Nusselt numbers for longitudinal laminar flow in shell-and-tube exchangers

The problem of determining shell side Nusselt numbers for a countercurrent, shell-and-tube configuration is examined in detail for square and hexagonal arrays of tubes when the shell side flow is laminar and parallel to the tubes. A multipole expansion method is employed to determine fluid velocity and temperature field for the fluid on the shell side. The numerical results for the shell side Nusselt numbers are compared with those by a cell theory and an asymptotic analysis. The cell theory agrees well with the numerical results at small area fractions and gives better estimates for hexagonal arrays. The results for the hexagonal arrays are in agreement with those of Sparrow et al. [1961]. The asymptotic analysis shows better agreement with the numerical results for wider range of area fractions of tubes in square and hexagonal arrays. In addition, we determine shell side Nusselt number as a function of the ratio of thermal conductivities of the fluids on the tube and shell side. Finally, we present formulas for determining Nusselt numbers for the periodic arrays.     

椭圆形换热管安放形式对换热效率的影响

对椭圆形换热管横置和竖置两种安放形式的换热器进行了数值模拟,分别研究了不同进口速度时,换热器壳程的出口压差、管壳程出口温度和壳程单位压降下的换热系数。对计算结果进行分析。结果表明:在该计算工况下,换热管竖置时其单位压降下的传热系数要高于换热管横置时单位压降下的传热系数。     

三叶孔板换热器壳程流体流动和传热特性数值研究

利用FLUENT软件对三叶孔板换热器壳程流体流动和传热特性进行了数值模拟研究,通过建立周期性全截面计算模型,对其强化传热机理进行了研究,分析了壳程内不同位置换热管壁面对流传热系数与换热管至壳体轴心距离的关系。结果表明:流体流经三叶孔时产生的射流以及在三叶孔板后产生的二次流动使壳程传热得到强化;三叶孔板换热器壳程内随着换热管至壳体轴心距离的增大,换热管壁面对流传热系数逐渐减小。     

圆盘板开孔对缩放管盘环式换热器壳程性能影响的数值研究

利用CFD技术对圆盘板上开不同直径孔的缩放管盘环式换热器壳程进行了数值模拟。结果表明,圆盘板开孔能在一定程度上改善圆盘板后侧流体的流动状况,且开孔后圆盘与圆环折流板之间的流体压力分布较均匀;开孔直径越大,换热器的壳程传热系数和压降均越小,综合性能越好;开孔直径不宜过小,适当地开孔才能有效地提高换热器的综合性能。     

管壳式换热器壳侧湍流流动与换热的三维数值模拟

综合应用体积多孔度、表面渗透度和分布阻力方法建立了适用于准连续介质的N-S修正控制方程.用改进的k-ε模型考虑管束对湍流的产生和耗散的影响,用壁面函数法处理壳壁和折流板的壁面效应, 对一管壳式换热器的壳侧湍流流动与换热进行了三维数值模拟.对计算结果进行了归纳,并与换热器冷态实验、前人的研究结果进行了对比分析,从而证明了该方法能更有效地模拟管壳式换热器壳侧的流动特性,压降实验数据和计算结果符合良好.     

管壳式换热器壳程流动和传热的三维数值模拟

提出了一种管壳式换热器壳程单相流动和传热的三维模拟方法 .用体积多孔度、表面渗透度、分布阻力和分布热源来考虑壳程复杂几何结构造成的流道缩小和流动阻力、传热效应 ,通过数值求解平均的流体质量、动量、能量守恒方程 ,得到壳程流动和换热的分布 .用该方法对一实验换热器进行了流动和传热的模拟 ,计算结果和实验结果吻合良好 .     

