Pulsed fluidization—a DEM study of a fascinating phenomenon

An investigation of pulsed fluidization by discrete element method simulation is presented. The particles used were 0.5 mm in diameter and 2650 kg/m³ in density. The pulsed gas flow consisted of a fluctuating component superimposed on a constant component. The effect of gas pulsation was assessed through examination of bed pressure drop and bubble patterns for a wide range of conditions. The influence on fluidized bed behaviour of frequency and amplitude of pulsation, superficial gas velocity of the base flow, and the nature (type) of pulsation was studied. Transition from chaos to ordered behaviour and formation of regular bubble patterns were reproduced. It was found that regular bubble patterns arise from periodical formation of horizontally aligned voids near the distributor plate.     

Collapsible-tube pulsation generator for crossflow microfiltration: Fatigue testing of silicone-rubber tubes

Fatigue tests have been performed for 18 silicone-rubber tubes operating as the active element of a crossflow pulsation generator. It was found that the silicone-rubber tube can be used as the main part of a cheap and reliable oscillator with good reproducibility of the downstream pressure. The operating life of the tubes was between 146 h and 378 h of continuous work for peak-to-peak downstream pressure amplitude 220 kPa to 310 kPa and frequency 5.2 Hz to 8.5 Hz. 0 1996 John Wiley & Sons, Inc.     

AN ADVANCING FRONT MODEL OF TRANSPORT INTO PULSATING LAMINAR FLOW

The diffusion and transport of a gas species is analyzed for a slowly pulsating fluid in laminar tube flow. It is assumed that the fluid velocity is in phase with the pulsating pressure gradient. A reversible chemical reaction within the fluid is modeled using advancing front theory. The pulsation is assumed to affect only the diffusion boundary layer thickness, which may be out of phase with the flow pulsation. The solution is approximate for small perturbations of the boundary layer thickness. With these assumptions a closed form solution for the bulk transport is obtained. The results are expressed as differences in the bulk transport relative to a steady flow having the same average flow conditions. The pulsation effect is expressed as a function of the non-dimensional flow amplitude and frequency parameters for various average conditions. Comparison with experimental measurements of oxygen diffusing into water and hemoglobin solutions shows good agreement. For the range of parameters used for the measurements, there is no effect of the pulsation on time-averaged transfer rates.     

Flame “blow-back” in the combustion of premixed gases in flow-through systems

Conclusions In the pulsating combustion of premixed mixtures the flame may be blown back from the combustion chamber into the mixing zone at velocities exceeding the flash-back velocity by more than an order. The chief factor in the determination of flame blow-back is the amplitude of the pressure pulsation, whose value depends on the combustion chamber dimensions, flow velocity, the temperature and composition of the mixture, and the resistance of the combustion chamber outlet system.Fizika Goreniya i Vzryva, Vo. 4, No. 2, pp. 209–214, 1968     

Pulsating Jet Impingement Heat Transfer Enhancement

This article presents results from a numerical study of pulsating jet impingement heat transfer. The motivation is to seek conditions offering a significant enhancement compared to steady flow impingement drying. The CFD software package FLUENT was used for simulating slot-type pulsating jet impingement flows with confinement. The parameter study included velocity amplitude ratio, mean jet velocity, and pulsation frequency. The distance from nozzle exit to surface was three times the hydraulic diameter of the nozzle. The Reynolds number based on the nozzle hydraulic diameter and jet temperature was 2,460 with a mean jet velocity of 30 m/s, which is the base case of the numerical experiments. Results showed that time-averaged surface heat transfer increased with increasing velocity amplitude for the same mean jet velocity. Large velocity amplitudes helped enhance heat transfer by two mechanisms: high jet velocity during the positive cycle and strong recirculating flows during the negative cycle. For the cases with different mean jet velocities but the same maximum velocity, time-averaged surface heat flux decreased with decreasing mean jet velocity. As for the effects of pulsation frequency, with high-velocity amplitude ratio, time-averaged surface heat fluxes were at the same level regardless of frequency. However, at low-velocity amplitude ratio, high frequency caused stronger recirculating flows resulting in greater heat transfer compared to the cases with a lower frequency.     

