基于停留时间分布的缠绕管内二次流研究

缠绕管内二次流能够显著强化管内传质、传热性能。通过测量缠绕管内液体的停留时间分布,利用无量纲方差σ²表征二次流强度,研究了缠绕直径、缠绕角度、缠绕管管径等结构参数对二次流的影响。结果表明：在径向流动未达到极限迁移距离的缠绕管中,随着液体Reynolds数Re的增大,σ²先减小后增大,对应缠绕管内依次出现的湍流作用区和二次流作用区;在径向流动达到极限迁移距离的缠绕管中,随着Re的增大,σ²先减小后增大最后趋于平稳,对应缠绕管内依次出现的湍流作用区、二次流作用区和二次流极限区。从湍流作用区转变为二次流作用区的临界Reynolds数Re_S随缠绕管缠绕直径和管径的减小而减小,缠绕角度对Re_S的影响较小。     

Liquid phase residence time distribution for gas–liquid flow in Kenics static mixer

The residence time distribution (RTD) of the liquid phase for co-current gas–liquid upflow in a Kenics static mixer (KSM) with air/water and air/non-Newtonian fluid systems was investigated. The effect of liquid and gas superficial velocities on liquid holdup and Peclet number was studied. Experiments were conducted in three KSMs of diameter 2.54 cm with 16 elements and 5.08 cm diameter with 8 and 16 elements, respectively, of constant L_e/D_e = 1.5 for different liquid and gas velocities. A correlation was developed for Peclet number, in terms of generalized liquid Reynolds number, gas Froude number and liquid Galileo number, where as for liquid holdup, a correlation was developed as a function of gas Reynolds number. The axial dispersion model was found to be in good agreement with the experimental data.     

The residence time distribution for laminar flow in helically coiled tubes

Sizable errors exist in previously published studies on the residence time distribution for ideal laminar flow in a helically coiled tubes. The numerical methods used to give corrected results are generally useful for flow situations where the projections of the streamlines on the channel cross-section are closed curves.An asymptotic solution, valid at long residence times, has been obtained. This solution confirms the numerical observation that the tail of the distribution is similar to that for laminar flow in a straight tube. If molecular diffusion is ignored, this form of asymptotic behavior is shown to be a general characteristic of all flow systems involving a fixed wall. When diffusion is considered, the asymptotic residence time distribution will be a decaying exponential in time. This confirms the widespread experimental observation that residence time distributions have exponential tails.     

SK静态混合器内各截面停留时间分布实验研究

采用脉冲示踪法对SK静态混合器内各截面的停留时间进行了实验测试,比较了流量对各截面停留时间分布的影响.结果表明:在同一截面上,随着流量增加,平均停留时间减小,停留时间分布密度曲线变得高而窄;在相同流量下,沿着轴线方向,平均停留时间增大,量纲一方差减小,流体流动趋向于活塞流.同一截面上,随着流量增加应答峰初期的斜率较陡,...     

撞击流气化炉内颗粒停留时间分布的随机模拟

根据多喷嘴对置式气化炉流场测试,将气化炉划分为若干区域,运用时间离散、状态离散的马尔可夫链随机模型,模拟了气化炉内颗粒相的停留时间分布(RTD)。当颗粒在撞击区和射流区间的回流比为0.5,向下撞击流股区和管流区为平推流模型,其他区域按全混流模型处理时,模拟值与实验值吻合较好。随着进料流量的增大,平均停留时间减小,量纲1方差减小;随着回流比的增加,平均停留时间增大;气固两相平均停留时间接近,但RTD存在一定差异。     

Measurement of residence time distribution in a rotary calciner

Rotary calcination is widely used in catalyst manufacturing and many other industrial processes. In this article, the influence of operational variables and material properties on the mean residence time (MRT), hold up, and axial dispersion was investigated in a pilot plant rotary calciner. Residence time distributions (RTD) of spherical, cylindrical, and quadrilobe catalyst particles were measured and contrasted. The Saeman's model was successfully applied to predict the experimental bed depth and the MRT as particles flowed through the calciner. It was observed that increasing the feed rate did not significantly affect the MRT. Results for the different particles indicated that cylinders and quadrulobes exhibited less axial dispersion than spheres due to the decreased flowability. A reliable method was developed to provide a reasonable RTD prediction in rotary calcination systems. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4068–4076, 2013     

