水平管环状流液膜厚度与波动参数分布

环状流是常见的一种气液两相流流型,基于双平行电导探针阵列传感器设计了环状流液膜动态测量系统,以水和空气为介质,进行了气相表观流速15~35 m/s、液相表观流速0.1~0.4 m/s范围内的水平管环状流周向液膜测量实验,分析了水平管环状流的液膜厚度、相界面波动参数的空间分布与发展变化规律。结果表明,水平管环状流底部液膜厚度随气相表观流速的增加而减小,随液相表观流速的增加而增大,但在高液相表观流速时有饱和趋势,对应条件下周向其他位置的液膜厚度持续增大,尤其在45°位置显著增大,下半周液膜分布趋于平缓;由底部到顶部,液膜波速和波频在周向上均呈逐渐减小趋势,与液膜厚度的分布规律一致,大幅度的扰动波主要分布在底部;底部液膜波速和波频随气相表观流速增加而增大,液相表观流速增加时,波速随之增大,但波频无明显变化,对应波长增大。     

Liquid film thickness in annular flow

An analytical solution relating the mass flow rate to the film thickness and other flow parameters have been developed for the general case of gas—liquid two-phase annular flow in a horizontal pipe line for power-law fluids. The equation developed reduces to the equation derived for the Newtonian fluid when the flow index is equated to unity. In the absence of a detailed knowledge of the flow dynamics, the data were interpreted tentatively in terms of superficial gas—liquid mass flow rate.     

Modelling of phase redistribution of horizontal annular flow divided in T-junctions

A phase redistribution model for annular flow divided in T-junctions is developed. The model consists of two sub-models accounting for phase distribution in the inlet to the junction and phase redistribution in the junction, respectively. In modelling phase redistribution in the junction, the dividing streamlines for each phase were determined separately assuming that the phenomena is mainly governed by the fluid inertia and pressure distribution in the junction. The overall model is capable of predicting phase redistribution in T-junctions having horizontal inlet and any branch orientation. The predictions of this model and those of other available models are compared with experimental data. The predictions of the present model were found to be in better agreement with the experimental data than other available models and correlations.     

Film thickness in unstable modes of upward cocurrent flow

The film thickness is one of the most important parameters characterizing the operation of film-type gas-liquid heat-and mass-transfer apparatuses.     

Effect of inclination on circumferential film thickness variation in annular gas/liquid flow

Time-varying, circumferentially local liquid film thickness data have been collected on a 38 mm internal diameter pipe at inclinations of 0^°, 30^°, 45^°, 60^° and 85^° from the horizontal using flush-mounted and parallel wire conductance probes. Analysis of these data permits time-averaged thicknesses, probability density functions, and power spectral densities to be determined. Results show that the distribution of the liquid film is not symmetrical with thicker films on the lower part of the pipe, which are dominated by large disturbance waves. Moreover, as the inclination angle deviates from horizontal, the film thickness distribution becomes systematically less asymmetric. The probability density functions show a strong narrow peak where the liquid film is less disturbed by the presence of waves. The power spectra show that a large portion of wave energy at the bottom is carried by waves of frequency . There is no influence of liquid velocity on the shape of the spectra. However, the dominant frequency appears to decrease with increasing liquid flow rate. The frequency of the disturbance waves at the bottom decreases with increasing inclination. Moreover, the spectra tend to flatten out with increasing inclination, due to the more uniform distribution of energy among waves of a broad frequency range.     

Modeling of low viscosity oil-water annular flow in horizontal and slightly inclined pipes: Experiments and CFD simulations

To characterize the effect of pipe inclination, low viscosity, flow rate and inlet water cut on annular flow pattern, a low viscosity oil-water two-phase annular flow in horizontal and slightly inclined (+1°, +3° and +5°) pipes with diameter of 20 mm has been experimentally investigated. A modified VOF model based on the CFD software package FLUENT was used to predict the in-situ oil fraction and pressure drop. The experimental data indicate that annular flow appears at a medium-high water cut. The slip ratio increases with flow rate increase but decreases with increasing water cut. The changes are more significant as the degree of inclination increases. Pressure drop is strongly dependent on flow rate, as it increases rapidly as inlet flow rate increase. Good agreement between the experimental data and calculated results of slip ratio and pressure drop was obtained.     

