Experimental and numerical investigations of scale-up effects on the hydrodynamics of slurry bubble columns

Experiments and simulations were conducted for bubble columns with diameter of 0.2 m(180 mm i.d.), 0.5 m(476 mm i.d.) and 0.8 m(760 mm i.d.) at high superficial gas velocities(0.12–0.62 m·s-1) and high solid concentrations(0–30 vol%). Radial profiles of time-averaged gas holdup, axial liquid velocity, and turbulent kinetic energy were measured by using in-house developed conductivity probes and Pavlov tubes. Effects of column diameter, superficial gas velocity, and solid concentration were investigated in a wide range of operating conditions. Experimental results indicated that the average gas holdup remarkably increases with superficial gas velocity, and the radial profiles of investigated flow properties become steeper at high superficial gas velocities. The axial liquid velocities significantly increase with the growth of the column size, whereas the gas holdup was slightly affected. The presence of solid in bubble columns would inhibit the breakage of bubbles, which results in an increase in bubble rise velocity and a decrease in gas holdup, but time-averaged axial liquid velocities remain almost the same as that of the hollow column. Furthermore, a 2-D axisymmetric k–ε model was used to simulate heterogeneous bubbly flow using commercial code FLUENT 6.2. The lateral lift force and the turbulent diffusion force were introduced for the determination of gas holdup profiles and the effects of solid concentration were considered as the variation of average bubble diameter in the model. Results predicted by the CFD simulation showed good agreement with experimental data.     

基于颗粒动理学的气固喷射器流动特性三维数值模拟

A computational study on the flow behavior of a gas-solid injector by Eulerian approach was carried out. The gas phase was modeled with k-ε turbulent model and the particle phase was modeled with kinetic theory of granular flow. The simulations by Eulerian two-fluid model （TFM） were compared with the corresponding results by discrete element method （DEM） and experiments. It was showed that TFM simulated results were in reasonable agreement with the experimental and DEM simulated results. Based on TFM simulations, gas-solid flow pattern, gas velocity, particle velocity and the static pressure under different driving jet velocity, backpressure and convergent section angle were obtained. The results showed that the time average axial gas velocity sharply decreased and then slightly increased to a constant value in the horizontal conveying pipe. The time average axial particle velocity increased initially and then decreased, but in the outlet region of the convergent section the particle velocity remarkably increased once more to the maximal value. As a whole, the static pressure distribution change trends were found to be independent on driving gas velocity, backpressure and convergent section angle. However, the static pressure increased with increase of convergent section angle and gas jet velocities. The difference of static pressure to backpressure increased with increasing backpressure.     

The turbulent behavior of novel free triple-impinging jets with large jet spacing by means of particle image velocimetry☆

A novel reactor that achieves rapid liquid–liquid mixing via free triple-impinging jets(FTIJs) is developed to improve mixing efficiency at unequal flow rates for liquid–liquid reactions. The flow characteristics of FTIJs were investigated using particle image velocimetry(PIV). The instantaneous and mean velocities data at different Reynolds numbers(Re) were analyzed to provide insights into the velocity distributions in FTIJs. The effect of jet spacing on the stagnation points, instantaneous velocity, mean velocity, profiles of the x- and ycomponents of mean velocity, and turbulent kinetic energy(TKE) distributions of FTIJs were investigated at Re = 4100 with a volumetric flow rate ratio of 0.5. The characteristics of the turbulent flows are similar for all jet spacings tested. Two stagnation points are observed, which are independent of jet spacing and are not located in the center of the flow field. However, velocity and TKE distributions are strongly dependent on the jet spacing.Decreasing jet spacing increases the expansion angle and the values of TKE, leading to strong turbulence, improving momentum transfer and mixing efficiency in FTIJs. The present study shows that optimization of the operating parameters is helpful for designing FTIJs.     

气-液-固三相下喷环流反应器局部气相流动特性

The local gas-phase flow characteristics such as local gas holdup (εg), local bubble velocity (Vb) and local bubble mean diameter (db) at a specified point in a gas-liquid-solid three-phase reversed flow jet loop reactor was experimentally investigated by a five-point conductivity probe. The effects of gas jet flow rate, liquid jet flow rate, solid loading, nozzle diameter and axial position on the local εg, Vb and db profiles were discussed. The presence of solids at low solid concentrations not only increased the local εg and Vb, but also decreased the local db. The optimum solid loading for the maximum local εg and Vb together with the minimum local db was 0.16 × 10-3 m3, corresponding to a solid volume fraction, εS = 2.5%.     

