Prediction of pressure drop and void-fraction in annular two-phase flows

The polynomial vvelocity profile due to Pai (1953), applicable to transition and turbulent conditions in single phase flow, has been used to solve the equations of two-phase annular flow with appropriate matching conditions at the interface and at the boundaries. The results obtained enable one to predict the pressure drop and the void-fraction, when the interfacial friction factor is specified. Entrainment of the liquid in the core of the flowing gas has been taken into account by using the empirical relation of Hutchinson and Whalley (1973). It has been shown that the theory provides results in agreement with horizontal tube and vertical tube adiabatic flow data, in addition to predicting the liquid film thickness accurately.     

Texture influence on liquid-side mass transfer

The impact on different packing material textures to liquid-side controlled mass transfer for the CO₂ absorption into silicone oil is analyzed experimentally. Typical industrially applied textures with bidirectional pyramidal and unidirectional wavy topographies are investigated for a wide range of liquid loads and viscosities. It is found that the texture design has a significant influence on the mass transfer efficiency, e.g. mass transfer intensification up to 80% can be performed by a textured surface compared with a flat inclined plate. Additionally, counter current gas flow affects the mass transfer positively as expected, but the texture impact is significantly higher. Silicone oil is widely used for fluid dynamic analysis. Here, fundamental data and procedures for mass transfer determination are provided for the first time in order to characterize present textures of column internals.     

Electrodiffusional determination of momentum transfer in annular flows: Axial developing and swirling decaying flows

Two types of developing flows were studied: the axial developing flow occuring downstream of fixed beds of spheres, and the swirling decaying flow induced by a tangential inlet. In both flows, the momentum transfer was investigated for different axial distances and for a Reynolds number range of 90 to 3780 using an electrochemical method. Measurement of the wall shear stress was achieved by means of the limiting electrodiffusional current on circular microelectrodes. Comparisons of swirling flow and axial developing and developed flows are made in terms of velocity gradients and friction factors.Notation A microcathode surface area - C _s potassium ferricyanide concentration - d microelectrode diameter - e=R ₂–R ₁ thickness of the annular gap - F Faraday constant - f _ax,f _df,f _e friction factors - I _L limiting diffusional current - k _F local mass transfer coefficient - L _e entrance length - R ₁ external radius of the inner cylinder - R ₂ internal radius of the outer cylinder - Reynolds number - S _ax,S _df,S _e velocity gradients - U _m mean axial velocity - x axial coordinate - kinematic viscosity This paper was presented at the Workshop on Electrodiffusion Flow Diagnostics, CHISA, Prague, August 1990.     

制冷剂充注量对微通道蒸发器换热性能影响的实验研究

以微通道换热器作为柜式空调蒸发器,设计并加工了微通道蒸发器(MCE,扁管和翅片均为铝材),搭建实验测试系统。研究了三种入口形式(Z-Inlet, Y-Inlet, U-Inlet)、五种制冷剂充注量(800~1600 g)下微通道蒸发器的表面温度分布、制冷量(Q)、输入功率(Pin)和能效比(EER),并与管翅式换热器(FTE,铜管-铝翅片)进行对比分析。结果表明,Z-Inlet形式MCE内部制冷剂行程基本相同,流量分配较均匀,其表面温度分布比较均匀,换热效果最佳;Z-Inlet形式MCE的制冷量和EER最高,与另外两种形式相比,制冷量和EER最高分别提高了8.8%和5.7%;MCE的制冷量和EER比FTE大,制冷量平均超出了11%,最高达13.3% (约600 W),EER平均提升了9.36%,最大约为12.4%;此外MCE的EER达到峰值对应的制冷剂充注量与FTE相比减少了200 g。     

气-液-液萃取传质效率的研究

在液-液萃取过程中,提高分散相的表面更新速率可有效提高萃取的传质效率.研究发现,在萃取过程中使用气体搅拌可以增加液液之间的接触面积,促进液相内的湍动和循环.据此,本文在气-液-液萃取条件下对不同填料的传质性能进行了测定.实验表明,通入气相后分散相液滴呈现稳定的“油包气”空心状态,这种结构大大降低了分散相液滴的传质层厚度,减小了传质距离,极大地强化传质效率.在适宜气速下,气-液-液萃取效率较传统萃取效率提高20％～40％.通过与散装填料对比,发现规整填料更利于强化萃取效果,传质效率提高约50％.     

