低气液流速下三相固定床的传质特性

在低气液流速下对三相固定床气液并流向上和并流向下操作过程的传质特性进行了对比研究。分别采用物理吸收法和化学吸收法测定液相体积传质系数k_L~a与气液相际比表面积a,利用电容层析成像系统测定了床层持液量,并分别使用直径为1.9,4.0,9.3 mm的球形填料进行实验,考察颗粒的尺寸效应。结果表明:k_L~a和a均随气液流速增大而增大,其中向上流动受气液流速影响均很明显,而向下流动受液速影响较大。颗粒尺寸越小,k_L~a与a越大。相同气液流速下,向上流动相比于向下流动,k_L~a高出45%,a高出75%。采用量纲分析法拟合实验数据,得到k_L~a与气液相Reynolds数、液相Weber数之间的关联式。根据稳定气泡理论模型,得到气液并流向上过程a的预测关联式。     

Mass transfer and current distribution on a metallic wire

The current and copper deposit distribution on a metallic fibre (stainless steel, diameter of 2 mm) was studied numerically and experimentally. The experimental copper deposit was measured with an optical microscope and the current distribution was deduced. The influence of electrolysis time on copper deposit distribution was also studied. A typical current tertiary distribution was observed. The experiment with a longer electrolysis time exhibited a larger current variation around the wire. A numerical study of this problem was also carried out. The simulation involved a laminar and turbulent flow solver together with a numerical model for the mass transfer of ionic species due to diffusion, migration and convection. A good correlation was found between simulated and experimental results for experiments with a short electrolysis time. This numerical model was then used to study the influence of the flow velocity and the diffusion coefficient on the current density and on the average mass transfer around a wire a few microns in diameter. The general relation: for 0.02 < Re_p < 14.22 and 1000 < Sc < 12,000 was obtained. Comparison with data available in the literature demonstrates good agreement between our model and previous results.     

竖直微肋管内LNG流动沸腾传热特性的分析

实验研究了竖直微肋管内液化天然气(LNG)流动沸腾传热特性,分析热通量、质量流量以及入口压力对LNG两相流动传热特性的影响。针对实验工况分别采用Kim关联式、Koyama关联式以及两种不同F_tp系数的Wellsandt关联式对传热系数进行预测,并将实验结果与预测结果进行比较。结果表明,在整个实验工况范围内,采用Koyama关联式预测准确性最好。     

Mass transfer in wavy liquid films

Theoretical analysis of mass transfer into wavy liquid films shows that the amplification in mass transfer due to the presence of the waves is a function of wave parameters, namely wave length, wave velocity, and wave amplitude and hydrodynamic conditions in the films. When the diffusing species do not penetrate deep into the film an expression is derived which shows that the increase in the mass transfer coefficient is a function of a single dimensionless group, Eν^{$\text{1}\text{2}$}, which, without the help of any adjustable parameter, can be found from the wave and flow parameters of the film. Comof this theory with some experimental data shows good qualitative agreement.     

Laminar flow of a liquid rivulet down a vertical solid surface

Conditions for an energy minimum of a liquid rivulet flowing down a vertical solid surface have been considered. The solution of the equation may yield a constant value equal to the maximum rivulet thickness and the curve z(x) representing the shape of the rivulet cross section (profile). The profile describes the smallest rivulet which may be formed for a given system. “Composed” solutions correspond to wider rivulets, e.g. finite films. Numerical integration has been performed over a wide range of the parameter characterizing the liquid, at contact angles ranging from 5° to 90°. The minimum rivulet concept, which makes it possible to describe a finite-width laminar film of minimum thickness, extends the Nusselt theory.     

环形通道内置螺旋线圈过冷流动沸腾传热特性

研究了置有螺旋线圈的内管加热环形通道中水的过冷流动沸腾传热特性。环形通道由铜制加热棒外套石英玻璃管构成,不同螺旋节距和线丝直径构成的5种螺旋线圈用于实验。对每种置入螺旋线圈管和光滑管分别进行了不同质量流速和热负荷下的16组实验,分析了线丝直径和线圈节距对传热系数的影响规律及机理。结果表明,螺旋线圈存在能使流体发生旋转分离,且使汽化核心密度增大,从而有效强化过冷沸腾换热;传热系数随着线圈节距的减小、线丝直径的增加,即节径比(P/e)减小而增大,当P/e最小为1.5时强化效果最好;最佳强化换热性能较光滑管的高出109%。将实验数据与4个经验关系式的预测值进行了比较,并给出了不同工况下各实验管的强化系数对比。     