气相温度脉动对碳黑生成反应的影响(英文)

The effects of gas temperature fluctuations on soot formation and oxidation reactions are investigated numerically in a reacting flow. The instantaneous variations of soot mass fraction with time are obtained under the time-averaged gas temperature of 1500-1700 K. The simulation results show that the gas temperature fluctuation has obvious influence on the instantaneous processes of soot formation and oxidation. Within the present range of gas temperature, the gas temperature fluctuation results in generally lower soot mass fraction comparing to that without gas temperature fluctuation. The increase in the fluctuation amplitude of gas temperature leads to decrease in time-averaged soot mass fraction and increase in time-averaged soot particle number density.     

鼓泡塔中水合物法分离混合气体的数值模拟

To develop a new technique for separating gas mixtures via hydrate formation,a set of medium-sized experimental bubble column reactor equipment was constructed.On the basis of the structure parameters of the ex- perimental bubble column reactor,assuming that the liquid phase was in the axial dispersion regime and the gas phase was in the plug flow regime,in the presence of hydrate promoter tetrahydrofuran（THF）,the rate of hydrogen enrichment for CH4＋H2 gas mixtures at different operational conditions（such as temperature,pressure,concentra- tion of gas components,gas flow rate,liquid flow rate）were simulated.The heat product of the hydrate reaction and its axial distribution under different operational conditions were also calculated.The results would be helpful not only to setting and optimizing operation conditions and design of multi-refrigeration equipment,but also to hydrate separation technique industrialization.     

Experimental Determination and Modeling of Bubble Size Distributions in Bubble Columns

Using a five point conductivity technique local values of bubble size,bubble velocity and gas fractionhave been experimentally determined in a 288 mmID and 4.3 m high bubble column as a function of axial andradial position for the air/water and CO_2/N_2/aqueous MDEA systems.The experimental results are comparedwith predictions from a fundamental two-fluid model.The implementation of a non-steady lateral drag term inthe two-fluid model has been shown.In addition to improving the physical realism of the model,it is found togive slight improvements in the predictions of the distributions of local bubble size.Predictions of bubble size arefound in reasonable agreement with experimental values in the heterogeous flow regime,whereas they are stil1found to be unreliable at low gas velocities.Local void predictions are found in reasonable agreement with experi-mental values,but deviations occur in the homogeneous flow regime towards the wall.This is attributed to defi-ciencies in the simplified bubble size mode     

气-雾超音速两相流过渡段内速度滑移和相间动量传递

Modelling and simulations are conducted on velocity slip and interfacial momentum transfer for supersonic two-phase (gas-droplet) flow in the transient section inside and outside a Laval jet(LJ). The initial velocity slip between gas and droplets causes an interfacial momentum transfer flux as high as (2.0-5.0) XXXXXx 104 Pa. The relaxation time corresponding to this transient process is in the range of 0.015-0.090ms for the two-phase flow formed inside the LJ and less than 0.5ms outside the LJ. It demonstrates the unique performance of this system for application to fast chemical reactions using electrically active media with a lifetime in the order of 1 ms. Through the simulations of the transient processes with initial Mach number Mg from 2.783 to 4.194 at different axial positions inside the LJ, it is found that Mg has the strongest effect on the process. The momentum flux increases as the Mach number decreases. Due to compression by the shock wave at the end of the LJ, the flow pattern becomes two d     

螺旋片导流式气液分离器的数值模拟与试验研究

The gas/liquid spiral separator, a key component in the compressed air system, was used to remove liquid and oil from gas stream by centrifugal and gravitational forces. To optimize the design of the separator, the relationship between the performance and structural parameters of separators is studied. Computational fluid dynamics （CFD） method is employed to simulate the flow fields and calculate the pressure drop and separation efficiency of air-liquid spiral separators with different structural parameters. The RSM （Reynolds stress model） turbulence model is used to analyze the highly swirling flow fields while the stochastic trajectory model is used to simulate the traces of liquid droplets in the flow field. A simplified calculation formula of pressure drop in spiral structures is obtained by modifying Darcy＇s equation and verified by experiment.     