四种不同比表面积双曲型高效填料的流体力学与传质性能

用4种不同比表面积双曲(SQ)型金属波纹填料进行冷模实验。通过研究SQ型填料的流体力学和传质性能可知,双曲填料特殊的弧形波纹结构降低了塔压降,改善了气液分布。研究表明,SQ填料的塔压降要明显的低于BHS型填料,当L=20.38m3/(m2·h)时,SQ800压降比BHS800降低81.5%。SQ1500在常压操作下每米理论级数最高可达到30,比SQ800的传质效率高,能耗高,但是操作范围小;板波纹SQ125填料操作范围最大但是分离效率低,适合原料粗分离;板波纹SQ250填料有较好的分离效率和较宽的操作范围,适合旧塔改造等。     

Electrochemical study of mass transfer in decaying annular swirl flow

This paper describes an investigation of local mass transfer behaviour at the inner rod and outer pipe wall of an annular test section in decaying annular swirl flow generated by axial vane-type swirl generators. Four swirl generators with vane angles of between 15–60° to the axis of the duct were used. The experiments were carried out in the Reynolds number range 3300–50 000 and at a Schmidt number of 1650. The axial distribution of the local mass transfer coefficients at both the inner rod and the outer wall were measured using an electrochemical technique. Current fluctuations were also recorded to gain information on the turbulence characteristics in the vicinity of the local electrodes. This paper was presented at the International Workshop on Electrodiffusion Diagnostic's of Flows held in Dourdan, France, May 1993.     

新型垂直筛板的传质点效率研究

探讨了ＮｅｗＶＳＴ的点效率数学模型，提出了气液并流平推流模型的气液相Ｍｕｒｐｈｒｅｅ点效率Ｅ_（ＯＧ）模型公式及Ｈａｕｓｅｎ点效率Ｅ_（ＯＨ）模型公式，依据实验数据，得出Ｅ_（ＯＧ），Ｅ_（ＯＬ），Ｅ_（ＯＨ）随清液层高度的变化规律，说明Ｈａｕｓｅｎ点效率比Ｍｕｒｐｈｒｅｅ点效率使用方便，同时得出传质单元数、单罩传质效率的经验关联式。     

Effects of turbulence on heat and mass transfer in stagnation flows

The influence of turbulence on local heat and mass transfer is systematically analyzed. Essential prerequisite is the availability of measurement methods, based on convective mass transfer, which allow the visualization of heat and mass transfer distributions. The results show that, even at low turbulence intensities, the flow and transport phenomena are decisively influenced by the specific design of turbulence grids and the orientation of the grid wires with respect to the stagnation line. Thereby, two different flow mechanisms occur which may interact. The anisotropy of grid turbulence leads to the “wire-gap effect” depending on the actual position of the grid wires in relation to the stagnation line. The other mechanism is based on the wavy wake velocity distribution behind screens and grid which, in conjunction with the centrifugal instability of stagnation flows, may lead to longitudinal vortices. The results indicate why turbulence has not been sufficiently considered in the available (semi-) empirical correlations. Information needed for an accurate prediction of heat and mass transfer in turbulence flows is discussed.     

Electrochemical study of mass transfer in decaying annular swirl flow Part II: Correlation of mass transfer data

This paper presents correlations of local mass transfer at the inner rod and the outer wall in annular decaying swirl flow generated by axial vane swirl generators. Four swirl generators with vane angles in the range 15–60° to the duct axis were used and experiments were carried out in a Reynolds number range 3300–50000 and at a Schmidt number of 1650. The results were correlated in the general form Sh _x = 0.0204 Re _x ^0.86 (1 + tan _i )^0.53 Sc ^1/3, for the inner rod, and Sh _x = 0.0224 Re _x ^0.86 (1 + tan _o)^0.55 Sc ^1/3, for the outer pipe. Comparison is made with heat transfer data for work with a similar entry configuration.     