帘式折流片换热器强化传热数值研究

为解决折流板换热器壳程流体阻力过大和折流杆换热器低Re下传热系数较小等管壳式换热器的不足,提出了壳程流体"斜向流"的新概念,研制了新型高效节能管壳式换热器?帘式折流片换热器,其壳程传热系数高于折流杆换热器20%～30%,而壳程压力损失大幅低于折流板换热器。以场协同原理分析了斜向流的强化传热机理,指出在帘式折流片换热器壳程中流体速度场与温度梯度场间的夹角小于折流杆换热器,是其强化传热的重要原因。对帘式折流片换热器中折流栅间距、折流片倾角、折流片宽度等重要几何参数对传热和压降的影响规律进行了数值模拟研究,并据此推导了壳程传热系数和流体阻力降准数关联式,为其工程设计和推广应用提供了参考依据。     

三叶孔板换热器壳程流动及传热数值模拟

三叶孔板换热器是一种新型纵流换热器,广泛应用于核电装备领域。针对目前使用较多的壳程“单元流道”模型的局限性,建立了三叶孔板换热器壳程整体模型,包括进出口接管。采用商用软件FLUENT14.0及RNG k-ε湍流模型对壳程流体流动与传热进行了数值研究,分析了三叶孔板换热器壳程流动与传热特性。结果表明：流经第一块支撑板后,流体已充分发展,并且随着壳程结构周期性变化,传热与压降也呈现周期性变化。在支撑板附近,流体流速变大,形成射流,并且由于支撑板阻挡,在支撑板前面和尾部产生二次流,能有效冲刷管壁,减薄流动边界层,起到强化传热作用。     

Coiled flow inverter as a heat exchanger

In the present work attempts are made to investigate the hydrodynamics and heat-transfer characteristics of a coiled flow inverter (CFI) as heat exchanger at the pilot plant scale. The experiments are carried out in counter-current mode operation with hot fluid in the tube side and cold fluid in the shell side. Experimental study is made over a range of Reynolds numbers from 1000 to 16,000 using water in the tube side of the heat exchanger. The shell side fluids used are either cooling water or ambient air. The coiled flow inverter is made up of coils and 90^° bends and inserted in a closed shell. The shell side is fitted with three types of baffles to provide high turbulence and avoid channeling in the shell side. The bulk mean temperatures at various downstream positions are reported for different flow rate on tube side, as well as the heat transfer efficiency of the heat exchanger is also reported. Pressure drop and overall heat-transfer coefficient is calculated at various tube and shell side process conditions. The outer and inner heat-transfer coefficients are determined using Wilson plot technique. The results show that at low Reynolds numbers, heat-transfer is 25% higher as compared to coiled tubes. At high Reynolds numbers, the configuration has less influence on heat transfer. New empirical correlations are developed for hydrodynamic and heat-transfer predictions in the coiled flow inverter.     

Numerical Simulation of Solute Dispersion in Laminar Tube Flow

Abstract

The dispersion of a solute slug in laminar tube flow has been simulated by a numerical method (the Flux Corrected Transport Algorithm) for a set of tube lengths embracing the pure convection through Taylor dispersion regimes. The results reveal unexpected double peak breakthrough curves for tubes of intermediate length. Experimental evidence of double peaks which qualitatively confirms the simulations is discussed.     

新型纵向流一体式翅片管换热性能数值模拟

为达到提高壳侧传热系数的同时又具有较低的压力损失的目的,提出了新型纵向流一体式翅片管。针对翅片与管壁夹角的变化影响传热效率的问题,根据有无翅片、翅片夹角的不同、翅片种类的不同,利用计算流体力学(CFD)软件建立了7种周期性单元流道,并对壳侧流场和传热场进行了三维数值模拟,分析了不同流速的介质在流道中的扰动、压降及温度变化。模拟结果表明,在所有的翅片管中翅片夹角为105°翅片管的换热系数最高,翅片夹角为105°翅片管换热系数约为光管的1.4—1.6倍,压降仅为横向掠过的圆形翅片管的0.018—0.15倍。翅片夹角为105°时,翅片管综合性能较优,对烟气换热器应用提出了数值保障。     

Exact Analysis of Field-Flow Fractionation

Abstract

A rigorous convective diffusion theory is formulated for the predictive modeling of field-flow fractionation (FFF) columns used for the separation of colloidal mixtures. The theory is developed for simulating the behavior of a colloid introduced into fluid in time-dependent flow in a parallel plate channel across which a transverse field is applied. The methodology of generalized dispersion theory is used to solve the model equations. The theoretical results show that the cross-sectional average concentration of the colloid satisfies a dispersion equation with time-dependent coefficients. The results of this work, in principle, are valid for all values of time since the introduction of the colloid. It is shown that these results asymptotically approach those of the nonequilibrium theory formulated by Giddings for large values of time.