Liquid film properties in two-phase annular flow

Experimental measurements have been made of the properties of falling liquid films of water with and without the addition of polymeric drag reducing agents. It was found that the presence of the drag reducing agent stabilised the film, delaying the disturbance wave transition to higher Reynolds numbers and decreasing the frequency of these waves. A small increase in wave velocity and base film thickness was also found.     

Non-ideal flow in an annular photocatalytic reactor

This study deals with the modelling of non-ideal flow in a tubular photocatalytic reactor with thin layer of TiO₂ photocatalyst. The objective was to analyse different level of mixing in the photoreactor applying basic principles of chemical reaction engineering. For this purpose photocatalytic oxidation of toluene was used as the model reaction. Photocatalytic reactor was operated in two different flow modes: classic type of an annular reactor with basically ideal (plug) flow with some extent of dispersion and annular flow reactor acted as stirred tank reactor with mixing of reaction mixture accomplished by recirculation. A series of experiments with step input disturbance at the entrance of the reactor with different air flow was performed in order to achieve better understanding of the reactor hydrodynamics. Several reactor models are applied, such as one dimensional model of tubular reactor at the steady state conditions, axial dispersion model at non-stationary conditions and the model of the continuous non-stationary stirred tank reactor. Numerical methods necessary for solving model equations and parameter estimation were described.     

Modeling an annular flow tubular reactor

A stochastic model representing annular flow in a tubular reactor is proposed. Numerical simulation was utilized to generate sample paths fitting the residence time distributions (RTD) of the system. The model was constructed from basic diffusion equations with the additional consideration of random effects disturbing the system, thus yielding a stochastic partial differential equation. The stochastic model is simulated using the Euler–Maruyama procedure. Experimental data from three tubular polymerization reactors were well fitted by the model. The model encompassed the two deterministic parameters, mean residence time and Peclet number, as well as three stochastic parameters: stochastic relevance (b), updated time (ΔT) and the seed that begins the Wiener process. The satisfactory results indicate that the model constitutes an important step toward comprehending the complex fluid dynamics of tubular flow systems.     

环隙式离心萃取器内部两相流动研究进展

环隙式离心萃取器是集成液-液混合与液-液分离于一体的高性能萃取设备,其广泛应用于核工业、化工环保、有色冶金、生物医药等领域。离心萃取器具有优异的水力学特性和传质特性,这主要得益于其环隙中的泰勒涡流以及转鼓内的离心分离流等特殊流动。本文主要依据离心萃取器结构和两相流动特点,综述了环隙内气-液界面变化规律、气泡流动特性、液-液两相流型、液滴流动特性,以及转鼓内的气-液界面等方面的研究进展,还总结了环隙螺旋隔板、转鼓径向叶片等结构的优化对于两相流动、混合或分离效果的影响。在后续研究中,可以从离心萃取过程中的液滴分散和聚并机理、三相流动测试及模拟、结构的模型化设计方法等方面开展更加深入的研究。     

Flooding velocity in a counter-current annular two-phase flow

Experimental work on flooding has been performed concerning the operation of wetted columns, falling film evaporators, updraft condensers, etc. However, owing to the complexity of the phenomenon of flooding and to numerous factors affecting it, a sufficiently definite theory has not yet emerged. In response to this, the authors performed an experiment whereby the many factors affecting flooding were carried out using four test tubes and four test liquids. It is known that, when the relative velocity of a falling liquid film to a counter-current gas reaches a certain value, a wave occurring on the film surface becomes unstable, its amplitude quickly increases, bridging the test tube and flooding occurs. Taking account of this observation, a theoretical equation was derived by dealing with the instability problem of such waves and an experimental correlation worked out on the basis of the derived theoretical equation and experimental data.     