杂质对丙酮-水吸收过程传质效率的影响

丙酮是一种广泛应用的有机溶剂,其回收具有重要的经济和环境意义。传统的活性炭吸附-解吸-精馏回收工艺存在工艺耗能高、工艺安全性和稳定性差等缺点,新型的水吸收-精馏回收丙酮工艺近年得到重视。但丙酮水吸收过程吸收用水的水质对吸收效果有较大影响,从而影响到后续精馏工艺的能耗。为此,考察了几种典型分散剂、表面活性剂和铵盐等杂质对丙酮水吸收效果的影响。结果表明,除六偏磷酸钠对吸收有弱增强作用外,所选择的表面活性剂和铵盐都不同程度地降低了吸收效果,因此,在吸收过程保持水质的纯净非常重要。同时,文中结合抑制气泡聚并、界面阻力、盐析等机理对杂质对丙酮-水吸收过程的影响进行了相应分析。上述实验结果对工业试验有很好的指导作用。     

Numerical simulation of influence of mass transfer on phase transfer catalytic reaction

Mass transfer with chemical reaction by liquid/liquid phase tranfer catalysis (LLPTC) for an isothermal batch reactor was analyzed. The results for the phase transfer catalyzed reaction system can be generally described by a pseudo first-order hypothesis, whereas the reaction system can be controlled by simultaneous mass transfer of the catalysts between two liquid phases and chemical reaction in the organic phase. The mass transfer limitation is mainly from the mass transfer step of QX from the organic phase to the aqueous phase. The concept of catalyst-effectiveness vs. physically meaningful parameters in a liquid/liquid phase transfer catalyzed reaction is introduced. The catalyst effectiveness is increased as the mass transfer factors increase, the ratio of reaction rate coefficients of aqueous forward reaction to organic increases, and the equilibrium constant in the aqueous solution increases.     

流化床液固两相传质过程的模拟研究进展

液固两相流化床具有液固相接触效率高、传质和传热性能好、颗粒分布均匀等优点,已被广泛应用于众多工业过程中。然而,流化床中与传质过程耦合的颗粒流化的复杂非线性特征及其湍动特性,使得对传质过程特性的研究十分困难。且仅依靠实验观测和理论预测难以揭示多相流相互作用规律,无法获得全面和详细的速度场和浓度场分布情况。近年来,数值模拟的快速发展为深入探索流化床中液固两相流动行为及其与传质过程耦合问题提供了重要的途径。本文对流化床液固两相流动与传质过程模拟方法进行了综述,并对其未来研究趋势进行了展望。借助于计算传质学理论可以更精确地预测局部浓度的分布情况,进而可以深入分析液固两相流化床中的传质过程规律与传质特性,为液固两相流化床的设计和优化提供理论基础。     

The effect of mass transfer on flow characteristics in vertical two-phase unstable flow

Studies carried out on the absorption of hexane from inert carrier gases into paraffin oil have shown that mass transfer can affect flow characteristics in unstable vertical two-phase flow. In the absence of mass transfer chain-flow patterns prevailed. These patterns were broken when mass transfer took place from the gas to the liquid phase. Bullet shaped slugs were then formed, the slugs pairing and coalescing as they rose up the column. The direction of mass transfer was critical no effect on flow pattern being observed when mass transfer took place from the liquid to the gas phase. Slug frequencies and slug sizes were found to be influenced by mass transfer and an effect of overall absorber length was observed. Individual slug sizes and separation distances were measured and varied widely even under constant flow conditions. Rise velocities were correlated using an equation of the form:where C₂ was a function of the fluid flow-rates and system physical-properties.     