竖直圆管内液氮流动沸腾传热特性的分析

实验研究了竖直圆管内液氮流动沸腾传热特性,分析壁面温度、流体温度、干度以及传热系数沿实验段管程的变化规律,考察热通量、质量流量和入口压力对液氮两相流动传热特性的影响。针对实验工况分别采用Chen、Klimenko、Shah以及Liu-Winterton关联式对传热系数进行预测,并将实验结果与预测结果进行比较,对不同传热系数区间内的相对误差进行了计算、分析,以评估实验工况范围内各关联式的准确性。结果表明,在传热系数较大的情况下,4个关联式的预测值普遍低于实验值,在整个实验工况范围内,采用Klimenko关联式预测时误差最小。     

Effect of counter current gas phase on liquid film

Liquid film flow is very important in many industrial applications. However, there are few reports about its characteristics on structured packings. Therefore, in this paper, liquid film phenomena were investigated experimentally to exploit new approaches for intensifying the performance of the structured packings. All experiments were performed at room temperature. Water and air were the working fluids. The effect of counter current gas phase on the liquid film was taken into consideration. A high speed camera, a non-intrusive measurement technique, was used. It is shown that both liquid and gas phases have strong effects on film characteristics. In the present work, liquid film width increased by 57% because of increasing liquid flow rate, while it decreased by 25% resulting from the counter current gas phase.     

Joint flow of two non-Newtonian liquid films down a vertical surface: Flow pattern analysis taking into account the effect of the gas stream

The distribution of flow elements over the residence time in the apparatus for the downward flow of two films of pseudoplastic (dilatant) liquids is analyzed taking into account the effect of the countercurrent gas stream. Velocity profiles are found for both films, and the density of distribution of flow elements over the residence time is determined for working film.     

Mass transfer in vertical liquid-solids flow of coarse particles

Wall-to-bed mass transfer in the hydraulic transport of spherical glass particles was studied. The experiments were performed by transporting spherical glass particles 1.20, 1.94 and 2.98 mm in diameter with water in a 25.4 mm I.D. tube. The mass transfer coefficients were determined by following rate of dissolution of a segment of the transport tube prepared from benzoic acid.In the runs in hydraulic transport, the Reynolds number of the tube varied between 1826 and 27597. The loading ratio (G_p/G_f) was between 0.026 and 0.474, and the fluid superficial velocity was between 0.267 · U_t and 4.904 · U_t, where U_t represents the single particle terminal velocity. For these ratios, the voidage ranged from 0.7123 to 0.9228.Also, wall-to-bed mass transfer in the single phase flow regime was studied. In the runs without particles, the Reynolds number of the tube varied between 122 and 39132. The data for the mass transfer factor (j_D) in single phase flow are correlated for turbulent flow regime, using the Chilton-Colburn's type equations, j_D = f(Re). Those investigations were conducted in aim to compare with results for wall-to-bed mass transfer in hydraulic transport.The data for wall-to-bed mass transfer (j_D) in hydraulic transport of spherical particles were correlated by treating the flowing fluid-particle suspension as a pseudofluid, by introducing a modified suspension-wall friction coefficient (f_w) and a modified Reynolds number (Re_m). The data for wall-to-bed mass transfer in the hydraulic transport of particles show that an analogy between mass and momentum transfer exists.     

Mass transfer from cylinders at various orientations to flowing gas streams

The effects of angle of cylinder orientation and cylinder length on mass transfer rates were studied for Reynolds numbers ranging from 6,000 to 18,000. Orientations (?) varied from 0° to 90° for cylinders whose L/D's ranged from 5.26 to 21.04. The relation j_m = m(N Re)ⁿ fit the data for a given orientation and L/D. Both j_m and N_Nu, data compared favorably to literature values for transverse and parallel flow. Sherwood numbers were shown to be [1] a function of (N Re × sin ?) [2] to correlate with Consin's “cosine law” and [3] to be a function of L and ?. Peak mass transfer occurred between 45° and 75°.     

Wetting of a vertical plate by a liquid flowing through a vertical nozzle

A model has been developed describing the wetting of a vertical plate by a liquid, starting from the exact hydrodynamic equations. By incorporating an approximately chosen wetting force term into the Navier-Stokes equations and using laminar boundary layer theory, the velocity field, the width and the actual contact angle characterizing the flow of the liquid were determined.The results predicted by the model were compared with those experimentally obtained for two limiting cases, and good agreement was demonstrated using the aqueous glycerol mixtures. The model was unsatisfactory for the water film since the boundary layer rapidly became turbulent and it was also observed that poor agreement was obtained with the mineral oil.     