Pulsating Jet Impingement Heat Transfer Enhancement

This article presents results from a numerical study of pulsating jet impingement heat transfer. The motivation is to seek conditions offering a significant enhancement compared to steady flow impingement drying. The CFD software package FLUENT was used for simulating slot-type pulsating jet impingement flows with confinement. The parameter study included velocity amplitude ratio, mean jet velocity, and pulsation frequency. The distance from nozzle exit to surface was three times the hydraulic diameter of the nozzle. The Reynolds number based on the nozzle hydraulic diameter and jet temperature was 2,460 with a mean jet velocity of 30 m/s, which is the base case of the numerical experiments. Results showed that time-averaged surface heat transfer increased with increasing velocity amplitude for the same mean jet velocity. Large velocity amplitudes helped enhance heat transfer by two mechanisms: high jet velocity during the positive cycle and strong recirculating flows during the negative cycle. For the cases with different mean jet velocities but the same maximum velocity, time-averaged surface heat flux decreased with decreasing mean jet velocity. As for the effects of pulsation frequency, with high-velocity amplitude ratio, time-averaged surface heat fluxes were at the same level regardless of frequency. However, at low-velocity amplitude ratio, high frequency caused stronger recirculating flows resulting in greater heat transfer compared to the cases with a lower frequency.     

脉冲对流化床气-固流动特性的影响

运用双流体法建立了三维脉冲流化床模型,模拟了不同脉冲下流化床内的气固流动特性。通过床层压降与气体空隙率分布的比较和分析,发现：脉冲可明显地降低床层压降,减小气泡尺寸;脉冲频率越大,床层压降越大;脉冲气流中有稳定气流的床层压降比无稳定气流的床层压降更小;脉冲振幅的增强可适量增加气体空隙率,但床层压降也增大。     

Heat transfer in a pulsed bubbling fluidized bed

Surface-to-bed heat transfer and pressure measurements were carried out in a 0.17 m ID pulsed bubbling fluidized bed with glass bead and silica sand particles having mean diameters ranging from 37 μm to 700 μm to investigate the effects of flow pulsation on heat transfer and bed hydrodynamics. A solenoid valve was used to supply pulsed air to the bed at 1 to 10 Hz. The bed surface was found to oscillate with the frequency of pulsation, the oscillation's amplitude decreasing with frequency. The standard deviation of the bed pressure drop in the pulsed bed was found to be larger than that in the conventional bed due to the acceleration force imposed by pulsation. For both Geldart B and A particles, high frequency pulsation (7, 10 Hz) enhances the heat transfer compared to continuous flow, the enhancement diminishing with superficial gas velocity and particle size. For Geldart B particles, the effect of pulsation on heat transfer ceases around U_o/U_mf = 3.5, whereas 24% improvement in heat transfer coefficient was obtained for 60 μm glass bead particles (Group A) at superficial gas velocities as high as U_o/U_mf = 27. Furthermore, in the fixed bed (U_o/U_mf < 1) for Geldart B particles, 1 Hz pulsation was found to be very effective resulting in two- to three-fold increase in heat transfer coefficient compared to continuous flow at the same superficial gas velocity. The flow pulsation loses its effect on heat transfer with increasing static bed height, i.e., when H_bed/D > 0.85.     

The effect of flow pulsations on the performance of cyclone personal respirable dust samplers

An experimental system incorporating an aerodynamic particle sizer was developed to measure the effect of pulsating flow on the performance of the personal cyclone sampler used to measure respirable dust in the U.K. The change in the penetration efficiency of the sampler caused by pulsating flow was measured for sinusoidal pulsations with frequencies between 20 and 280 Hz, for particles of aerodynamic diameter up to 5.5 μm. A simple empirical model, relating the change in penetration efficiency to the frequency and amplitude of the pulsation, was fitted to the data. This model was used to determine the effect of the pulsations produced by the portable sampling pumps most commonly used with the cyclone sampler. It is predicted that the pulsations produced by some of the pumps would change the penetration efficiency of the sampler by more than 10% but that the change could always be reduced to less than 3% by adding a small damper.     

