二元气-固流化系统临界流化速度的研究

在直径为4cm的有机玻璃流化床中,对由不同尺寸和质量分数的铁矿石和煤颗粒组成的二元气-固流态化系统进行了流化特征的实验研究,得到了该系统的流化特性曲线,给出了临界流化速度和二元系统混合颗粒平均直径之间的关系,用该关系式对临界流化速度进行了预测,并且将预测值和实验值进行了比较。结果表明,当煤颗粒的质量分数为10%时,临界流化速度umf与颗粒直径的平方dm2的关系为umf=0.13dm2+1.2;当煤颗粒的质量分数为20%时,两者的关系变为umf=0.12dm2+1.2,预测值与实验值误差在10%以内。     

Numerical simulation of hydrodynamic characteristics in a gas–solid fluidized bed

The fluidization of quartz particles as bed materials in the fluidized bed has significant influences on the combustion and gasification of refused derived fuels. Three-dimensional (3-D) simulations and analyses are performed for Geldart B particles using the computational fluid dynamics (CFD) method based on the kinetic theory of granular flows (KTGF) to investigate the hydrodynamic behavior. The drag models of Syamlal–O’Brien, Gidaspow, and Wen and Yu are selected to analyze the applicability of the kinetic model. The pressure drop, velocity distribution and solid volume fraction are studied numerically when the gas inlet velocity is changed. The results show that the increase of superficial gas velocity would lead to heterogeneous expansion of solid volume fraction and velocity distributions in both the dense phase zone and free board with a similar distribution pattern. The near wall particles form a dense phase structure with the solid volume fraction being greater than 0.3.     

Slugging Behavior of Geldart D Particles in a Vibrated Gas-Fluidized Bed

The periodic slugging behavior of Geldart D particles in a vibrated gas-fluidized bed (VFB) is favorable for some applications involving the processes of particles stratification, especially for fine coal beneficiation in a VFB. It is of great significance to conduct systematic studies on slug formation, slug coalescence and growth, slug rising velocity, and slugging frequency in a VFB of Geldart D particles. The slug formation model is proposed based on theoretical analysis and indicates that the vibration amplitude, the vibration frequency, and the superficial air velocity all have significant effects on the slugging behavior. Also, the comparison of predicted values and measured values of the slug height at its initial formation verifies that the slug formation model has high prediction accuracy. The dynamic of the coalescence of bubbles into an adjacent slug is qualitatively studied and the correlations of the slug height and the slug rising velocity are established, respectively. The results show that the slug height increases with the excess fluidizing air velocity and the height of slug location, and the slug rising velocity performs an exponential growth with the increase of slug height. The results also show that the slugging frequency is independent on the superficial air velocity.     

Viscosity and separation performance of a gas-solid separation fluidized beds

A theoretical model of viscosity in gas-solid separation fluidized beds is established according to the two-phase flow theory of fluidized beds. After comparing theoretical and measured values, the correlation coefficient between the two is as high as 0.99, showing that the model has good predictability for the viscosity of fluidized beds. Meanwhile, the viscosity and its influencing factors were studied using a Brookfield viscometer. The study shows that smaller medium particles (0.074–0.15?mm) can reduce the viscosity of fluidized beds, but they will aggravate the viscosity fluctuation at more than 5?wt% addition, which is unfavorable to the stability of fluidized beds. In addition, in the actual separation process, the external factors (such as moisture and coal powder content) also affect the viscosity of the fluidized beds. Increasing the moisture increases the viscosity of the fluidized bed, whereas coal dust has the opposite effect. In order to ensure the stability of the fluidized bed, the bed moisture content should be controlled below 1?wt%, while the content of coal powder should be limited below 5?wt%. Based on separation tests, reducing the viscosity will improve the separation performance of a fluidized bed at the proper fluidized gas velocity, with the lowest possible error E_p of 0.085.     

