伴有Marangoni效应的传质动力学

结合Marangoni对流的流体动力学条件，通过建立的半经验模型研究了伴有Marangoni效应的传质动力学，阐述了Marangoni效应增强传质的机理，得到了传质Sherwood数与Marangoni数之间的连续指数关联，从而得以解释不同实验过程中得到的不同Sherwood数与Marangoni数之间的关系.研究表明，由Marangoni效应而增强的传质系数与界面Marangoni湍动的表现形式有关.     

绕流具有光合生化反应管束的格子Boltzmann模拟

采用格子Boltzmann方法模拟了有机废水溶液绕流表面附着有光合生物膜管束的流动、传质及光合生化反应过程,并分析了管束排列,管间距及Reynolds数对流场、浓度场、平均阻力系数及底物的Sherwood数的影响。结果表明：溶液绕流小管间距时具有较高的Sherwood数,同时阻力系数也较高;与顺排管束相比,绕流叉排管束的平均Sherwood数增大了一倍左右,其增大幅度随Re的增大而增大,而阻力系数仅增大了约10%。一定条件下,绕流叉排管束时,出口处具有较低的底物浓度和较高的产物浓度。结果表明,很小的流场扰动可对浓度场产生较大影响,从而,叉排管束更有利于传输传质及生物降解。     

错排降膜阵列气液交叉流界面捕集PM2.5的传质类比模型

根据气溶胶颗粒拟流体性质提出了气液交叉流界面捕集PM2.5的传质类比模型.分析了颗粒Schmidt数及其指数m对气溶胶流体传热传质类比的影响机理.以常用的横掠错排管束对流传热Nu方程为基础,导出了横掠错排降膜阵列PM2.5传质Sherwood数方程,由此建立了以m为模型参数的PM2.5捕集效率预测模型.用横掠20列×90排ø3 mm降膜阵列PM2.5捕集效率实测数据回归获得m值为0.808.在Reynolds数50～650的范围内,模型预测传质Sh与实测值误差在±20%之内.     

超声微反应器内气液传质过程的介尺度强化机制

采用CFD方法对超声微反应器内的Taylor气液两相流的传质过程进行了模拟。针对传质过程中主要的介尺度结构,包括气泡表面波、空化声流、液相内的局部浓度,分析了其空间分布和时间演化规律。模拟结果有效捕捉了实验难以观测的液膜区域,并将液膜厚度与气泡表面波振动进行了关联,阐释了气液界面附近的空化声流对传质过程的强化作用。根据超声微反应器内Taylor流的传质特点,分别研究了不同流动和超声条件对液弹内和液膜处传质过程的影响,比较了各局部传质对整体传质效率的贡献。通过分析整体/局部Sherwood数与Peclet数间的关系,研究了超声效应对气液传质速率的影响。分析结果从介尺度角度验证了文献关于超声微反应器传质系数的计算,完善了超声微反应器内气液传质过程的强化理论。     

脉冲筛板萃取柱中传质过程的研究

在内径40mm实验脉冲筛板柱中,用30%磷酸三丁酯-硝酸-水体系测定了各种操作条件下两相稳态传质浓度剖面、液滴直径分布和分散相体积分数.根据多级返流模型拟合实际浓度剖面推算得到真实的体积传质系数K_(oDa)和两相返混参数,并进而计算出相际传质系数K_(oD).对标准板和分散-聚合型板两种柱结构内的传质过程进行了分析和讨论.研究结果表明,在本实验范围内,脉冲筛板柱内相际传质系数可按单液滴湍流内循环传质模型来预测.     

湿工况下泡沫金属内换热和压降的数值模拟和实验验证

泡沫金属应用到换热器空气侧有望提高析湿工况下的换热性能。为了了解湿空气在泡沫金属内的热质传递和压降特性,建立了泡沫金属内液滴形成、生长和运动特性的数值模型。基于液滴成核数目和成核临界半径得出液滴形成过程的传质率模型;通过建立液滴与湿空气相界面附近湿空气中水蒸气的组分守恒方程,得出液滴生长过程的传质率模型;通过对不同孔棱柱表面液滴的受力分析,建立在重力和风力的共同作用下的液滴接触角模型。将液滴形成及生长的传质率模型和接触角模型分别作为质量源项和表面张力源项,加入连续性方程、动量方程和能量方程组中,实现对泡沫金属内液滴生长、形成和运动过程模拟。模型的实验验证结果表明,换热量预测值与实验结果的最大偏差为11.9%,压降预测值与实验结果的最大偏差为17.7%。     

