Prediction of pressure drop in a modified gas-liquid downflow bubble column

A model for pressure drop in the ejector induced downflow bubble column based on mechanical energy balance within the framework of dynamic interaction of the phases has been formulated. The model includes the effect of bubble formation and form drag at interface on the pressure drop. It provides a functional form of equation for correlating pressure drop. The theoretical model proposed in the present study appears to predict the pressure drop satisfactorily for gas-liquid dispersed flow in the concurrent gas-liquid downflow bubble column.     

Studies on frictional pressure drop of gas-non-Newtonian two-phase flow in a cocurrent downflow bubble column

An exclusive study has been done on experimental investigation of the two-phase frictional pressure drop with air-non-Newtonian liquid (CMC solutions) system in cocurrent downflow bubble column. The effects of gas and liquid flowrate on two-phase frictional pressure drop have been illustrated. An attempt has been made to fit the experimental two-phase frictional pressure drop data by modified Lockhart and Martinelli correlation and Aoki correlation. In another approach, friction factor method was adopted to correlate the experimental results in terms of dimensionless groups of the operating and system variables and the predicted values were found to be in good agreement with the experimental result. The experiments were performed in the bubbly flow regime because of its stability and uniformity.     

Quality of mixing in a downflow bubble column based on information entropy theory

Quality of mixing in a modified downflow bubble column has been analyzed by using information entropy theory. Mass transfer efficiency based on quality of mixing has also been enunciated in this work. Empirical models have been developed for downflow system with the parameters which affect the quality of mixing and mass transfer efficiency. The developed correlation for quality of mixedness in the downflow bubble column was interpreted by the mass transfer phenomena. The present analysis on the quality of mixing in downward two-phase flow in bubble column may give insight into a further understanding and modeling of multiphase reactors in industrial applications.     

Pressure drop and hydrodynamic properties of pulses in two-phase gas-liquid downflow through packed columns

The hydrodynamics and the pulse properties in the pulse flow regime of gas-liquid downflow through a packed column were studied using 6 mm Raschig rings and 3 mm spheres as packings. The pulse flow regime is considered to be gas-continuous flow outside the pulses and more like dispersed bubble flow inside the pulses and the pressure drop is viewed as being contributed to by the gas continuous part outside the pulses and by the pulses themselves. Correlations for the total pressure drop, the pressure drop across the pulse and for the pulse velocity are obtained. The experimental data of the average holdup, the pulse holdup, the base holdup and the transition from gas continuous to pulse flow regime are compared with the literature values.     

Ejector performance in a co-current gas-liquid downflow bubble column

Performance of an ejector system in a two-phase downflow bubble column has been evaluated with respect to the energy dissipation during coaxial flow of liquid and gas in a parallel throat and divergent diffiser and during gas—liquid mixing in the column. Experiments were carried out in a 51.6 mm i.d. column with five different nozzles. Three systems, namely air—water, air-kerosene and air—paraffin were used. Correlations have been developed for predicting the ejector loss coefficient as well as mixing loss coefficient as a function of different physical and dynamic variables of the system.     

Gas-holdup distribution and energy dissipation in an ejector-induced downflow bubble column: the case of non-Newtonian liquid

A precise knowledge of gas-holdup distribution and energy dissipation is essential for designing gas-liquid contactors. A semi-theoretical approach has been presented to obtain the axial distribution of gas holdup through the column for gas-non-Newtonian liquid two-phase flow system. The whole column is distinguished to have three zones based on gas holdup, viz. top, middle and bottom. The middle section where significant accumulation of bubbles takes place, contributes higher gas holdup towards the total compared to the other two sections. Energy dissipation in the column have been calculated from two-phase gas-liquid frictional losses. A comparative study shows that substantial gas holdup are observed in the present system with considerably lower energy losses. The experimental data of gas holdup have been correlated in terms of pressure drop by the modified Lockhart-Martinelli equation.     

Hydrodynamics and simulation of air–water homogeneous bubble column under elevated pressure

A gas–liquid Eulerian computational fluid dynamics (CFD) model coupled with a population balance equation (PBE) was presented to investigate hydrodynamics of an air–water bubble column (1.8 m in height and 0.1 m in inner diameter) under elevated pressure in terms of pressure drop, gas holdup, mean bubble size, and bubble surface area. The CFD-PBE model was modified with three pressure correction factors to predict both the total gas holdup and the mean bubble size in the homogeneous bubbly flow regime. The three correction factors were optimized compared to experimental data. Increasing the pressure led to increasing the density, reducing the bubble size, and increasing the gas holdup. The bubble size distribution moved toward a smaller bubble size, as the pressure increased. The modified CFD-PBE model validated with experimental data and empirical models represented well hydrodynamics of the bubble column at P = 0.1, 1.5, and 3.5 MPa.     