空气重介质流化床的床层黏度研究

利用旋转法对空气重介质流化床的床层黏度进行研究,分析影响流化床床层黏度的因素,建立床层黏度与影响因素的数学模型。结果表明:流化床床层黏度随着加重质磁铁矿粉平均粒度的增大而增大,随着流化气速的增大而减小,随着床层高度及重介质水分含量的增大而增大;当煤粉质量分数大于12%时,床层黏度明显增大,煤粉质量分数为17%时,气-固两相流的流动性变差。     

The hydrodynamics of liquid-solid and gas-liquid-solid inverse fluidized beds with bioparticles

The hydrodynamic characteristics, such as minimum fluidization velocity (U_lmf for liquid-solid (LS) system and U_g,if for gas-liquid-solid (GLS) system) and bed expansion ratio (BER), of liquid-solid and gas-liquid-solid inverse fluidized beds (LSIFB and GLSIFB) with bare particles and particles with biofilm were investigated. In the LSIFB system, U_lmf and BER of the bare particles were independent of the solids loading. For bioparticles, the increase of the biofilm thickness reduced U_lmf and increased BER, suggesting that the fluidizability increases with the presence of the biofilm. In the GLSIFB system, the initial fluidization gas velocity (U_g,if) and the complete fluidization gas velocity (U_g,cf) both increased with increasing particle diameter and decreasing particle density under fixed superficial liquid velocities. Biofilm attachment led to a decrease of both U_g,if and U_g,cf, and an increase of bed expansion, again suggesting increased fluidizability with the presence of biofilm.     

循环流化床底部区域流动特性的数值模拟

基于欧拉两相流模型计算循环流化床底部区域的流动特性。在低气速(1.0～2.5m/s)、低循环量下(5.2～34.5kg/(m2·s)),模拟时黏性采用层流模型取得了较好的效果。实验采用光导纤维探头测量仪测量流化床底部区域3个截面的局部颗粒浓度,模拟计算了循环流化床底部3个截面的颗粒浓度的径向分布,并同循环流化床装置的实验数据进行了对比。结果表明,数值模拟计算与实验结果相吻合。     

Hydrodynamic Study of Gas–Solid Internally Circulating Fluidized Bed Using Multiphase CFD Model

In the present work, hydrodynamic study of gas and solid flow in an internally circulating fluidized bed (ICFB) was carried out using the CFD multiphase model. Two- (2D) and three-dimensional (3D) computational meshes were used to represent physical ICFB geometries of 0.186-m and 0.3-m diameter columns. The model approach uses the two-fluid Eulerian model with kinetic theory of granular flow options to account particle–particle and particle–wall interactions. The model also uses various drag laws to account the gas–solid phase interactions. The 2D simulation results by various drag laws show that the Arastoopour and Gibilaro drag models predict the fluidization dynamics in terms of flow patterns, void fractions, and axial velocity fields in close agreement with the Ahuja et al. (2008 Ahuja, G. N., and A. W. Patwardhan. 2008. CFD and experimental studies of solids hold-up distribution and circulation patterns in gas–solid fluidized beds. Chemical Engineering Journal 143:147–160.[Crossref], [Web of Science ®] , [Google Scholar]) experimental data. Three dimensional simulations were also carried out for a large-scale ICFB. The effects of superficial gas velocity and the presence of draft tube on solid holdup distribution, solid recirculation pattern, and gas bypassing dynamics for the 3D ICFB were investigated extensively. The mechanism governing the solid circulation and the pressure losses in an ICFB has been explained based on gas and solid dynamics obtained from these simulations. Predicted total granular temperature distributions in 3D ICFB draft tube and the annular zone are qualitatively in agreement with the experimental data. The total granular temperature tends to increase with the increase in solids concentration in the dilute region (? < 0.1) and decrease with an increase of solids concentration in the dense region (? > 0.1).     

Experimental and theoretical study on hydrodynamic characteristics of tapered fluidized beds

A novel fluidized bed ash cooler was developed for circulating fluidized bed boilers based on a proposed modified tapered fluidized bed. A cold model was built to study the hydrodynamic characteristics of the modified tapered fluidized bed, and its critical superficial gas velocity u_mf and critical velocity for full fluidization u_mff were particularly studied. The effects of taper angle α, static bed height H, air inlet section width δ and particle size d_p on the u_mf and u_mff were experimentally investigated. Furthermore, a theoretical model and an empirical correlation have been proposed to predict the u_mf and u_mff, respectively. The predicting capabilities of the model and correlation have been experimentally discussed. And the predicting capability of the model has also been compared with that of an existing representative model. It is found that both the u_mf and u_mff increase with the increase of taper angle α, static bed height H and particle size d_p, but decrease with the increase of air inlet section width δ, respectively. Additionally, the predicted values of u_mf and u_mff compare well with the experimental data, and the model has a better capability than the existing representative model in predicting the u_mf of the modified bed.     