微通道内单乙醇胺水溶液吸收CO_2/N_2混合气的传质特性

采用高速摄像仪对400μm×400μm T形微通道内单乙醇胺(MEA)水溶液吸收混合气中CO_2过程的气液两相流及传质特性进行了实验研究,微通道内的压力降采用压力传感器进行测量。考察了弹状流型下气液两相流量及MEA浓度对压力降、比表面积和传质性能的影响。结果表明,当MEA浓度不变,气液两相流量增大时,压力降、比表面积、传质系数、体积传质系数和增强因子均增大,并逐渐趋于恒定。当气液流量不变,MEA浓度增大时,压力降、传质系数、体积传质系数和增强因子增大,但比表面积减小。实验条件下,压力降范围为2.00~5.23 kPa,化学吸收过程的传质系数范围为7.74×10~(-4)~2.97×10~(-3) m·s~(-1)。对于伴有快速化学反应的传质过程,以Sherwood数、Reynolds数、Schmidt数及增强因子为变量建立了体积传质系数的预测关联式,平均偏差为5.09%,具有良好的预测性能。     

超临界反溶剂过程传质数学模型

建立了有机溶剂(甲苯)液滴与超临界反溶剂(超临界CO2)之间的传质模型,用于模拟超临界反溶剂制备微纳米粉体材料的传质过程。该模型考虑了双向传质过程,既有反溶剂向溶液的扩散过程,又有溶液中的溶剂向反溶剂的“汽化”过程。液滴的传质行为是影响颗粒形态和尺寸分布的关键因素。假定传质是在一个孤立的微小液滴与包围着它的反溶剂连续相间进行的,利用描述液滴内和液滴外某一点行为的连续方程、扩散方程、能量方程和动量方程,及界面上的守恒条件进行耦合,从而建立传质过程的数学模型,并给出求解方程和求解的边界条件和初始条件,进行数值求解。     

微通道内单乙醇胺水溶液吸收CO2/N2混合气的传质特性

采用高速摄像仪对400 μm×400 μm T形微通道内单乙醇胺（MEA）水溶液吸收混合气中CO₂过程的气液两相流及传质特性进行了实验研究,微通道内的压力降采用压力传感器进行测量。考察了弹状流型下气液两相流量及MEA浓度对压力降、比表面积和传质性能的影响。结果表明,当MEA浓度不变,气液两相流量增大时,压力降、比表面积、传质系数、体积传质系数和增强因子均增大,并逐渐趋于恒定。当气液流量不变,MEA浓度增大时,压力降、传质系数、体积传质系数和增强因子增大,但比表面积减小。实验条件下,压力降范围为2.00~5.23 kPa,化学吸收过程的传质系数范围为7.74×10^-4~2.97×10^-3 m·s^-1。对于伴有快速化学反应的传质过程,以Sherwood数、Reynolds数、Schmidt数及增强因子为变量建立了体积传质系数的预测关联式,平均偏差为5.09%,具有良好的预测性能。     

正丁醇萃取磷酸的微分散过程与传质系数研究

采用受力分析的方法建立了微分散液滴直径计算模型,以正丁醇/磷酸/水为实验体系,研究了微分散法萃取磷酸的传质特性,依据传质理论计算出了各实验条件下的传质系数及传质速率。研究结果表明:在微分散条件下,液滴直径随分散相流量的增大而减小;直接影响传质系数的因素主要是进口酸浓度、物性、相比、液滴直径、两相流量;采用准数关联出传质系数关联式为:Sh=9.01×10-8Sc1.612Re0.6876p1.027,计算值与实验值相对误差小于10%,为微分散萃取磷酸技术的深入研究提供了理论基础。     

A numerical study of the Dufour and Soret effects on unsteady natural convection flow past an isothermal vertical cylinder

The Dufour and Soret effects on the unsteady laminar free convective flow with mass transfer flow past a semi-infinite isothermal vertical cylinder were studied numerically. The governing partial differential equations were converted into a non-dimensional form and solved numerically by applying a Crank-Nicolson type of implicit finite-difference method with a tri-diagonal matrix manipulation and an iterative procedure. For the hydrogen-air mixture, which is a non-chemical reacting fluid, the profiles of the unsteady dimensionless velocity, temperature and concentration are shown graphically for the different values of thermal and mass Grashof numbers, thermal diffusion parameters (Soret numbers) and diffusion-thermo parameters (Dufour numbers). Finally, the simulated values of the average skin-friction coefficient, the average Nusselt number and the average Sherwood number are presented. The numerical results reveal that for an increasing Soret number or decreasing Dufour number, the time to reach the temporal maximum and the steady-state decreases for the flow variables. As the Soret number increases or the Dufour number decreases, both the skin friction and the Sherwood number increase, whereas the Nusselt number decreases.     

