【作者】 栾德玉；

【导师】 周慎杰；

【作者基本信息】 山东大学 ， 机械工程， 2012， 博士

【摘要】 许多过程工业都会涉及到高粘非牛顿流体的搅拌混合,这是搅拌技术的难点之一。具有一定初始屈服应力的假塑性流体是一种最常见的非牛顿流体,呈现出剪切稀化的流动特性,在层流搅拌时会产生洞穴效应,洞穴内部流体混合较好,而外部的流体则处于停滞或缓慢流动状态,这对流体混匀极为不利。另外,当流体粘度较大或者对剪切力敏感时,搅拌只能处于层流或较低雷诺数的过渡流状态,此时在叶片的上下方附近会形成环形的混合隔离区,隔离区内部介质只能依靠分子的扩散运动与外部实现传质过程,混合效果很不理想。为此,本文通过数值模拟方法,采用错位六弯叶涡轮搅拌器,对假塑性流体的宏观流场特征、洞穴变化规律以及混沌混合特性展开研究,为高效搅拌装置开发打下理论基础。通过流变仪测试了黄原胶水溶液(一种典型的假塑性流体)的流变学参数,分析了流体表观粘度与切应变速率的关系,探讨了质量浓度和温度等因素对流变参数的影响规律。黄原胶水溶液的表观粘度随切应变速率的增大而减小,表现出良好的剪切稀化性,表观粘度与切应变速率的关系符合Herschel-BuIkIey流变模型,流变指数随流体浓度升高呈下降趋势,受温度的影响较小,实验测试得到的流变参数为后续的模拟研究提供了依据。选用甘油进行了层流搅拌功率实验,用纯净水进行了PIV湍流实验,实验数据与相应的数值模拟结果均吻合较好,因此可以用CFD方法来预测流体的流动及混合特性,从而验证了本文所建立的层流和κ-ε湍流模型计算结果的可靠性。基于层流粘性模型,对错位桨和六弯叶桨在假塑性流体搅拌过程中的洞穴变化规律进行了研究,给出了较为适宜的洞穴边界速度的确定方法,分析了洞穴形状、直径以及高径比随表观雷诺数Re*的变化规律,描述了洞穴的发展过程。研究表明,洞穴形状随Re*的增大均呈现圆柱形的变化规律,符合洞穴圆柱形模型(EN模型)的描述；在双对数坐标中,Dc/D-NpRey的变化规律以及洞穴高径比的变化与假塑性流体流变指数无关。然而,洞穴高度的轴向扩展系数β明显不同,六弯叶桨β=0.55,错位桨β=0.79,错位桨搅拌更有利于消除流场洞穴效应。对错位桨和六弯叶桨在牛顿流体和假塑性流体中的搅拌功耗及搅拌槽内的压力分布进行了数值模拟研究,探讨了错位桨搅拌效果改善的机理,建立起错位桨功率常数K。和Metzner-Otto数Ks与搅拌桨几何尺寸的关联式。在层流区域内,由表观粘度法得到假塑性流体的功率曲线和牛顿流体的功率曲线是重合的,牛顿流体的临界雷诺数为10,假塑性流体为30；错位桨Metzner-Otto数Ks与流变指数n基本无关,随转速的升高,错位桨节能效果逐渐显现；在Coriolis力的作用下,错位桨搅拌槽内流体可形成较为强烈的旋流运动,这对搅拌效果的改善是极为有利的；利用K。和ks的关联式,可以较为准确地预测错位桨搅拌假塑性流体的功率消耗和槽内平均剪切特性。采用k-ε湍流模型,对错位桨、六弯叶桨及双层桨组合形式在假塑性流体搅拌中的流场特性和混合过程进行了对比研究,阐述了错位桨诱发混沌流动的特性,探讨了错位桨搅拌转速对流场的切应变速率、排出流率以及泵送效率的影响,分析了组合桨层间距对搅拌流场结构的影响,确定了在不同搅拌转速下较为适宜的层间距。研究表明,桨叶错位能够产生不对称流场结构,有效消除混合隔离区,诱发流体的整体混沌流。混合过程与流场结构密切相关,错位桨在混合速率和混合效率方面均高于六弯叶桨,其中错位桨的单位体积混合能只有六弯叶桨的52%,体现出它的优越性。底部为错位六弯叶桨、上层采用45°斜叶桨的组合形式,用于假塑性流体搅拌混合过程具有明显优势,并将该组合形式应用于中型FCC催化剂成胶搅拌过程,催化剂颗粒的耐磨性得到了较大提高,颗粒磨损指数由2.6提高到了1.9,产品质量得以提高,证明该组合形式可以满足催化剂成胶搅拌的需求,值得推广应用。更多还原

