重要的现实意义。鉴于目前有关受污染气泡水动力学特性的研究(包括机理解释)尚存在一定缺陷,本文将表面活性剂作为污染源,采用理论分析、数值模拟和实验相结合的方法,对受污染气泡的界面参数和上浮运动特性进行了深入研究。

对于理论分析,为描述较大气泡雷诺数(Reb > 1)下气泡界面污染物浓度的分布情况,基于Langmuir吸附动力学方程和非简化停滞帽模型,给出了动态平衡后帽角随气泡终端上浮速度和气泡直径的变化规律、停滞角和停滞帽中界面浓度关于溶液平均浓度的表达式、部分污染界面上污染物浓度分布的表达式,以及动态平衡前停滞角关于气泡上浮时间的表达式,并对停滞帽中界面浓度随气泡上浮随时间的变化进行了讨论。

为了在理论分析的基础上进一步研究受污染气泡的界面参数及其周围流场,提出了一种数值模拟受污染界面的处理方法。该方法假设吸附于气泡界面的污染物分布在紧邻气泡界面的浓度边界层中,且气泡界面与流域间的传质过程也发生于此。为此,界面浓度输运方程仅在浓度边界层中求解,动量方程和流域浓度输运方程于整个流域中求解,求解后气泡界面污染物浓度的具体值源于紧邻气泡界面第一层网格单元上的值。随后,使用该方法对不同气泡雷诺数下受污染气泡的界面参数及其周围流场进行了数值研究。研究发现,该方法下的界面浓度和界面切向速度与停滞帽模型相比更接近于直接数值模拟时的结果;气泡受污染后其界面切向速度的降低会增大受污染界面前缘附近,流场中切向速度在周向和法向上的变化率,从而使界面切应力和界面涡量在受污染界面前缘界面浓度分布的拐点处出现一个峰值,峰值的大小与前缘位置距离气泡界面最大对流处的远近有关;界面涡量的产生会增大气泡尾部流域中涡量场的强度和影响范围,从而增大该流域中逆压梯度区与气泡界面间的距离,并与界面涡量(包括均值和最大值)共同对尾涡尺寸产生影响。

为了对数值研究内容进行补充,实验研究了表面活性剂的电离性以及溶液极性,对气泡变形程度、上浮速度和上浮轨迹的影响。研究发现,极性溶液中非离子型表面活性剂对气泡上述运动特性的影响略强于离子型表面活性剂,且非极性溶液中表面活性剂的加入不会对气泡的上浮运动特性产生宏观影响。结合数值模拟结果可知,气泡上浮速度的减小,源于其界面上沿流向切应力与静压均值的增加,所导致的阻力系数的增加;气泡宽高比的减小,源于其界面上沿流向切应力与静压均值之比的增加,所导致的其尾部射流强度的减弱;气泡横向迁移距离的减小,源于上述界面参数的改变而导致的气泡所受升力的减小。

Bubbly flows exist widely in manyindustrial processes, such as froth flotation, heat transfer, chemical reactionand so on. In these processes, the liquid is inevitably contaminated bycontaminants. For the contaminated bubble, the variations of their dynamicproperties are sure to affect the structures of bubbly flows as well as thenatures of heat and mass transfer at the interface. Therefore, deeperinvestigations are needed on the dynamic properties of contaminated bubbles fortheir actual meanings. Here, intensive studies are performed on the interfacialcharacteristic parameters and the rising dynamics of a contaminated sphericalbubble via theoretical analysis, numerical simulation and experiment,respectively. Here, the surfactants are taken as the contaminant in thefollowing researches.

For the theoretical analysis, the Langmuiradsorption kinetic equation and the non-simplified stagnant cap model wereused, in order to describe the concentration distribution of contaminants onthe bubble surface at finite Reynolds numbers (Reb > 1). Then, thevariations of the cap angle with the terminal velocity and the diameter of arising bubble were discussed; the formulas of the stagnant angle and theinterfacial concentration on the rear stagnant cap about the bulk contaminantconcentration after the dynamic balance were proposed. Additionally, the timeevolution of the stagnant cap was evaluated. Moreover, according to theinstantaneous bubble rising velocity, the interfacial concentration on the rearstagnant cap against time was discussed as well.

For the numerical simulation, a treatmentfor the contaminated interface was proposed, in order to further study theinterfacial characteristic parameters and the surrounding flow field of acontaminated bubble on the basis of theoretical analyses. As for the presentnumerical treatment, the contaminants adsorbed on the bubble surface wereassumed to be distributed over the concentration boundary layer adjacent to thebubble surface. Additionally, the mass transfer processes between the bubblesurface and the bulk around it were also supposed to be existed in theconcentration boundary layer. Then, the transport equation of interfacialcontaminant concentration was solely solved in the concentration boundarylayer. The concentration on the bubble surface was involved in the transportequation of interfacial contaminant concentration, and its specific value wasexpressed by the value on the first layer grid adjacent to the bubble surface.The current results show that the retardation of the interface increases thevariation rate of the tangential velocity both in the circumferential directionat the interface and in the normal direction near the interface. The abovephenomena result in the formation of a peak point on the curves of interfacialtangential velocity and interfacial vorticity, respectively. The values ontheir peak points are closely related the location of their peak points to thestrongest convection point in the bulk around the bubble. Moreover, the generationof interfacial vorticity enhances the strength and the incidence of vorticityin the flow field behind the bubble.

As a supplement to the numerical study,experimental investigations were carried on about the influence of surfactantionizing and solution polarity on bubble rising velocity, deformation andtrajectory. It is found the surfactant hardly affect the rising dynamics of abubble when the bubble is rising in nonpolar solutions. After the combinationof experimental and numerical simulations, it can be drawn that the decrease ofbubble rising velocity originates from the increase of average values oftangential stress and pressure along the streamwise direction on the bubblesurface. Then, the decrease of bubble deformation results from the weakness ofvortex strength which is related to the pressure along the streamwise directionon the bubble surface.

关键词:受污染气泡;界面特征参数;流场分析;上浮运动特性

contaminated bubble; interfacialcharacteristic parameters; flow field analyses; rising dynamic

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