折流板设备中流体停留时间分布与流动形态的对比研究

折流板结构在大空间设备中形成折流通道,以防止短路,从而使流体与换热管和颗粒床等内构件充分接触。停留时间分布(RTD)实验具有简单便捷的优势,针对设备中的短路与如何确定设备合理结构的问题分别设计了几组结构来进行RTD实验与流场数值模拟研究。对比分析结果表明,设备中的短路和死区是相对存在的,通过RTD密度函数曲线的出峰时间及拖尾情况能判断设备中短路与死区的情况,该结论可以为设备设计提供理论依据。利用多釜串联模型的釜数及RTD曲线的出峰时间和方差分析了流量、折流板缺口面积及板距对设备中流体流动形态的影响;研究了设备中的压降,综合考虑设备的能耗和性能,提出了以RTD实验来确定折流板设备合理结构时最合适的釜数、出峰时间及方差。     

Liquid phase residence time distribution for a two-phase countercurrent flow in a packed column with a novel internal

The liquid phase residence time distribution (RTD) for gas–liquid countercurrent flow in a packed column with a novel internal was measured by conductivity measurements and an air–water system. The RTD of a liquid tracer is well represented by the ADM and PDE models. At lower gas flow rates, the Peclet number of the liquid in the packed column with the internal is lower than that without the internal; at higher gas flow rates, it is vice versa, especially with an internal with a higher volume fraction. The distribution of the liquid RTD can be improved by using suitable geometric parameters of the internal to give a larger volume fraction and a lower stage height.     

气液旋流器内液相平均停留时间

旋流设备内伴有传热、传质或反应过程时,介质停留时间是其关键参数。在冷模实验装置上,采用持液量法,对气液旋流器内液相平均停留时间进行了研究。结果表明,液相停留时间随入口含液率增大明显降低,随入口气速增大降低较小。气液界面剪切力远小于液相重力是入口气速对液相停留时间影响较小的主要原因。基于液膜受力平衡,建立了气液旋流器内液相平均停留时间模型。模型预测停留时间与实测值总体吻合良好,在液膜Reynolds数Re_l< 1200范围内,模型预测停留时间偏大,讨论了模型预测偏差与液膜流型的关系。     

Measurement of residence time distribution in microfluidic systems

We present a method for measuring the residence time distribution (RTD) in microfluidic systems. A piezoelectrically actuated sample injector releases approximately 100 nl of tracer liquid into a microchannel of rectangular cross section. The spreading of the tracer pulse in pressure-driven microflows is monitored with fluorescence microscopy measurements. Residence time distributions are determined for single-phase liquid and segmented gas-liquid microflows, with the RTD being significantly narrower for the latter case. The selected flow conditions are relevant to synthesis in microreactors with residence times up to several minutes.     

Residence time distribution of a purely viscous non-Newtonian fluid in helically coiled or spatially chaotic flows

This work is dedicated to an experimental study of residence time distributions (RTD) of a pseudoplastic fluid in different configurations of helically coiled or chaotic systems. The experimental system is made up of a succession of bends in which centrifugal force generates a pair of streamwise Dean cells. Fluid particle trajectories become chaotic through a geometrical perturbation obtained by rotating the curvature plane of each bend of ±90° with respect to the neighboring ones (alternated or twisted curved ducts). Different numbers of bends, ranging from 3 to 33, were tested. RTD is experimentally obtained by using a two-measurement-point conductimetric method, the concentration of the injected tracer being determined both at the inlet and at the outlet of the device. The experimental RTD is modeled by a plug flow with axial dispersion volume exchanging mass with a stagnant zone. RTD experiments were conducted for generalized Reynolds numbers varying from 30 to 270. The Péclet number based on the diameter of the pipe is found to increase with the Reynolds number whatever the number of bends in the system. This reduction in axial dispersion is due to both the secondary Dean flow and the chaotic trajectories. Globally, the flowing fraction, which is one of the characteristic parameters of the model, increases with the Reynolds number, whatever the number of bends, to reach a maximum value ranging from 90% to 100%. For Reynolds numbers less than 200, the flowing fraction increases with the number of bends. The stagnant zone models fluid particles located close to the tube wall. The pathlines become progressively chaotic in small zones in the cross section and then spread across the flow as the number of bends is increased, allowing more trapped particles to move towards the tube center. Results have been compared with those previously obtained using Newtonian fluids. The values of the Péclet number are greater for the pseudoplastic fluid, the local change of apparent viscosity affecting the secondary flow. For pseudoplastic fluids, the apparent viscosity is lower near the wall and higher at the center of the cross section. The maximum axial velocity is flattened as the flow behavior index is reduced, inducing a decrease of the secondary flow in the central part of the pipe and an acceleration of it near the wall, which reduces the axial dispersion. These results are encouraging for the use of this system as continuous mixer for complex fluids in laminar regime, particularly for small Reynolds numbers.     