Pressure drop in pulsed extraction columns with internals of discs and doughnuts

A study on the pressure drop in pulsed extraction columns with internals of immobile discs and rings, usually called Discs and Doughnuts Columns (DDC) is carried out. The local pressure at a desired level of the column is obtained by resolving of turbulent flow model based on Reynolds equations coupled with k–? model of turbulence. Consequently, the pressure drop for a column stage or for a unit of column length is determined. The results are used for development of correlations for determination of pressure drop as a function of plate free area, interplate distance and pulsation parameters – amplitude and frequency. Good correspondence to experimental data is observed. The developed quantitative relations are useful for non-experimental numerical optimization of stage geometry in view of lesser energy consumption.     

Reasonable Aeration Tap Setting of Burned Oil Sand Particles in Standpipe of a Fluidized Bed

A large scale cold model apparatus of CFB riser reactor was established according to the method of directly fluidized coking oil sand. By changing the position of aeration orifice and amount of aeration air, the solids flux was measured and the location of sensitive aeration orifice above a butterfly valve was fixed(50 mm above the valve). At last, the amount of aeration air for allocation(1~1.5 m3/h) was known and the distance between each pair of consecutive general aeration orifices(about 3 m). And a prediction equation for estimating the relation between the amount of aeration air and the solids flux was established based on experimental data and theoretical analysis, which was Q=6.17Ws0.8529(D/Do)2(L/Lo). The prediction was in good agreement with the experimental data. By using a multi-point pressure pulsation instrument to measure the pulsation at different altitudes of the standpipe, the pressure pulsation curves at different aeration orifices under different amounts of aeration air were drawn. By Fourier transformation, it was found that there was no obvious dominant frequency when oil sand flowed through the circulation standpipe, and each frequency seemed to make equal contribution to the pressure pulsation. The periodical concentration fluctuation of the particles did not exist in the circulating standpipe, and the pulsation intensity was small.     

Drying of a Porous Material in a Pulsed Fluid Bed Dryer: The Influences of Temperature,Frequency of Pulsation,and Air Flow Rate

In this work, a four-section pulsed fluid bed apparatus with a 0.18 m² cross-section area was used to investigate the influence of pulsed-fluidization variables on the drying process of molecular sieves, a test material that was chosen because it presents an initial constant drying rate period. A two-level factorial design was developed to evaluate the influences of the inlet gas temperature—40 and 70°C—the frequency of pulsation—250 and 900 rpm—and the air flow rate—500 and 600 m³(STP)/h—on the drying rate. In addition, a comparison was made between the drying rates achieved with conventional and pulsed fluidization. Results showed that all the investigated variables affect the drying rate. Moreover, drying rates with conventional fluidization are considerably higher, which shows that one must expect a lower drying rate when pulsation is used in a drying process controlled by the external evaporation. Concerning fluid dynamics, this work also analyzed the influence of the frequency of pulsation on the pressure drop across the bed. The higher the frequency, the higher the pressure drop. That result can be explained by the reduction of channeling.     

Dynamics of a pulsating monodisperse mixture

Dynamics equations for a monodisperse two-phase mixture are derived with regard to interphase forces due to a difference in velocity, the associated mass effect, and changes in the velocity field near particles. Numerical solutions are given for low-frequency (10^-1-10² Hz) pulsation of the carrier phase. It was found that the residence time of the disperse phase in a column depends primarily on its retention capacity and pulsation frequency.     

