高粘度流体二维搅拌流场

用有限元法求解二维搅拌流动的Navier-Stokes方程。采用极坐标系,引用罚函数法处理流体不可压缩条件,并消去压力项,成功地计算了锚式桨搅拌槽二维流场的速度分布、流型以及剪切率分布。计算的流型与实验拍摄的结果符合良好,还对不同Re数下的搅拌流场进行了数值模拟,并描述了典型Re数下的计算结果及Re数对流场的影响。     

锚式搅拌槽中高粘弹性流体的流速分布及其功率消耗

用照相法测定了锚式搅拌槽中高粘弹性流体的流型和流速分布,另测定了搅拌功率消耗,结果发现:1.与牛顿流体相比,在低Re数下,粘弹性流体的切向速度较大,而径向速度则较小.2.转速相同时,在高剪切率区域,粘弹性流体的剪切率大于牛顿流体.由CEF方程导出功率计算式N_pRe_af_s~(1-n)=k_pf_vf_s~2[1+F_1avf_s~(m-n-3)Wi/K_s~2]用实验数据确定f_(?)和F_(1av),得到可适用于牛顿流体、假塑性流体和粘弹性流体的普适功率计算式,计算结果与实验值比较接近.     

锚式搅拌槽中高粘弹性流体的流速分布及其功率消耗

用照相法测定了锚式搅拌槽中高粘弹性流体的流型和流速分布,另测定了搅拌功率消耗,结果发现:1.与牛顿流体相比,在低Re数下,粘弹性流体的切向速度较大,而径向速度则较小.2.转速相同时,在高剪切率区域,粘弹性流体的剪切率大于牛顿流体.由CEF方程导出功率计算式N_pRe_af_s~(1-n)=k_pf_vf_s~2[1+F_1avf_s~(m-n-3)Wi/K_s~2]用实验数据确定f_(?)和F_(1av),得到可适用于牛顿流体、假塑性流体和粘弹性流体的普适功率计算式,计算结果与实验值比较接近.     

搅拌槽中高粘流体的速度分布

本文使用示踪粒子照相法对双螺带、框-螺带-锚、交叉桨式和MIG-锚搅拌器,测定了搅拌槽中高粘流体的流型和速度分布,用单个悬浮粒子测定了循环时问。结果表明在槽水平断面上,流体主要是切向流动,径向流动很弱,轴向循环流量系数N_q的大小显著地依赖于轴向泵送流体的叶片面积的大小。双螺带搅拌器的轴向流动速度和循环流量系数大于其它三种搅拌器。框-螺带-锚搅拌器的切向速度最大,而轴向速度很小,全槽范围的上下循环流动很弱。交叉桨叶的轴向速度在某些径向位置接近于零。本文结合速度分布讨论了高粘流体的混合机理和搅拌器结构对混合速度的影响。搅拌槽中流体的流型和速度分布信息对解析槽中流体的温度和浓度分布是非常重要的。一只高混合效率搅拌器,应该具有较强的剪切性能和良好的轴向循环性能。以往有关搅拌槽中流体力学的研究绝大多数是针对搅拌功率消耗、混合和循环时间,而流型和速度分布的研究只有少量报道。本文研究了四种搅拌器的流型和速度分布,结合早先的研究结果,评价了它们的剪切、混合和循环性能以及搅拌器结构的合理性,并为化工生产中选择和改进搅拌器提供依据。     

折叶涡轮桨搅拌器流型及温度分布数值模拟

采用MIXSIM对折叶涡轮桨搅拌器搅拌槽内流体流型及紊流状态下的温度分布进行模拟.结果显示:随Re数的增大,流体流型由径向流逐渐向轴向流发展;在温度变化趋于稳定时,温度在搅拌轴两侧基本呈对称分布,且分别存在一个低温区域.     