Velocity of full fluidization of a bed of polydisperse granular materials

Based on the principle of hydrodynamic equivalence, a technique to calculate the velocity of complete fluidization of polydisperse granular materials of different densities with allowance for the transformation of their initial size distribution has been worked out. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 82, No. 2, pp. 296–300, March–April, 2009.     

Investigation of gas-solid flow characteristics in a novel internal fluidized bed combustor by experiment and CPFD simulation

Based on the coal self-preheating combustion technology, this research proposed a novel internal fluidized bed combustor (IFBC) with an internal separator for stable preheating of fuel. In order to verify feasibility and operation stability of IFBC, cold experiment, electrical capacitance tomography (ECT) and computational particle fluid dynamic (CPFD) simulation were performed in a laboratory-scale IFBC. The effects of superficial air velocity (U_g) and return valve structure on the operation and gas-solid flow characteristics were investigated. The results revealed that the CPFD prediction agreed well with the experiment values. The pressure balance curve presented an “8″ shape distribution, and the particle volume fraction (PVF) showed ‘core-annular’ distribution features. With the increase of U_g, the PVF in the standpipe increased, and the discharge pattern of the return valve changed from continuous discharge to intermittent discharge, and the solid circulation flux showed a trend of increasing first and then decreasing. With the decrease of the outlet opening of return valve (Φ), the gas–solid flow behavior in standpipe experienced a transition from gas leakage, stabilizing material seal, and blocking state. For U_g = 2 m/s, Φ = 50 %, an effective solid seal in the return valve was established and IFBC has a stable circulation and operation.     

Free-surface granular flows down heaps

The present paper extends the granular-flow constitutive model of Savage (1998 J Fluid Mech 377:1–26) to treat spherical particles. Savage accounted for both quasi-static and collisional stresses by considering: (i) strain-rate fluctuations embodied in a critical state plasticity model, as well as, (ii) individual particle velocity fluctuations modelled by granular-flow kinetic theory. In the present work, the governing equations of the kinetic theory of Jenkins (1998 In: Hermann HJ, Luding S (eds) Physics of Dry Granular Media. Kluwer Academic pp. 353–370) for identical spherical, smooth, inelastic particles are supplemented with additional quasi-static terms that have forms patterned after the corresponding terms in the equations of Savage for two-dimensional disk-like particles. The resulting equations along with side-wall and free-surface boundary conditions are applied to examine free-surface granular flow down a heap contained between two frictional vertical side walls. Width-averaged equations of motion are integrated to obtain depth profiles of mean velocity, granular temperature, solids fraction and the Savage–Jeffrey parameter. Detailed comparisons are made with particle-tracking experiments. When the gap between the vertical side walls is fairly narrow, good agreement is found between the predicted and the measured profiles of mean velocity and granular temperature.     

Depth-averaged relations for granular-liquid uniform flows over mobile bed in a wide range of slope values

The behavior of liquid-granular flows, driven by gravity, is experimentally analyzed. Two types of free-surface uniform flow can take place, having different boundary conditions at the bottom. The first one runs over a fixed surface behaving as a solid (non-deformable) impermeable wall; the second one runs over a mobile-bed at rest, formed by the same loose grains and liquid of the flowing mixture. In the paper we will mark the differences between the two, but focus on the latter one. The experiments span over, and characterize, the possible flow regimes. In mobile-bed uniform flows it has been found that the Froude number reduces as the slope increases. Accordingly, there is an increment of the solid-concentration. These results are meaning that as slope increases a progressive dominance and thickening of frictional layers over collisional ones is taking place through the flow depth. Same behaviours have been observed by changing the type of grains in the flowing mixture. These findings contrast with the case of flows over a solid wall, where different trends are observed. Application of force balances by means of Coulomb law provides interesting confirmation of what observed and allows to take into account the surface-tension effects, which come into play when the particles on top are going to desaturate. Experimental data have also been employed to assess the applicability of kinetic theories to wet granular flows. Energy and momentum balances, under the hypothesis of no contribution in the liquid phase (except for the added mass concept) to shear stress and to the energy processes, are applied throughout the flow depth of the solid phase. Although depth-averaged quantities come out to have a trend similar to the experimental one, deficiencies in the theoretical approach, mainly due to its inability to represent frictional contacts, are clearly detected. Same conclusions may be drawn by applying the quite simple Bagnold theory. Altogether, a more appropriate theory able to deal with both collisional and frictional mechanisms, including the transition between, is demanded.