Effect of ultrasound on liquid phase adsorption of azeotropic and non-azeotropic mixture

Relative adsorption experiments of azeotropic and non-azeotropic mixture were separately conducted at constant temperature with and without the presence of ultrasound. The ultrasound wave was applied at a frequency of 21 kHz with 25% amplitude at atmospheric pressure. The adsorption of azeotropic and non-azeotropic mixture was analysed using monolayer adsorption theory and at lower liquid equilibrium concentration it followed a Langmuir type of equation. The adsorptive capacity of azeotropic and non-azeotropic mixtures on activated carbon was found to decrease but the selectivity was increased. The result also showed that the azeotropic point of relative adsorption was noticeably moved due to the ultrasound effect.     

脉动流参数对旋流器分离性能的影响

介绍了脉动实验系统及相关的实验参数,研究了脉动流的一些主要参数对水力旋流器分离性能的影响,详细分析了脉动周期比、流量脉动幅值比和雷诺数等参数对水力旋流器用于细颗粒分离性能的影响。结果表明,在脉动周期比为0.68及流量脉动幅值比为2%左右等条件下,流量的脉动反而会提高水力旋流器的分离效率。     

Mixing and solid‐liquid mass transfer characteristics in a three phase pulsed plate column with packed bed of solids in interplate spaces—a novel aerobic immobilized cell bioreactor

BACKGROUND: The pulsed plate column (PPC) with packed bed of solids in the interplate spaces finds use as a three phase aerobic bioreactor and is a potential heterogeneous catalytic reactor. Good knowledge of the extent of mixing in the liquid phase and solid‐liquid mass transfer coefficient are essential for modeling, design and optimization of these columns. The present work aims at the study of liquid phase mixing and solid–liquid mass transfer characteristics in a three phase PPC. RESULTS: Residence time distribution studies were performed. Dispersion number was found to increase with increase in liquid superficial velocities, frequency of pulsation, amplitude of pulsation and the vibrational velocities. Increase in frequency and amplitude of pulsation, and hence increase in vibrational velocity, resulted in increase of the solid–liquid mass transfer coefficient. CONCLUSIONS: The mixing behaviour in this contactor approximated a mixed flow behaviour. The three phase PPC was found to outperform many other kinds of three phase contactors in terms of solid liquid mass transfer characteristics. Empirical correlations developed can be used for the determination of solid–liquid mass transfer coefficients for three phase PPC and hence can facilitate the design, scale‐up and modeling of these columns, when used as chemical or biochemical reactors. Copyright © 2011 Society of Chemical Industry     

Formation of powdered copper deposits by constant and pulsating overpotential electrolysis

The possibility of electrodepositing copper powder by pulsating overpotential is shown. These powders are compared to the powders obtained by constant overpotential electrodeposition. It is also shown that the grain size and the morphology of deposited powder is a function of overpotential, and of amplitude and frequency of pulsation in constant and pulsating overpotential deposition respectively.     

Formation of powdered copper deposits by square-wave pulsating overpotential

Copper powder deposits were obtained by square-wave pulsating overpotential electrolysis and by constant overpotential electrolysis. It is shown that the particle grain size and the morphology of deposited powders are functions of amplitude overpotential, frequency of pulsation and pulse to pause ratio in pulsating potential deposition, and of overpotential in constant overpotential deposition.     

Response of Cavity Pressure to Vibration Parameters During VAIM Process

Summary In this study, a new type of vibration-assisted injection molding machine was introduced including the principle, structure and application. The responses of cavity pressure to the piston rod vibration amplitude and vibration frequency were discussed in detail. The cavity pressure oscillation frequency was the same with the piston rod frequency. The piston rod vibration frequency had little effect on cavity pressure oscillation amplitude. The cavity pressure oscillation amplitude increased with the increase of piston rod amplitude. The introducing of vibration forces field shortened the filling time, postponed the gate-frozen time and quickened the pressure build up process. More materials could be packed into the mold as the mean cavity pressure was higher which improved the quality of the products.     

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。