Efficient dispersion in a modified two-phase non-Newtonian downflow bubble column

A comprehensive study of dispersion of gas-non-Newtonian fluid has been made in a modified downflow bubble column. Distribution of gas holdup in axial location has been anticipated from hydrostatic force balance. The experimental gas holdup data in such modified bubble column have been analyzed by slip velocity and drift-flux models. A correlation in terms of various physical, geometric and operating parameters of the present system has also been developed to analyze the gas holdup.     

垂直上升绝热管内汽、液两相流的压降

针对蒸发过程中沸腾层内汽、液两相流的流动状态,对垂直上升绝热管内汽、液两相流流动状态(泡状流和弹状流)中压降的主要部分采用与实际情况相近的模型进行分析,对压降的次要部分采用均相模型和分相模型进行分析,通过积分推出两相流体在区间[Z_1,Z_(1+1)]内总压降的计算式。本文理论计算值与实验结果符合较好。     

Direct contact heat transfer with phase change: Bubble growth and collapse

A theoretical expression has been presented for the prediction of the instantaneous heat transfer coefficient of a condensing two-phase bubble. The modified form of this model predicts very well the available experimental data. Another expression, presented for the total time taken by an evaporating two-phase drop, also gives very good agreement with the available experimental data.     

Energy Efficiency of Two‐Phase Mixing in a Modified Bubble Column

Energy efficiency for gas liquid mixing in a modified downflow bubble column reactor has been analyzed in this paper. Efficiencies of the different parts of the bubble column have been assessed on the basis of energy dissipation. Prediction of the energy dissipation coefficient as well as energy utilization efficiency due to gas‐liquid mixing as a function of different physical, geometric and dynamic variables of the system has been done by correlation method. The distribution of energy utilization in the different zones of the column has also been analyzed. Experiments were carried out with air‐water and air‐aqueous solutions of carboxy methyl cellulose with different concentrations.     

Determination of holdup by air purge in φ50 mm discs and doughnuts pulsed extraction column

在柱径为50 mm的折流板脉冲萃取柱中,首先利用吹气法研究了硝酸水溶液和30％TRPO-煤油水溶液体系的单相流时均摩擦压降Δp_f特性.实验结果表明当A′ω≠0,且u_c＝0时,Δp_f可以忽略;而当Aω＝0时,Δp_f与u_c满足Noh模型的线性关系.在Noh模型的基础上,给出了计算Aω≠0时单相流时均摩擦压降的模型,实验结果与模型预测相一致.在单相流摩擦压降的基础上,又研究了连续相分别为硝酸溶液和30％TRPO-煤油溶液体系的两相流时均摩擦压降Δp_f特征.实验结果表明：两相流时均摩擦压降Δp_f可用于计算分散相存留分数,其近似由连续相所对应的Noh单相流模型求得,并可用于折流板脉冲萃取柱分散相存留分数的确定.     

φ50 mm折流板脉冲萃取柱吹气法测量存留分数

在柱径为50 mm的折流板脉冲萃取柱中,首先利用吹气法研究了硝酸水溶液和30％TRPO-煤油水溶液体系的单相流时均摩擦压降Δp_f特性.实验结果表明当A′ω≠0,且u_c＝0时,Δp_f可以忽略;而当Aω＝0时,Δp_f与u_c满足Noh模型的线性关系.在Noh模型的基础上,给出了计算Aω≠0时单相流时均摩擦压降的模型,实验结果与模型预测相一致.在单相流摩擦压降的基础上,又研究了连续相分别为硝酸溶液和30％TRPO-煤油溶液体系的两相流时均摩擦压降Δp_f特征.实验结果表明：两相流时均摩擦压降Δp_f可用于计算分散相存留分数,其近似由连续相所对应的Noh单相流模型求得,并可用于折流板脉冲萃取柱分散相存留分数的确定.     

Comparison of hydrodynamic parameters for countercurrent and cocurrent flow through packed beds

Hydrodynamic parameters have been determined in common equipment, i.e., same column and liquid and gas distributors, for cocurrent and countercurrent two-phase flow through fixed beds. The piston/dispersion exchange model (PDE) with usual Danckwerts' boundary conditions (closed/closed system) has been used to describe the liquid flow. A new imperfect pulse method has been used to estimate the PDE model parameters directly from the experimentally nonideal input and output response. The transition between trickle flow and pulse flow, for two-phase downflow, and the occurrence of flooding, for countercurrent flow, has been investigated using a macroscopic model for the two-phase flow.     