Illustrative numerical results are obtained for the case of steady laminar flow and a uniform initial distribution. The behavior of the coefficients in the dispersion equation is explained on physical grounds. Of particular interest is the fact that at large values of the transverse Peclet number P, Taylor dispersion in the FFF column is very small. Under these conditions, axial molecular diffusion as well as Taylor dispersion in the connecting tubing could make a substantial contribution to the axial dispersion observed in practical FFF columns.

The theoretical predictions are compared with the experimental data of Caldwell et al. and Kesner et al. on electrical FFF columns. The comparisons indicate that the theory has potential in predicting the performance of such systems.     

一种新型受热面传热和流动特性的数值模拟及实验研究

针对余热利用过程中低温热源的含尘量高、不连续及不稳定等特点, 提出了一种新型菱形受热面结构。在传热过程中, 该受热面表现出管束叉排布置的特征, 传热过能力较强, 流动阻力较大, 壳侧对流换热表面传热系数较高。在实施吹灰过程中, 该受热面呈现出管束顺排布置的特征, 易清洗, 吹灰效率高。采用数值模拟和实验方法研究了新型受热面结构的传热和流动特性, 给出了壳侧的Nusselt数和摩擦因子随Reynolds数的变化规律。实验结果和数值分析均表明, 该受热面能够适应现有余热利用过程的基本要求, 在便于清灰和除垢的同时实现高效传热。     

新型板壳式换热器壳程流动与换热的数值模拟

提出一种新型的板壳式换热器,建立2种不同板束截面形式的换热器模型,利用FLUENT软件对壳程流体的流动和换热进行数值模拟,从多个方面对板壳式换热器壳程湍流流动与强化传热进行了探讨。模拟结果表明,由于换热板片特殊的蜂窝结构,靠近板片壁面的流体产生了明显的周期性波浪式流动,这种流动加剧了流体的湍流强度及边界层的扰动,起到了壳程强化传热的效果。对于2种不同截面形式的换热器,圆形截面形式的换热器壳程空间利用率较高,流体流动充分,热交换效果更好,在同流量下,其壳程对流换热系数比方形截面形式的高35%—40%,压降高17%—19%,单位压降下的壳程对流换热系数高15%—19%。该数值模拟结果对板壳式换热器的研究具有一定的理论意义和工程实用价值。     

Electro-Osmotic Dehydration of Hangzhou Sludge with Selected Electrode Arrangements

Electro-osmosis is considered a promising technique for the dehydration of sludge with low permeability and high water content. With little information available on the optimal electrode arrangement for electro-osmosis, comparisons of rectangular, hexagonal, and staggered arrays were performed based on laboratory experiments and theoretical analysis. Experimental results identified that favorable electro-osmotic effects were produced by the hexagonal array with regard to drainage, water content, and shear strength of treated soils compared to the other two arrays examined; the average coefficient of energy dissipation of the hexagonal array was 39% lower than that of the rectangular array and 53% lower than that of the staggered array. This finding demonstrates that the hexagonal array consumed less energy. To describe the electro-osmotic efficiency, the effective electric field theory previously proposed was used to calculate proportions of effective field areas; the results show that the hexagonal array performed best, followed by the rectangular and finally the staggered arrays. It is thus concluded that the hexagonal array was most effective and was thus recommended as the primary pattern for application. The validity of the effective electric field theory in estimating electric field distributions for electro-osmosis was also confirmed by comparing theoretical predictions and experimental results.     