Inclination effects on wave characteristics in annular gas–liquid flows

Measurements of wave characteristics have been conducted in a 0.0762 m internal diameter (ID) pipe at inclinations of 0°, 10°, 20°, 45°, 60°, 75°, and 90° from horizontal. Wave celerity and frequency are very strongly dependent on modified Lockhart–Martinelli parameter, X*, and the inclination angle. Wave amplitude increases with increasing liquid film thickness at the bottom of the pipe. Wave amplitude depends on liquid film thickness for any pipe diameter, surface tension, and viscosity. Strouhal number (dimensionless wave frequency) decreases with increasing X*. Effect of pipe diameter, surface tension, and liquid viscosity on the liquid film Reynolds number, Re_LF, was studied. Re_LF variation with X* is not sensitive to the surface tension and less sensitive to the pipe diameter. However, Re_LF is very sensitive to the viscosity of the flowing liquid. Correlations for wave celerity, amplitude, frequency, and liquid film Reynolds number are proposed. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Bubble size and mass transfer characteristics of sparged downwards two-phase flow

When gas is continuously fed through a sparger into a downflowing liquid in a pipe a ventilated cavity is often formed. The cavity remains attached to the sparger even in the presence of high liquid flow rates that would wash away a free slug bubble. Small bubbles are shed from the base of this cavity by the falling liquid film at the wall of the pipe and these bubbles are swept downwards forming a bubbly flow that is highly effective for mass transfer. The ventilated cavity is undesirable since it reduces the driving force for liquid circulation when the pipe is the downcomer of an external air loop fermenter or analogous gas/liquid reactors. The cavity also reduces the available interfacial area for mass transfer. It has been shown [Thorpe et al., 1997. Proceedings of the Fourth International Conference on Bioreactor and Bioprocess Fluid Dynamics; Lee, 1998. Ph.D Thesis, University of Cambridge, UK], that the length of the cavity can be reduced by replacing the common industrial design of a horizontal sparger (HS) with two novel spargers; a peripheral sparger (PS) and a plunging jet sparger (PJS) (Fig. 3). In this paper we investigate the effect of PS and PJS on mass transfer and the resulting bubble size.Experiments were carried out with air and water in a large circulating rig with a 0.105 m diameter test section. The local average bubble size in the bulk two-phase flow region below the ventilated cavity was determined using photography for three combinations of liquid and gas volumetric flow rates. The average bubble size was essentially the same (differences within 10%) for the PS, central spranger (CS) and HS. The PS created the largest bubble in all cases examined. The PJS created smaller bubbles than all the other spargers and did not allow the formation of cavities, which suggests that it has the superior performance. The estimated increase in k_La due to the smaller bubble size for the PJS was by a factor of 1.3.In order to check this result, the effects of sparger type on the volumetric mass transfer coefficient (k_La) were also measured. The k_La was determined with a dynamic method, by using unsteady state absorption of oxygen. The results confirmed the apparent superiority of PJS over the other spargers. An average increase of 19% in the k_La was observed when the PJS was used instead of the industrial design (HS). The CS and PS showed similar k_La values again within 10% of the HS.However the power consumption is larger when the PJS is used instead of the industrial design HS. Hence an attempt was made to adjust the bubble size and mass transfer coefficients of the PJS to account for the differences in energy consumption. When this is done the PJS and HS produce roughly the same bubble size and have the same mass transfer performance. Still the PJS had the important operational advantages of producing shorter cavities and having the greater resistance to stall at low liquid flow rates.     

微通道内液-液两相流流型及传质的研究进展

微通道内液-液两相流流动在微化工系统中占有重要的地位,了解微通道内液-液两相流体流动和传质规律对推动其工业化应用有重要作用。本文以微通道内液-液两相流系统为研究对象,简述了不同工况下微通道内液-液两相流流型和混合传质效率,分析了微通道特征、流体性质和流体流动速度等对流型形成和传质效率的影响。指出目前对于微通道内液-液两相流的研究多处于定性研究,定量研究仅针对某一体系展开,所得结果具有一定的局限性。关于微通道内液-液两相流传质研究实验较多而数值模拟方法相对较少,接下来的研究工作中应该考虑建立微通道内液-液两相流基础研究的数据库,通过分析大量的数据获得有效的流型划分准则和相关经验式以此推动微通道内液-液两相流的工业化应用。同时在传质研究过程中应研究开发相应的数值模拟模型,保证实验和数值模拟相结合,提出有效的传质效率评价机制。