Mass transfer and shear stress on a vertical electrode with gas evolution

The relationship between the mass transfer coefficient and the shear stress along the vertical electrode was investigated under electrolytic gas evolution, either oxygen or hydrogen, from alkaline solution containing ferrocyanide and ferricyanide ions. The shear rate obtained from the shear stress measurement was empirically correlated with the gas evolving current density, the electrode height and the liquid kinematic viscosity. The dimensionless correlation of the mass transfer coefficient with the shear stress under oxygen evolution condition agreed formally with the correlation of the mass transfer in turbulent-free convection. On the other hand, the experimental results under hydrogen evolution varied greatly from those under oxygen evolution.     

Gas–liquid interfacial area and mass transfer coefficient in a co‐current down flow contacting column

The gas–liquid interfacial area and mass transfer coefficient for absorption of oxygen from air into water, aqueous glycerol solutions up to 1.5% (w/w) and fermentation medium containing glucose up to a 3% concentration were determined in a co‐current down flow contacting column (CDCC; 0.05 m i.d. and 0.8 m length). Experimental studies were conducted using various nozzle diameters at different gas and re‐circulation liquid rates. Specific interfacial area (a) is determined from the fractional gas hold‐up (ε_G) and the average bubble diameter (d_b). Once the interfacial area is determined, the volumetric mass transfer coefficient (k_La) is then used to evaluate the film mass transfer coefficient in the CDCC. The effects of operating conditions and liquid properties on the specific interfacial area were investigated. The values of interfacial area in air–aqueous glycerol solutions and fermentation media were found to be lower than those in the air–water system. As far as experimental conditions were concerned, the values of interfacial area obtained from this study were found to be considerably higher than those of the literature values of conventional bubble columns. The penetration theory is used to interpret the film mass transfer coefficient and results match the experimental k_L data reasonably well. Copyright © 2006 Society of Chemical Industry     

Mass transfer behaviour of a fixed bed electrochemical reactor with a gas evolving upstream counter electrode

Mass transfer rates were determined at a horizontal screen cathode stirred by oxygen bubbles evolved at a horizontal anode placed below the screen by measuring the limiting current of the cathodic reduction of ferricyanide ion from alkaline solution. Variables studied were oxygen discharge rate, ferricyanide concentration and number of closely packed screens forming the cathode. For a single screen cathode the data were correlated by the equation: J = 0.249 (Re Fr)^-0.25 The mass transfer coefficient was found to decrease with increasing the number of screens forming the cathode. Implications of the present work for improving the performance of the flow-through packed bed electrochemical reactor were highlighted.     

Mass transfer to liquid films on an inclined plane

Flow visualisation studies on a small inclined plane show that gravity-controlled roll waves cause mixing of the surface layers of liquid but have little effect on the liquid adjacent to the solid surface.Mass transfer experiments are described in which case β-naphthol and benzoic acid surfaces are dissolved by a water film; in other tests carbon-dioxide-air mixtures are absorbed into the surface of a flowing film.Mass transfer from solid to liquid is adequately predicted by the theory of a smooth laminar film, despite the presence of waves in the real case. Certain anomalies are present in the behaviour of dissolving β-naphthol.In the range of liquid film Reynolds numbers from 20 to 300, mass transfer from gas to liquid is markedly enhanced by waves. For each channel inclination, the mass transfer coefficient reaches a maximum value relative to the predictions of laminar flow theory when the Reynolds number is about 100. The maximum corresponds to a 250 per cent increase of mass transfer coefficient for a vertical surface, a 160 per cent increase for channel inclined at 45°, and a 90 per cent increase for channel inclined at 7°30′. The countercurrent gas flowrate has a moderate effect on mass transfer coefficient in the range of gas Reynolds numbers from 6000 to 32,000.Superimposed vibrations have very little effect on mass transfer to an already-rippling film, though the surface is disturbed by a complex pattern of sinusoidal waves. This supports the contention that the major contribution to mass transfer is from gravity-controlled roll waves and not from the more regular sinusoidal waves.     

垂直管内液固并流向上传热特性的研究及其应用

采用河砂与水作试验介质,研究了垂直管内液固两相并流向上的传热和阻力特性。实验结果表明,垂直管内液固两相并流向上具有特殊的流阻特性和较强的传热作用。