液膜性质的小尺度研究

Structured packing is a good candidate for CO₂ capture process because of its higher mass transfer efficiency and lower pressure drop.Now,the challenging problem of CO₂ capture and storage demands more and more efficiency equipment.The aim of the present study is to investigate the liquid film characteristics under counter current gas phase and throw some insight into the enhancing mechanism of mass transfer performance in structured packing.A high speed digital camera,non-intrusive measurement technique,was used.Water and air were working fluids.Experiments were carried out for different gas/liquid flow rates and different inclination angles.The time-average and instantaneous film widths for each set of flow parameters were calculated.It is shown that the effects of gas phase could be neglected for lower flow rate,and then,become more pronounced at higher flow rate.According to instantaneous film width,three different stages can be distinguished.One is the constant width of liquid film.The second is the slight decrease of film width and the smooth surface.This kind of character will lead to less interfacial area and deteriorate the packing mass transfer performance.For the third stage,the variation of film width shows clearly chaotic behavior.The prediction model was also developed in present work.The predicted and experimental results are in good agreement.     

旋流燃烧室内气体-颗粒两相湍流流动的数值模拟

综合应用代数Reynolds应力模型和流体相脉动速度大小和方向均具有随机性的颗粒相随机轨道模型,对旋流燃烧室内有直流射流与旋转射流相互作用的气-固两相湍流流动进行了数值模拟.得到的气相轴向与切向速度和轴向脉动速度均方根值分布以及颗粒相轴向总质量流通量和轴向与切向速度分布与实验基本相符合,并比对气相湍流采用k-ε模型的相应计算结果有较明显的改进.     

SIMULATION OF PARTICLE TRANSPORT AND DEPOSITION IN A COMBUSTOR

A computational model for Lagrangian particle tracking for studying dispersion and deposition of particles in a combustor with swirling flow and chemical reaction is developed. The model accounts for the effect of thermophoretic force, as well as the drag and lift forces acting on particles, in addition to the Brownian motion and gravitational sedimentation effects. The mean turbulent gas flow, temperature fields and chemical species concentration in the combustor are evaluated using the stress transport turbulent model of the FLUENT code. The instantaneous fluctuation velocity field is generated by a Gaussian filtered white noise model.

The simulated axial, radial and tangential mean gas velocities are compared with the existing experimental data. Ensembles of particle trajectories are generated and statistically analyzed. The effects of size and initial distribution on particle dispersion and deposition are studied. The particle concentration at different sections are also evaluated and discussed. The results shows that the turbulence dispersion effect is quite important, while the thermophoresis effect is small.     

Simulation of swirling turbulent combustion in the TECFLAM combustor

An algebraic concentration moment (ACM)-PDF turbulent combustion model is proposed and formulated in this paper. The presumed PDF approach is adopted for the closure of the time-averaged temperature relevant quantity. It is integrated with the algebraic expression for the second-order-moment of concentration fluctuations. The obtained ACM-PDF model is employed in the simulation of swirling turbulent diffusion combustion in the TECFLAM combustor. The calculated gas axial, radial and tangential velocities, turbulent kinetic energy, species mass fractions, temperature, and fluctuating temperature are compared with the measured test data. Agreement between the calculation and the measurement is achieved.     

Stereoscopic PIV studies on the swirling flow structure in a gas cyclone

Understanding the swirling flow in a gas cyclone is of great importance in improving the cyclone design. Once the three-dimensional strong swirling flow is fully understood, cyclone performance such as pressure drop and separation efficiency can be improved by optimizing the cyclone design. The swirling flow was investigated by the stereoscopic particle image velocimetry (Stereo-PIV) in this work. The instantaneous whole-field tangential, axial, and radial velocities were measured simultaneously in the cylindrical and conical separation zone, and in the dust hopper area of the cyclone with gas inlet velocity of .The time-averaged flow pattern in the cylindrical and conical sections of the cyclone showed: a typical Rankine vortex with inner quasi-forced vortex and outer quasi-free vortex which is generated by tangential gas velocity; inner upward flow and outer downward flow of axial gas velocity; and centripetal flow in the region close to the wall due to the presence of radial gas velocity. In the dust hopper, a secondary longitudinal circular flow is formed in the annulus area between the conical body and the cylindrical wall. Experimental results indicate that the separated particles may be re-entrained into the cyclone from the bin to degrade the separation efficiency of the cyclone.     