Sparged loop reactors

Circulating bubble columns or loop reactors form one of the important classes of modified bubble columns. The present paper analyses the performance of external loop air lift reactors (EL-ALR). The EL-ALR has many advantageous features especially at large scale. These arise from its feature of having controlled liquid circulation which is the key parameter for the design and operation of EL-ALR contactors. Therefore, a reliable and generalised circulation model has been presented. The reliability of the model depends on the accuracy of the predictive methods for the gas hold-up and the two-phase fractional pressure drop. Detailed analysis of these fundamental parameters of gas-liquid flows has been presented. The effects of design (area ratio of riser to downcomer, height to diameter ratio and volume of reactor) and operating (gas flow rates and sparging locations) parameters on the performance of the EL-ALR have been analysed in detail. A rational basis has been developed for the estimation of pressure drop, mixing time and mass transfer coefficient. An optimum combination of design and operating parameters has been suggested and a criterion has been developed for the optimum location of spargers. An attempt has been made to provide critical analysis of the published information and to construct a coherent picture of EL-ALR.     

附加惯性力作用下竖直矩形流道内过冷流动沸腾的数值模拟

摇摆条件下附加惯性力的作用会对两相流动的压降及汽泡受力产生影响。考虑相变能量和质量输运,采用流体体积(VOF)多相流模型对附加惯性力条件下竖直矩形流道内过冷流动沸腾进行了数值模拟。汽液界面位置通过分段线性插值(PLIC)的方法获得。模拟结果获得了孤立汽泡周围压力、速度、温度分布以及二次流动现象,分析了汽泡聚合过程汽泡形态及内部速度矢量的演变过程,模拟结果与文献中结论吻合良好。附加惯性力作用使得流动压降比静止条件下要大,过冷流动沸腾压降由于汽相产生会在单相流动的基础上产生波动,且热通量越大,压降波动幅度越大。摇摆产生的附加惯性力相对汽泡所受的其他力而言可以忽略不计,而摇摆导致的流量波动会改变汽泡受力大小,进而影响沸腾换热。     

气液两相流摆动特性实验的数值模拟

为了给鼓泡塔反应器设计提供依据,运用计算流体力学(CFD)软件模拟了鼓泡塔气液两相流动态行为。采用双欧拉法对鼓泡塔矩形反应器内不同曝气量下气液两相流的摆动特性进行了模拟考察,液相采用标准κ-ε紊流模型,气相采用分散相零方程模型,分析了网格尺寸、时间步长以及相间作用力对模拟结果的影响,模拟的曝气量为42.5~237 m L/s。结果表明,当相间作用力仅考虑阻力时,气液两相流呈现周期性摆动规律;随着气流量的增加,气泡羽流的摆动幅度和频率增大,同时液体的气含率也在增加;模拟的气液两相流摆动频率数据与实验值吻合较好,两者的相对误差为7.2%~12.9%。     

微通道分流弹状流的界面过程及压力演变规律

弹状流分流不仅能调控两相流流型从而强化传热,同时也是生物化工、制药行业的传统过程。针对壁面微通道分流液相、调控两相流型的过程进行数值模拟,获得局部参数变化规律,是获得两相流流型演变机理的基础。采用VOF模型耦合动态自适应网格精准追踪气液界面,模拟气液界面在分液口的界面运动;获得轴向及壁面静压、动压的演变规律。通过模拟可知微通道分流弹状流的关键是气弹在分液口的类活塞运动;同时由于界面拉普拉斯压力差的存在,弹状流压降具有不连续性;且此不连续压力随气弹在分液口的类活塞运动具有周期波动性。而弹状流液桥部分的局部压降是影响总压降的关键;近气弹头部的液相区压降显著,近气弹尾部的液相区域由于液速降低其压降明显衰弱;此为弹状流有别于其他两相流流型的压降特点。     

Pressure drop and its reduction of gas–non-Newtonian liquid flow in downflow trickle bed reactor (DTBR)

The influences of concurrent flow of air–Newtonian and non-Newtonian liquid systems on pressure drop and on its reduction in downflow trickle bed reactor are presented in the present work. The pressure drop at different flow regimes in the trickle bed is enunciated by the dynamic interaction model based on the framework of the momentum balance. From the analysis, it is observed that the non-ideality factor of bubble flow regime is higher than that of pulse and trickle flow regimes which may influence efficiency of the reactor. The present work also concludes that the percentage of pressure reduction increases with increasing the surfactant concentration. However there is a limitation of change of concentration, above which no more reduction can be obtained. The present study may be useful for further understanding and modelling of multiphase reactor with non-Newtonian liquid, which has great industrial applications.     

大输量多相混输管路气液两相流实验研究

为了研究大输量条件下多相混输管路的流动特性,以水和空气为实验介质,在长江大学多相流实验平台上进行了水平状态的高气液量两相流模拟实验研究。实验采用内径为60 mm、长9.4 m的透明有机玻璃管,并利用高速摄像仪记录实验过程中的流型。通过对实验流型进行整理,将水平管内的气液两相流流型划分为分层流、泡状流、段塞流和环状流,并与典型的Mandhane流型图进行对比分析。另外,对实验范围内的几种典型流型下的压降梯度变化规律进行了研究,泡状流区域压降梯度随气流速的增大而减小,段塞流区域压降梯度随气流速的增大而缓慢增大,环状流区域压降梯度随气流速的增加而继续增大。