Effect of the viscosity ratio on mass transfer from a fluid sphere at low to very high Peclet numbers

The effect of the viscosity ratio on mass transfer from a fluid sphere is examined in this paper. Numerical solutions of the Navier-Stokes equations off motion and the equations of mass transfer have been obtained for the unsteady state transfer from a fluid sphere moving in an unbounded fluid medium of different viscosity. The effects of the viscosity ratio and the flow on the concentration profiles were investigated for Reynolds number, viscosity ratio and Péclet number ranges of 0?Re?400, 0?κ?1000 and , respectively. The local and average Sherwood numbers are also presented graphically. The steady state results show that the average Sherwood number is increasing as Peclet number increases for a fixed viscosity ratio. However, for a fixed Peclet number, the average Sherwood number is decreasing as the viscosity ratio increases and reaches a limit value corresponding to the average Sherwood number for a solid spherical particle. From the numerical results, a predictive equation for the Sherwood number in terms of the Peclet number, the Reynolds number and the viscosity ratio is derived.     

Kinetic theory based computation of PSRI riser: Part II—Computation of mass transfer coefficient with chemical reaction

The design of circulating fluidized bed systems requires the knowledge of mass transfer coefficients or Sherwood numbers. A literature review shows that these parameters in fluidized beds differ up to seven orders of magnitude.To understand the phenomena, a kinetic theory based computation was used to simulate the PSRI challenge problem I data for flow of FCC particles in a riser, with an addition of an ozone decomposition reaction. The mass transfer coefficients and the Sherwood numbers were computed using the concept of additive resistances. The Sherwood number is of the order of 4 × 10⁻³ and the mass transfer coefficient is of the order of 2 × 10⁻³ m/s, in agreement with the measured data for fluidization of small particles and the estimated values from the particle cluster diameter in part one of this paper. The Sherwood number is high near the inlet section, then decreases to a constant value with the height of the riser. The Sherwood number also varies slightly with the reaction rate constant. The conventionally computed Sherwood number measures the radial distribution of concentration caused by the fluidized bed hydrodynamics, not the diffusional resistance between the bulk and the particle surface concentration. Hence, the extremely low literature Sherwood numbers for fluidization of fine particles do not necessarily imply very poor mass transfer.     

Mass transfer from a contaminated fluid sphere

The unsteady mass transfer from a contaminated fluid sphere moving in an unbounded fluid is examined numerically for unsteady‐state transfer. The effect of the interface contamination and the flow regime on the concentration profiles, inside and outside a fluid sphere, is investigated for different ranges of Reynolds number (0 < Re < 200) and Peclet number (0 < Pe < 10⁵), viscosity ratio between the dispersed phase and the continuous phase (0 < κ < 10), and the stagnant‐cap angle (0° < θ_cap < 180°). It was found that the stagnant‐cap angle significantly influences the mass transfer from the sphere to a surrounding medium. For all Peclet and Reynolds numbers and κ, the contamination reduces the mass transfer flux. The average Sherwood number increases with an increase of stagnant‐cap angle and reaches a maximum equal to the average one for a clean fluid sphere at low viscosity ratio and large Peclet numbers. A predictive equation for the Sherwood number is derived from these numerical results. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Computation of the mass transfer coefficient of FCC particles in a thin bubbling fluidized bed using two- and three-dimensional CFD simulations

In this study, the standard kinetic theory based model with a modified drag correlation was successfully used to compute the mass transfer coefficients and the Sherwood numbers of FCC particles in a thin bubbling fluidized bed column using the additive diffusional and chemical reaction resistances concept. Also, the effects of the computational domain (two- or three-dimensional) and the reaction rate constant (low and high) are discussed.The computations show that the Sherwood numbers are in agreement with the measurement ranges for small particles in the fluidized bed system. The mass transfer coefficients and the Sherwood numbers are high near the inlet section, and decrease to a constant value with increasing height in the column. The two-dimensional computational domain simulations provide enough information to explain the phenomena inside a symmetrical system, but three-dimensional computational domain simulations are still needed for asymmetrical systems. Finally, the mass transfer coefficients and the Sherwood numbers increased with the larger reaction rate constant.     

