Voidage measurement of gas–liquid two-phase flow based on Capacitively Coupled Contactless Conductivity Detection

Based on Capacitively Coupled Contactless Conductivity Detection (C⁴D) technique, a new method for the voidage measurement of conductive gas–liquid two-phase flow is proposed. 15 Conductance signals, which reflect voidage distribution of gas–liquid two-phase flow, are obtained by a six-electrode C⁴D sensor. With the conductance signals, the flow pattern of gas–liquid two-phase flow is identified by flow pattern classifiers and then the voidage measurement is implemented by a corresponding voidage measurement model (for each typical flow pattern, a corresponding voidage measurement model is developed). The conductance measurement of the six-electrode C⁴D sensor is implemented by phase sensitivity demodulation (PSD) method. The flow pattern classifiers and the voidage measurement models are developed by partial least squares (PLS) technique and least squares support vector machine (LS-SVM) technique. Static voidage measurement experiments and dynamic voidage measurement experiments show that the proposed voidage measurement method is effective, the developed six-electrode C⁴D sensor is successful and the measurement accuracy is satisfactory.     

Numerical Simulation of a Spouted Bed Using Computational Fluid Dynamics (CFD)

In this article, computational fluid dynamics (CFD) technology is used to model a spouted bed(SB). The multifluid Eulerian-Eulerian approach based on kinetic theory of granular flows and Gidaspow's drag model for the interaction between gas and particles are applied in the modeling. The effects of the SB properties—that is, cone angle, particle size, cylinder diameter, and static bed height of particles—on its dynamics performance are investigated. The simulated results—that is, flow pattern of particles, fountain height, voidage, and particle velocity of the spout zone—are presented. It is shown that periodic fluctuation of spouting appears in an SB with conical angle of 30° and inlet velocity at 16.6 m/s. When the SB cylinder diameter becomes 0.52 m, periodic fluctuation appears, too. The stable spouting of the SB with a 90° cone angle could be obtained at an inlet air velocity of 24.3 m/s. The fountain height of particles decreased with an increase in particle size and the static bed height of particles. It is kept at about 0.19 m when different SB cylinder diameters in the range of 0.36 to 0.48 m are used. In the spouting region, the voidage decreased with static particle height in bed, but the particle velocity increased. For a certain particle size, the voidage decreased with an increase in particle height, but the velocity of the particles increased. It was also found that the cylinder diameter did not affect the volume fraction of particles except for the cylinder diameter 0.52 m and the change in particle velocity was minimal in the spout zone. With the different static bed height of particles used, the voidage and particle velocity did not change much at the same level of spout zone.     

喷动流化床流动特性研究

选用密度与尿素相近而粒度不同的三种模拟物料，在内径为１８２ｍｍ的有机玻璃喷动流化床中，对喷动流化床的流体力学行为进行了研究。测定了不同物料最大操作区所对应的静床高及喷动流化气速与床层压降、床层空隙率分布的关系。为喷动流化床尿素造粒热态试验提供了可资借鉴的设计及操作依据     

Experimental and numerical study of fluid dynamic parameters in a jetting fluidized bed of a binary mixture

The solid circulation pattern, the voidage profile, and the jet penetration height have been investigated experimentally and computationally in a cold-flow model of jetting fluidized beds (JFBs) of a binary mixture in this paper. This rectangular two-dimensional bed is 0.30 m wide and 2.05 m high with a central jet and a conical distributor, which roughly stands for the ash-agglomerating fluidized-bed coal gasifier. A video camera and coloured particle tracer method were employed to explore the fluid dynamics in the bed. In terms of the average physical properties of binary mixtures, a hydrodynamic model describing the gas-solid flow characteristics in a jetting bed is resolved by using a modified Semi-Implicit Method for Pressure-Linked Equation (SIMPLE) algorithm. This paper focuses on three features of the fluid dynamics—solid circulation pattern, voidage profile, and jet penetration height. The solid circulation pattern is composed of three regions: the jetting region, the bubble street, and the annular region. Above the central nozzle the time-averaged isoporosity contours are almost elliptic, while near the walls of the bed, the voidage in high solid concentration region is approximately equal to that at the minimum fluidization state. The jet penetration height increases with increasing jet gas velocity and with decreasing average particle diameter. The increase in weight percentage of the lighter component in the binary system reveals that reduction of average density causes the enlargement of jet penetration height. The simulated results show good agreement with the experimental data.     