桨叶型式对中心龙卷流型搅拌槽内流动场的影响

采用计算流体力学(CFD)对中心龙卷流型搅拌槽内部流场进行了数值模拟,从速度、压力、湍动能、功耗等方面研究三种不同的搅拌桨叶对搅拌槽工作性能的影响。研究结果表明:6PBDT在整个搅拌槽内速度分布相对较均匀;6SBDT在轴截面处的压力分布较为均匀,湍动能分布面积相对较大;在相同条件下,6SBDT所消耗的功率大,即槽内流体的混合性能最好。     

搅拌槽内三相混合特性的数值模拟

利用Ansys CFX软件对自吸式龙卷流型搅拌槽内的气、液、固三相混合特性进行了研究,从速度场、固相分布、湍动能、湍动能耗散及切应变速率等方面对其进行数值模拟,并与标准搅拌槽进行对比发现:自吸式龙卷流型搅拌槽具有良好的速度分布和固液悬浮性能,其湍动能和湍动能耗散分布合理,有利于物料之间的充分接触与混合,且其主要混合区域的剪切力小、功耗低,对介质的损伤小,节能效果明显。     

基于深度学习的气液固三相反应器图像分析方法及应用

气液固三相反应器中复杂的颗粒背景给流动参数的图像检测带来巨大挑战。发展了一种基于深度学习的气液固三相反应器图像分析方法,包括采集图像、制作训练集、建立图像识别模型和提取流动参数四个步骤。采用全卷积神经网络,在学习率为0.005、训练次数为2000次、训练集大小超过400张图像的条件下,图像识别误差小于5%。利用该方法可以获取三相反应器中局部相含率（气相分数和液相分数）及其空间分布、时间序列等信息,再采用时域分析、频率分析、小波分析等分析方法提取二次参数,可用于流型识别、压降预测和气液分布的均匀性判别等。将该方法用于涓流床中流动参数的检测,结果表明,局部液相分数的时间序列信号及其功率谱、概率密度分布均能清晰地区分涓流、脉冲流、鼓泡流等典型流型;时间序列信号的均值、标准差、极差和概率密度分布曲线半峰宽等特征参数可用于确定流型边界;平均液相分数可以用于预测涓流区的压降,计算值与实验测量值的平均相对偏差约为15%;液相分数空间分布的标准差可用于表征涓流床中不同流型的气液分布均匀性。该方法为气液固三相反应器的研究提供了新的工具。     

三相循环流化床中气泡大小及其分布的实验研究

用光纤探头技术对三相循环流化床中的气泡大小及其分布进行了系统研究 ,实验测定了操作条件对气泡大小及其分布的影响规律 .实验结果表明 ,三相循环流化床中气泡的大小分布可用对数正态分布表征 ,在实验条件下气泡平均直径在床中心区域较小且沿半径方向由中心向边壁逐渐增大 ,并随表观气速的增大而减小 ,随固含率的增大而增大 ,表观液速对气泡平均直径的影响较小     

入口流速对间断微通道内流型演变及流量分配特性影响

微通道内气液两相流动规律是影响微通道换热器换热系数和流场温度均匀性的主要因素。以N2和H2O为工质，对间断、并联矩形微通道换热器内气液两相流型的起始、发展、稳定过程的演化以及并联通道内流量分配不均匀特性进行了数值模拟研究。结果表明，不同的进口Re对微通道内流型的演变过程和流动周期有重要影响，当进口Re为450时，气相工质在均流腔内以离散的散团状形态脉动扩散至微通道中，并联通道内气相工质从弹状流型态逐渐转变为泡状流型态；当进口Re增至为1600时，气相工质在均流腔内以较连续的椭圆状型态扩散至微通道中，并联通道内气相工质从环状流型态逐渐转变为泡状流型态。通道结构还将影响并联通道间的流量分配的均匀性，间断微腔的存在使微通道内工质质量流量分布均匀性提升38.7%，通过研究通道内压力分布规律，发现通道内静压的分布不均匀是导致两相工质从均流腔进入微通道时发生不均匀分配的重要原因。     