Gas—Liquid Flow Through Helical Coils in Horizontal Orientation

Experimental investigations have been carried out to evaluate the two‐phase pressure drop and the holdup for gas‐Newtonian liquid flow through helical coils. 24 helical coils and three different liquids were used for the experiments. Empirical correlations have been developed to predict the two‐phase friction factor and the liquid holdup as functions of various physical and dynamic variables of the system. Statistical analysis of the correlations suggests that they are of acceptable accuracy.     

Residence time distribution analysis from a continuous couette flow device around critical taylor number

This paper presents an experimental study of residence time distribution (RTD) analysis by pulse response technique in a continuous Couette flow device with rotating inner cylinder and stationary outer cylinder. Two kinds of experimental tests using pulses of tracer dye solution and particles resulting from a fast precipitation were performed in the region near the critical Taylor number characterizing boundary between laminar and laminar vortex flow. For most experiments performed in laminar and laminar vortex flow regime around the critical Taylor number over the ranges 0 < T_a < 120 and 0 < R_e < 5.5 the normalized response can be described by a dispersion model. The results of the critical Taylor number as characterized by the minimum dispersion number appear consistent with both theoretical predictions and other empirical observations.     

同向双螺杆挤出机的停留时间分布及填充度

引 言 双螺杆挤出机在高分子材料加工中已被广泛地应用于聚合物共混改性、反应挤出及高分子可控降解等各个方面[1].但先前对挤出过程研究较少,一般仅停留在"黑箱"型经验操作的层面,主要以定性的机械设计为主.     

微通道内低黏聚合物流体的停留时间分布研究

以低分子量的聚丙二醇为流动介质、酸性橙的N,N-二甲基乙酰胺溶液为示踪剂,采用脉冲响应法测定了螺旋型微通道（MC）反应器内的停留时间分布（RTD）,验证了平行多釜串联（PTIS）模型与RTD实验数据的匹配性,系统考察了MC长度和Reynolds数（Re）对RTD的影响,并讨论了流体径向速度分布指数y和Peclet数（Pe）的变化规律。结果表明,RTD随MC长度的增加而变窄。当通过降低黏度来增大Re时,RTD随之变窄。当通过增大流速来增大Re时,若管径较大,则RTD随之变窄。但若管径较小且管长不长时,则RTD随Re的增加而变宽;如管长较长,则RTD随Re（或流速）的增加先变宽后变窄,即存在临界Reynolds数（Re_c）。RTD的这些变化规律表明,二次流动和径向分子扩散对细小管径的MC的RTD有显著的影响。     

假塑性流体在双螺带桨搅拌釜内停留时间分布

采用电导率法研究了羧甲基纤维素水溶液假塑性流体在双螺带搅拌釜内的停留时间分布(RTD)情况,考察了物料黏度、流量、搅拌转速和转向对停留时间分布的影响.发现假塑性流体在釜内的停留时间分布类似于全混釜,但有一定的拖尾现象.物料黏度对平均停留时间和当量全混釜数影响不大;随着流量增大,平均停留时间减小,而当量全混釜数基本不变;...     

连续操作密相流化床颗粒停留时间分布特性模拟放大研究

采用基于GPU（graphics processing unit）大规模并行的粗粒化CFD-DEM（computational fluid dynamics-discrete element method）方法,耦合多分散、非球形颗粒曳力模型,对连续操作的三维流化床进行了长时间颗粒停留时间模拟。通过对不同尺寸（长度）流化床的模拟发现不同粒径颗粒平均停留时间（mean residence time,MRT）与流化床长度呈线性关系,该关系可以用来预测更大尺寸流化床内的颗粒停留时间。随着流化床长度的增加,不同粒径颗粒MRT的差异变大,说明流化床长度的增加对不同尺寸颗粒的停留时间具有一定的调控能力。     

挤出过程物料停留时间分布的在线测量和模型

采用荧光检测方法在线测量了同向双螺杆挤出机中物料的停留时间分布(τ)，研究了示踪剂浓度和电压信号强度的关系以及螺杆转速(ω)和喂料速率(Q)对双螺杆挤出机τ的影响，比较了多级理想混合(TIS)模型、回流多级理想混合(BFC)模型和双回流多级理想混合(DBFC)模型描述该挤出机的符合程度。结果表明，当9为10．0～16．0kg/h，ω为60～120r／min时，示踪剂浓度和电压信号具有线性关系；ω和Q对平均停留时间有显著的影响，对轴向混合强度的影响不明显。BFC模型和DBFC模型适合描述双螺杆挤出机的流动状况，而TIS模型偏差较大。