循环流化床稀相流动局部空隙率的测量

本文采用激光相让多普勒颗粒分析仪（PDPA）测量流化床气固两相流局部瞬时流动结构．通过测量瞬时颗粒直径、速度及颗粒通过测量体的渡越时间，分析导出了一种计算局部瞬时空隙率的方法实验证明即使在很稀的稀相流动中仍存在着不均匀的流动结构．在同一流动条件下，还用光纤探针测量了瞬时空隙率．结果表明PDPA测量方法能够测量出光纤探针无法测量到的稀相流动中微观瞬时流动结构的变化．     

Time-dependent gas holdup variation in an air-water bubble column

Time-dependent gas holdup variation in a two-phase bubble column is reported with air and tap water as the working fluids. The results indicate that time-dependent gas holdup is closely related to the water, whose quality is unsteady and changes, not only during the two-phase flow, but also during idle periods. The significance and characteristics of the time-dependent gas holdup variation are influenced by the bubble column operation mode (cocurrent or semi-batch), the sparger orientation, the superficial gas velocity, and the superficial liquid velocity. It is proposed that a volatile substance (VS), which exists in the water in very small concentrations and inhibits bubble coalescence, evaporates during column operation and results in a time-dependent gas holdup. The influence of bubble column operation mode, sparger orientation, superficial gas velocity, and superficial liquid velocity on the time-dependent gas holdup variation are explained based on their effects on bubble size, bubble contacting frequency and mixing intensity. This work reveals that regular tap water may cause significant reproducibility problems in experimental studies of air-water two-phase flows.     

Void fraction in vertical gas-liquid slug flow: Influence of liquid slug content

In this study we develop a model for computing the mean void fraction and the liquid slug void fraction in vertical upward gas-liquid intermittent flow. A new model for the rate of gas entrained from the Taylor bubble to the liquid slug is formulated. It uses the work done by the pressure force at the rear of the Taylor bubble. Then an iterative approach is employed for equating the gas entrainment flux and the gas flux obtained via conservation equations. Model predictions are compared with experimental data. The developed iterative method is found to provide reasonable quantitative predictions of the entrainment flux and of the void fraction at low and moderate liquid slug void fraction conditions. However, with an increased liquid slug void fraction experimental data indicate that the flow in the liquid slug transits to churn-heterogeneous bubbly flow thus gas entrainment flux tends to zero. Considering this effect in the iterative model significantly improved the predictions for large liquid slug void fraction conditions.     

快速流化床入口结构对床层流动特性的影响

在直径０．１４ｍ、高１１ｍ的快速流化床装置上，考察了床层入口结构及进风系统对床层流动特性，尤其是截面平均空隙率轴向分布的影响。实验结果表明，由于入口结构的改变，床内两相流动将受到严重影响。当入口结构为弱约束时，截面平均空隙率可为Ｓ型轴向分布。     

The influence of moisture on the fluidization behaviour of porous pharmaceutical granule

The influence of moisture content on the fluidization behaviour of placebo pharmaceutical granule has been studied in a 14 cm diameter cylindrical fluidized bed column. The dry granule has a mean diameter of and exhibits a bimodal size distribution with modes of 169 and . Bed pressure drop profiles and tapped density measurements were generated for granule moisture contents between 5 and 30 wt%, which corresponds to typical final and initial moisture contents experienced during drying. At high moisture contents, the wet granule exhibits Geldart C type powder behaviour as channelling and defluidization exist. As the moisture content is reduced, the granule fluidity improves and demonstrates behaviour characteristic of Geldart B powders. The changing fluidization behaviour was quantified using parameters such as the full support velocity, full support bed voidage and Hausner ratio. These parameters were found to increase significantly above granule moisture contents of 10 wt%. The increase in the Hausner ratio suggests that the interparticle force load in the bed increases. This change in interparticle force load is responsible for the increase in the full support velocity and bed voidage.     

Discrete element method modeling of non-spherical granular flow in rectangular hopper

Discrete element method (DEM) was developed to simulate the corn-shaped particles flow in the hopper. The corn-shaped particle was described by four overlapping spheres. Contact force and gravity force were considered when establishing the model. In addition, the velocity distribution and voidage variance of corn-shaped and spherical particles were investigated. The results show that the vertical velocity difference between centre and side wall and the horizontal velocity of corn-shaped particles are relatively larger than that of spherical particles. The mean voidage for corn-shaped particles is smaller than for spherical particles in any hopper. And the mean voidage values decrease with the increase of the ratio of width and length (D/L) and the ratio of height and width (H/D) for both corn-shaped and spherical particles. The local voidage profiles in hoppers with different D/L were also studied. It demonstrates that the wall effect on the voidage of spherical particles is more remarkable than that of the corn-shaped particles. The voidage fluctuations of corn-shaped and spherical particles decrease obviously with increasing D/L when the particles are far away from the wall. And when the particles are discharging, the wall effect on the spherical particles is more remarkable than the condition of packing naturally.