旋流数为1.0的强旋湍流两相流动的PDPA实验

采用相位多普勒颗粒测速仪 (PDPA)对旋流数为 1 0的轴向和切向进风的圆柱形旋风筒内强旋湍流气粒两相流动进行了测量研究 ,并与旋流数为 0 47、 1 5和 2 0 8的实验结果进行了对比分析 ,指出了旋流数变化对两相流场及两相湍流特性的影响 .     

旋风分离器内气相旋转流动态特性分析与表征

旋风分离器内的气相旋转流具有很强的动态特性,表现为流场瞬时参数随时间波动变化。为了对旋风分离器内气相旋转流动态特性进行表征,本文基于热线/热膜风速仪（HWFA）和动态压力传感器测量的旋风分离器内瞬时切向速度和瞬时压力,从时域和频域两个方面进行了分析。结果表明,旋风分离器内瞬时切向速度信号时域上的波形分布与旋转流的摆动存在联系,时域上的标准偏差可以直观地表征旋风分离器内旋转流的波动强度;频域的主频和功率谱密度（PSD）可以表征旋转流动态参数波动的准周期行为、传递行为和强度衰减特征,也是旋转流摆动行为及其影响范围的反映。基于瞬时切向速度和瞬时压力的时域和频域分析能较好地反映旋转流流场的波动特点,均可用于表征旋风分离器内气相旋转流的动态特性。     

The Bed Expansion and Particle Velocity Analysis in the Swirling Fluidized Bed Combustor (SFBC) Cold Model

Flow visualization and analysis was extensively conducted in a bench‐scale swirling fluidized bed combustor (SFBC) cold model. An advanced laser‐based particle image velocimetry (PIV) system was used to visualize the gas‐particle flow in SFBC cold model. The robust experiment design method was applied to study the bed expansion in the SFBC cold model in relationship with secondary airflow ratio. It was found that the secondary airflow ratio did not affect the bed expansion in the SFBC cold model. Based on the PIV velocity profiles analysis, it was observed that particle velocities increased when secondary airflow ratio is increased. The secondary air was definitely generating the high tangential particle velocity.     

Experimental study of gas swirling flow instability characteristics in a cyclone using the hot-wire anemometry technique

The instability characteristics of gas swirling flow in a cyclone were investigated experimentally by measuring the instantaneous tangential velocity with the hot-wire anemometry. The results showed that the instantaneous tangential velocity fluctuated continuously with time at both low and high frequencies. Further analysis of measured data regarding time and frequency domain by probability density and spectral methods revealed that the velocity fluctuation was affected not only by the turbulence flow itself but also by gas swirling flow instability. Also, the distributions of dominant frequency and amplitude indicated that low-frequency velocity fluctuation caused by the instability had the transfer behavior and attenuation character, which could be characterized by the dominant frequency that varied little along the radial position and decreased gradually along the axial direction, while the amplitude increased significantly with decreased radial position. Due to the gas swirling instability, the turbulence intensity and the fine particle diffusion were enhanced, which would degrade the separation efficiency of cyclone.     

顶置单喷嘴气化炉旋流场特征研究

受限旋转射流流场特征的研究对顶置单喷嘴气化炉的开发、结构优化以及长周期稳定运行有重要意义。本文对不同旋流数下顶置单喷嘴气化炉内的速度场和停留时间分布进行了实验研究。结果表明：随着旋流数的增加,切向旋转速度显著增加,气化炉内轴向速度衰减加快,回流区减小;气量一定时,随着旋流数的增加,气化炉内气体的最短停留时间显著增加,停留时间标准差减小。     

旋风分离器内气相旋转流不稳定性的实验研究

旋风分离器内气相旋转流具有较强的不稳定性,其表现形式是旋转流的旋转中心围绕几何中心的偏心摆动,导致流场的瞬时速度随时间发生脉动变化。这种旋流的不稳定特性难以用时均流场参数进行描述,需要用动态流场参数描述,为此采用热线风速仪测量了?300 mm旋风分离器内瞬时切向速度随时间的变化。实验结果表明瞬时切向速度是由气体湍流形成的高频脉动和旋转流偏心摆动形成的低频脉动两部分叠加构成,据此探讨了旋转流摆动形成的机制。瞬时切向速度的低频脉动来源于刚性涡的偏心摆动,脉动幅值与偏心距成正比。通过瞬时切向速度频域建立了旋转流的摆动频率与入口速度、筒体直径和排气管直径的计算模型。