新型螺旋盘管换热器的流动及传热性能研究

一种新型的高换热效率的螺旋盘管换热器。将该新型换热器与普通的管壳式螺旋盘管换热器进行理论分析对比,该新型换热器具有以下主要三个方面优势:可轻松拆卸,定期清除管外壁上的污垢;能有效提高管外侧的对流换热系数;有效减少了管壳体本身的蓄热耗能。通过理论分析计算,比较了设计新型螺旋盘管换热器的性能优劣;并进行了实验研究,在实验中,通过对比实验发现新型螺旋盘管换热器管外侧水流速是普通螺旋盘管换热器的2.3倍,管外侧对流表面换热系数是普通螺旋盘管换热器的1.69倍。验证了新型螺旋盘管换热器能够有效提高换热能力,为螺旋盘管换热器的结构设计、性能优化提供有益的理论和实践依据。     

管壳式换热器壳程高黏度流体的传热强化

传统的管壳式高黏度流体换热器采用光滑管设计制造,其壳程传热及流动效能低,壳程传热阻力占总热阻的80％以上,且流动阻力损失较大,提高壳程流体的传热膜系数和降低流动阻力是提高高黏度流体换热器效能的技术关键．     

三维翅片管外螺旋流动传热强化

流体在螺旋隔板换热器的壳程类似于塞状流流动,几乎没有返混和流动死区.在相同压降下,其传热系数比普通的弓形隔板换热器高得多.以润滑油作为实验介质,研究了润滑油在螺旋隔板单管换热器的壳程传热和压降性能,并与光滑管进行了性能对比.采用Wilson图解法分别分离出了螺旋隔板花瓣管和光滑管单管换热器的管程传热系数,并计算出各自的壳程传热系数,壳程传热系数相对误差为±3％.实验结果表明,在相同Reynolds数下,螺旋隔板花瓣管单管换热器的Nusselt数和压降Δp分别是螺旋隔板光滑管单管换热器的2～2.7倍和1.3～1.4倍.与螺旋隔板光滑管单管换热器相比,螺旋隔板花瓣管单管换热器的传热性能的提高远高于压降的提高,证明在螺旋流条件下,花瓣管具有很好的传热强化性能.     

Mixing induced by buoyancy‐driven flows in porous media

A theoretical model for fluid mixing in steady and transient buoyancy‐driven flows induced by laminar natural convection in porous layers is presented. This problem follows a highly nonlinear dynamics and its accurate modeling poses numerical challenges. Based on the Taylor dispersion theory, a one‐dimensional analytical model is developed for steady and transient velocity fields. To investigate steady‐state mixing, a unicellular steady velocity field is established by maintaining a thermal gradient across a porous layer of finite thickness. A passive tracer is then introduced into the flow field and the mixing process is studied. In the case of transient flows, as the convective flow grows and decays with time the behavior of the dispersion coefficient is characterized by a four‐parameter Weibull function. The simple analytical model developed here can recover scaling relations that have been reported in the literature to characterize the mixing process in steady and transient buoyancy‐driven flows. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1378–1389, 2013     

涡发生器与螺旋片强化不同曲率的壳侧传热

为考察涡发生器与螺旋片对不同曲率的套管式换热器壳侧的传热强化效果,在不同中径的壳侧安装了相同密度的螺旋片和三角翼型涡发生器。以空气为介质,在Re=680～16000范围内,采用实验和数值模拟方法研究了壳侧的传热和阻力特性,考察了不同曲率下复合强化的壳侧的综合性能,分析了传热强化机理。结果表明,曲率越大,壳侧传热系数越高,摩擦因数越大。对曲率分别为0.131、0.321和0.440的3种换热器壳侧,涡发生器将其传热系数平均提高了31.52%、20.83%和18.33%。小曲率和复合强化的壳侧综合性能更好。涡发生器改变了换热器壳侧的流场结构,提高了速度场和温度场之间的协同性,从而提高了换热器壳侧的传热效果。