填充密度对中空纤维膜接触器精馏过程壳程传质的影响

探讨了填充密度对新型聚丙烯中空纤维膜接触器精馏过程中壳程传质的影响。中空纤维膜接触器在填充密度为4.7%、9.5%、19%时均可在远离常规填料液泛线以上的气速范围操作。低填充密度下,壳程的Sherwood数理论值大于实验值,但填充密度较高时,实验值大于理论值。高Reynolds数有利于壳程传质,低填充密度时的Sherwood数高于高填充密度时。随着F因子逐步增大,壳程气相传质阻力随之变小,且膜接触器的填充密度越高,气相传质阻力越大。3种不同填充密度膜接触器的气相总传质系数随着接触器填充密度的增大而迅速减小,由391.84×10^-5 m·s^-1降低为83.28×10^-5 m·s^-1。     

Determination of oxygen transfer from single air bubbles to liquids by oxygen microelectrodes

For the investigation of mass transfer from gas bubbles into liquids the concentration gradient of oxygen migrating from air bubbles was measured by means of oxygen microelectrodes. For this purpose a single air bubble was fixed by a platinum wire spiral with the liquid flowing downward. Thus the ascent of the bubble in an aerated liquid was simulated. Liquid-side mass transfer coefficients determined from concentration gradients were higher than values calculated from theory. Sherwood numbers obtained from experimental results for bubbles of larger diameters were distinctly higher than those for smaller bubbles (diameter 1 mm); the difference corresponds approximately to that predicted theoretically between bubbles with mobile and those with rigid interfaces.     

Approximate theoretical solution for the Sherwood number of oscillating bubbles at different Reynolds numbers

In this paper a theoretical approach to the effect of bubble oscillations on the mass transfer rates has been carried out to get a better understanding of the effect of bubble oscillations in multiphase gas–liquid contactors. The perturbation method has been used to approximate the velocity profile surrounding the bubbles while they oscillate. The shape of the oscillating bubble was modelled taking into account the effect of the liquid viscosity on bubble oscillation frequency and amplitude. The modelled shapes match the photographs of bubbles oscillating in liquids with different viscosities. As a result, new approximate theoretical models for the Sherwood number in viscous fluids at different flow regimes have been proposed. The models extend the work already available in the literature for mass transfer rates from oscillating bubbles in inviscid fluids and provides good results in predicting the Sherwood number at high and moderate Reynolds numbers, the preferred regimes in many industrial operations where, as a result of the hydrodynamics processes involving the bubbles. Their oscillations do not completely decay.     

Countercurrent plug flow mass exchange with internal particle diffusion

The model for countercurrent steady state mass transfer between uniform dispersed phase particles and a continuous phase, both assumed to move in plug flow, with diffusion resistance inside the particles, and film resistance around the particles, is solved analytically for three particle geometries: infinite plates, infinite cylinders and spheres. In the solution, the direct relation between the local continuous phase concentration, and the average dispersed phase concentration is accounted for in the boundary condition. Concentration profiles in the mass exchanger are derived from the analytical solution, as well as asymptotic Sherwood numbers. The analytical solution requires the determination of roots of the characteristic equations, which is cumbersome for spherical particles. Correlations are provided for the asymptotic Sherwood numbers for easy computation. The model can easily be used for the analogous direct heat exchange problem.     

The influence of hydrodynamic and mass transfer entrance effects on the operation of a parallel plate electrolytic cell

Experimental measurements of mass transfer in an electrochemical flow cell of rectangular cross section with different hydrodynamic entrance and electrode lengths have been made. For fully developed flow, average Sherwood numbers under laminar conditions vary with Graetz number to a power 0·30. For turbulent flow, fully developed mass transfer conditions occur about twelve equivalent diameters along the electrode and are best represented by the Chilton-Colburn analogy which predicts Sherwood numbers varying with Reynolds number to a power of 0·8 and Schmidt number to a one-third power. For shorter electrodes Sherwood numbers can be adequately correlated by an expression with Reynolds number to a two-thirds power and dimensionless electrode length to a power of −0·2. For hydrodynamic entrance lengths of not less than eight equivalent diameters, data in the laminar region can be expressed by an emperical boundary layer type of equation which includes terms for the hydrodynamic entrance length and electrode length. In the turbulent regime substantially developed flow occurs after eight entrance lengths and correlations with fully developed flow equations are satisfactory