最大叶片式桨流体动力学性能的数值模拟

运用CFD方法研究了最大叶片式桨在层流区域内的流体动力学性能,模拟体系为高粘度牛顿流体和非牛顿流体,主要考察桨叶的功耗特性、Metzner常数、剪切性能和排液性能. 结果表明,功耗计算值与文献实验值基本一致;桨叶的Metzner常数ks=10与流变行为指数n无关;搅拌形成双循环流型,釜中部桨叶所在区剪切速率大、排液量大,产生的漩涡流也大,导致剪切效率低于0.5;随雷诺数增加(Ren=1.4, 5.0, 7.6),全釜平均剪切速率(3.40, 9.91, 15.05 s-1)和全釜平均排液量(0.0014, 0.0033, 0.0052 m3/s)逐渐增加,尤其是桨叶下端两翼区平均剪切速率(4.36, 11.48, 16.35 s-1)和平均排液量(0.0026, 0.0064, 0.0095 m3/s)增加相对较大. 说明Ren增加,搅拌混合作用加强,剪切速率大产生的界面积大,排液量大使高低剪切区内流体快速循环,有利于流体高效混合.     

流体流变性对错位六弯叶桨流场特性的影响

利用计算流体力学(CFD)的方法,以高黏牛顿流体和假塑性非牛顿流体为研究对象,对错位六弯叶桨在层流域的搅拌流场特性进行了研究。结果表明:数值计算得到的功率值与实验测量值吻合较好,搅拌雷诺数对假塑性流体搅拌流场的量纲一速度和切应变速率影响较大,因此提高转速对改变流场的速度及切应变速率分布是一个有效的办法;而流体的流变性只对假塑性流体的量纲一速度有明显影响,对切应变速率影响较小,当流变指数n<0.3时与n基本无关。当流动由层流向过渡流转变时,搅拌桨的流量准数及泵送效率有显著提高;假塑性流体的流变指数降低时,其泵送效率显著下降。研究进一步认识了错位桨在不同流体中的搅拌流场特点,为高黏假塑性流体搅拌桨的设计、应用以及开发新型搅拌桨提供了参考。     

Energy Dissipation Rates of Newtonian and Non‐Newtonian Fluids in a Stirred Vessel

Energy dissipation rates of water and glycerol as Newtonian fluids and carboxyl methyl carbonate solution as non‐Newtonian fluid in a stirred vessel are investigated by 2D particle image velocimetry and compared. Mean velocity profiles reflect the Reynolds (Re) number similarity of two flow fields with different rheological properties, but the root mean square velocity profiles differ in rheology at the same Re‐number. Energy dissipation rates are estimated by direct calculation of fluctuating velocity gradients. The varying energy dissipation rates of Newtonian and non‐Newtonian fluids result from the difference in fluid rheology and apparent viscosity distribution which decides largely the flow pattern, circulation intensity, and rate of turbulence generation.     

Velocity profiles and frictional pressure drop for shear thinning materials in lid-driven cavities with fully developed axial flow

A finite element numerical study has been carried out on the isothermal flow of power law fluids in lid-driven cavities with axial throughflow. The effects of the tangential flow Reynolds number (Re_U), axial flow Reynolds number (Re_W), cavity aspect ratio and shear thinning property of the fluids on tangential and axial velocity distributions and the frictional pressure drop are studied. Where comparison is possible, very good agreement is found between current numerical results and published asymptotic and numerical results. For shear thinning materials in long thin cavities in the tangential flow dominated flow regime, the numerical results show that the frictional pressure drop lies between two extreme conditions, namely the results for duct flow and analytical results from lubrication theory. For shear thinning materials in a lid-driven cavity, the interaction between the tangential flow and axial flow is very complex because the flow is dependent on the flow Reynolds numbers and the ratio of the average axial velocity and the lid velocity. For both Newtonian and shear thinning fluids, the axial velocity peak is shifted and the frictional pressure drop is increased with increasing tangential flow Reynolds number. The results are highly relevant to industrial devices such as screw extruders and scraped surface heat exchangers.     