Prediction of flow patterns in horizontal two-phase pipe flow

A new concept of a sequence of plots is used for the separation of flow regimes in two-phase flow. The empirical correlations themselves are based on extensive tabulations of flow-type data from the AGA-API Data Bank. Provided these data and the tabulations are accurate, the accuracy of the proposed method is better than 5%. The accuracy for any separation can easily be obtained from the information given. The probability of being able to separate flow types by the various correlations was determined and used to select the best correlations. In actual use, there will be times when a complete separation cannot be made. However, one will know this and be able to say that the flow is one of two or, rarely, more types.     

Slug flow characterization in horizontal annulus

To characterize slug flows in annuli channels and highlight the effect of the eccentricity on the flow behaviors, experiments were conducted in two horizontal annuli setups (a) concentric and (b) fully eccentric using air and water as the testing fluids. The range of air and water superficial velocities investigated were 0.45–3.49 m/s and 0.15–2.77 m/s, respectively. Slug parameters measured using conductance probes designed for this study include slug length, translational velocity, slug frequency, and slug holdup. It is found that the slug translational velocity is unaffected by the annulus eccentricity; however, parameters including slug frequency, slug holdup, and slug lengths have a higher value in the fully eccentric annulus when compared with the concentric one. We introduced a new definition of hydraulic diameter, which reconciles the correlation between the dimensionless mean slug length and the mixture velocity of the horizontal annuli with different setups.     

Mass transfer in annular cylindrical reactors in laminar flow

Radial diffusional mass transfer is studied in a fluid flowing in fully developed laminar flow in an annular cylindrical reactor in which a first order heterogeneous reaction is taking place at the wall. The asymptotic behaviour of the concentration decrease far from the reactor inlet is especially investigated. Limiting values of Sherwood numbers are numerically determined and represented by semi-empirical expressions. Additivity relationships between homogeneous and heterogeneous contributions are established. Theoretical results are found in excellent agreement with those yielded by a new experimental method based on heterogeneous decomposition of ozone. Annular reactors exhibit a mass transfer efficiency which is noticeably higher than that of empty tubes. This efficiency may be characterized by three criteria related to inner space utilization, catalytic surface utilization and/or mechanical energy degradation.     

水平和倾斜管内气液分层流界面稳定性

A viscous Kelvin-Helmholtz criterion of the interfacial wave instability is proposed in this paper based on the linear stability analysis of a transient one-dimensional two-fluid model. In thismodel, the pressure is evaluated using the local momentum balance rather than the hydrostatic approximation. The criterion predicts well the stability limit of stratified flow in horizontal and nearly horizontal pipes. The experimental and theoretical investigation on the effect of pipe inclination on the interfacial instability are carded out. It is found that the critical liquid height at the onset of interfacial wave instability is insensitive to the pipe inclination. However, the pipe inclination significantly affects critical superficial liquid velocity and wave velocity especially lor low gas velocities.     

A multidimensional model for annular gas-liquid flow

A finite volume method-based CFD model has been developed to simulate steady, turbulent, two-dimensional annular gas-liquid flow in a duct. The gas flow is treated as being equivalent to flow through a rough-walled duct. The effect of the liquid film on the gas phase is included in the form of modified wall functions which incorporate the well-known triangular relationship (Annular Two-Phase Flow, Pergamon Press, Oxford, 1970) that exists among wall shear stress, film flow rate and film thickness in annular flow. The presence of droplets is accounted for by solving an additional scalar transport equation for the mass fraction of the droplets. Entrainment and deposition of droplets are included as source term and boundary condition, respectively, in the mass fraction equation. It is shown that the resulting model, while retaining simplicity of formulation, gives good predictions of the literature data of annular flow parameters under equilibrium and non-equilibrium conditions.