【Abstract】 Mixing of viscous non-Newtonian fluids are commomly encountered operation in many process industries, which is one of the difficulties for stirring technology. Pseudoplastic fluids possesing a yield stress are most common class of non-Newtonian fluids, exhibiting the shear-thinning viscosity property. Mixing of such fluids result in the formation of a well-mixed region (so called cavern) around the impeller surrounded by stagnant and/or slow-moving fluids elsewhere in the mixing vessel. These poor mixing conditions have been implicated in lower mass tranfer rates. Moreover, Mixing is often obliged to be conducted under low Reynolds number conditions due to the high viscosity fluids and shear-sensitiveness of materials. These low Reynolds number flow conditions may bring the isolated mixing regions (IMRs), which comprise two troidal vortices formed respectively above and below an impeller, in stirred tank. These regions exchange materials with the bulk of the flow only through diffusion, which is slow relative to convective processes. Therefore, the flow feature, cavern development and efficiently chaotic mixing performance of pseudoplastic fluids in a stirred tank equipped with the6PBT impeller are investigated by numerical simulation method in this paper. The research results promote the understanding of the mixing mechanism of pseudoplastic fluids, lay a theoretical basis and provide valuable instructions for the design of the efficient agitation equipments.The rheological measurements of xanthan gum solutions (a typical kind of pseudoplastic fluid possessing a yield stress) were carryed out by rheometer for analyzing the relationship between apparent viscosity and shear rate and the influence of mass concentration and temperature on the rheological parameters. It is found that apparent viscosity of the solutions decreases sharply as shear rate increases, which exhibits a good shear-thinning rheology, and the solutions rheology is well fitted to the Herschel-Bulkley rheological model. The flow behavior index of solutions decreases as mass concentration increases, but it almost independent of temperature variations. The resulting rheological data can provide a basis for numerical simulation later. The computed data of power consumption agree well with the measured data in experiment of stirring glycerine, and the numerical results of the flow macro structure and velocity distribution by k-ε turbulent model also agree well with PIV experimental data in water, which validated the CFD model developed in laminar flow and turbulent flow regime. These results show that CFD calcalations can pick up the features of flow fields and can be effectively used to predict the flow and mixing characteristics of fluids.Based on laminar viscosity model, cavern development during agitation of pseudoplastic fluids with the impeller of perturbed six-bent-bladed turbine (6PBT) and impeller of six-bent-bladed turbine (6BT) was studied by numerical simulation. The cavern boumdary velocity was properly determined and the cavern dimensions of shape, diameter and height to diameter ratio versus Reynolds number were analysed, in the meantime, the course of cavern development was also described. The results show that the cylindrical cavern development can be found as Reynolds number increases, which shows a good agreement with the representation of the cylindrical model (Elson s model). The slope from a log-log plot of Dc/D verse NpRey, as well as the variations of cavern height to diameter ratio, have nothing to do with the rheological behavior index of fluids all together. However, the rate constant of axial fluid expansion describing cavern height once the cavern touches the vessel base and wall,β, is not alike,β=0.55for6BT impeller,β=0.79for6PBT impeller, respectively, so6PBT impeller is more beneficial to eliminate the cavern effect in stirring pseudoplastic fluids.The power consumption and pressure distribution of Newtonian and pseudoplastic fluids in stirred tank with6PBT and6BT impeller were calculated by the numerical model established. The efficient agitation mechanism of6PBT impeller was made a thorough discussion, and the complete correlations of kp and ks to impeller geometry were established. The results show that, in the laminar flow regime, all the power consumption data obtained on the basis of apparent viscosity concept for various shear thinning fluids are in agreement with those for Newtonian fluids, the critical Reynolds number of Newtonian and pseudoplastic fluids in laminar flow regime is10and30, respectively. In addition, there is no correlation between ks and rheological behavior index, n, on the whole, and6PBT impeller can lead to a saved energy as rotating speed increases. The stronger swirling motion may emerge in stirred tank with6PBT impeller under the action of Coriolis force, which is exceedingly advantageous to promotion of mixing efficiency. Moreover, the power consumption and average shear rate in stirring pseudoplastic fluids may be accurately predicted by the correlations of kp and ks, comparatively.The flow fields and mixing characteristics of pseudoplastic fluids in stirred tank with6PBT impeller,6BT impeller and dual-impeller combination were numerically investigated by standard k-ε turbulent model, respectively. The properties of the chaotic flow induced by6PBT impeller were made a thorough inquiry, and the influences of the rotating speed with6PBT impeller on shear rate, flow number and pumping capacity of fluids were also made a detailed discussion. Moreover, the influences of the different layer clearances with dual-impeller combination on the flow structure were analyzed, the proper layer clearance at different speed was determined in the meantime. The results show that, compared with6BT impeller,6PBT impeller can destroy the isolated mixing regions with its dissymmetrical structure of flow field and induce the chaotic flow in whole vessel. The tracer mixing process is close dependent on the flow field structure,6PBT impeller has a major superiority on mixing rate and mixing effiency to6BT impeller, in which, concerning the mixing energy per unit volume,6PBT impeller merely accountes for a52%proportion of6BT impeller. Besides, the dual-impeller combination of6PBT impeller as lower impeller and impeller of three-pitched-bladed turbine as upper one is discovered to have a clear superiority on mixing process of pseudoplastic fluids. At the end, this kind of dual-impeller combination is applied to the medium-sized trial-production stirring process of FCC catalyst gum formation on-the-spot. The wearability of catalyst particle is greatly improved with the increase of particle wear index from2.6to1.9, and the product reaches the high quality. It is concluded that the dual-impeller combination can meet the productive demands in agitation of catalyst gum formation, so is worth application and dissemination.更多还原