Numerical simulation of fiber orientation distribution in round turbulent jet of fiber suspension

The Reynolds averaged Navier–Stokes equation was solved numerically with the Reynolds stress model to get the mean fluid velocity and the turbulent kinetic energy in a round turbulent jet of fiber suspension. The fluctuating fluid velocity was described as a Fourier series with random coefficients. Then the slender-body theory was used to calculate the fiber orientation distribution and orientation tensor. Numerical results of mean axial velocity and turbulent shear stress along the lateral direction were validated by comparing with the experimental ones. The results show that most fibers are aligned with the flow direction as they go downstream, and fibers are more aligned with the flow direction within the region near the jet core. The fibers with high aspect ratio tend much easier to align with the flow direction, and the fiber orientation distribution is not sensitive to fiber aspect ratio when fiber aspect ratio is larger than 5. Fiber density has no obvious influence on the fiber orientation distribution and fiber orientation tensor. The randomizing effect of turbulence is insignificant in the regions near outside and jet core, and becomes significant in the region between outside and jet core. The randomizing effect increases with the increasing of the distance from the jet exit. Different components of fiber orientation tensor show a similar distribution pattern.     

Mechanical coagulation in emulsion polymerizations

Coagulum formation for different emulsion polymerizations was correlated to various agitation parameters. For low Reynolds numbers, rotational speed was shown to be important, whereas, for high Reynolds numbers, power consumption was the important parameter. These results were theoretically tied to first-order coagulation kinetics by incorporating shear rate relationships for flow in an agitated tank. For low Reynolds numbers, the average shear rate was assumed to be proportional to the rotational speed of the impeller. And for high Reynolds numbers, Kolmogorov's theory of locally isotropic flow was employed to relate the average shear rate to the power consumption.     

Study on the flow characteristics and mixing performance of multi-blade combined agitator

Developing an agitator suitable for wide viscosity range is of great significance to the energy saving and efficiency improvement by the intensification of fluid flow and mixing process. The power characteristics, flow field distribution, turbulence characteristics and mixing performance of multi-blade combined (MBC) agitator under laminar to turbulent flow state were studied experimentally and numerically at the level of large eddy simulation. The predicted power curve is consistent with the experimental results. Tangential flow is the main flow in laminar flow. With the increase of Reynolds number (Re), axial and radial flows in the vessel gradually increase. When Re reaches 486, the velocity field distribution is basically the same as that in the turbulent flow. At the same energy consumption level, MBC agitator is superior to the commercial Maxblend agitator in mixing high viscosity fluid. The intensification of axial and radial flows is due to the dispersed arrangement of the blades, enabling the MBC agitator to achieve larger axial and radial flows from the transitional flow to the turbulence state. Moreover, the turbulent kinetic energy is evenly distributed and the mixing process is significantly accelerated.     

多叶片组合式搅拌桨釜内流动特性和混合性能研究

开发可适用于较宽黏度范围的搅拌桨,强化釜内的流体流动和混合过程对于搅拌釜的节能增效具有重要的意义。实验与数值模拟相结合,在大涡模拟层面研究了多叶片组合式搅拌桨（MBC桨）从层流到湍流状态下,釜内的功率特性、流场分布、湍流特性和混合性能。结果表明：预测的功率曲线与实验结果一致;层流状态下釜内以切向流动为主,随着Reynolds数（Re）的增大,釜内轴向和径向流动逐渐增强,当Re达到486时,速度场分布与湍流状态下基本一致;在相同的能耗水平下,MBC桨对高黏度流体的混合性能优于商业Maxblend桨。桨叶的分散组合布置,强化了釜内的轴向和径向流动,使得MBC搅拌桨在从过渡流到湍流状态下均可实现较大的轴径向流动,湍动能分布较为均匀,混合过程显著加快。