Particle scale study on heat transfer of gas–solid spout fluidized bed with hot gas injection

ABSTRACT

In this paper, the heat transfer characteristics of a 2D gas–solid spout fluidized bed with a hot gas jet are investigated using computational fluid dynamics-discrete element method. The initial temperature of the background gas and particles in the spouted bed was set to 300?K. The particle temperature distribution after injection of 500?K gas from the bottom, center of the bed, is presented. The simulation results indicate well heat transfer behavior in the bed. Then, statistical analysis is conducted to investigate the influence of inlet gas velocity and particle thermal conductivity on the heat transfer at particle scale in detail. The results indicate that the particle mean temperature and convective heat transfer coefficient (HTC) linearly increase with the increase in inlet gas velocity, while the conductive HTC and the uniformity of particle temperature distribution are dominated by the particle thermal conductivity. The conductive and convective heat transfer play different roles in the spout fluidized bed. These results should be useful for the further research in such flow pattern and the optimization of operating such spouted fluidized beds.     

Gas flow influences on bubbling and flow characteristics of fluidized bed with immersed tube under electrostatic effects

Industrial bubbling fluidized beds are used to fluidize particles. When particles are fluidized, electrostatic effects will cause the particles to form obvious agglomerates, thus reducing fluidization performance. For better fluidization performance, internal component immersed tubes are usually placed in fluidized bed to limit the bubble size and reduce particle agglomerates. Meanwhile, pulsed gas flow can increase particle disturbance, which is also an effective method to reduce particle agglomerates. In this paper, the CFD-DEM model under electrostatic effects is constructed to research the bubbling and flow characteristics in fluidized beds. Firstly, particle mixing qualities with and without the immersed tube are compared. Then, the effects of different superficial gas velocities are investigated with an immersed tube. Finally, different frequencies are applied to study the energy loss and flow characteristics around the immersed tube. The results show that the addition of the immersed tube can reduce bubble size to facilitate particle mixing. Due to the obstruction of the immersed tube, the bubbles are generated near the wall. As the superficial gas velocity increases, the larger bubbles are generated. Moreover, the electrostatic force applied to the particles varies periodically with the frequency of incoming pulsed gas flow, with fluctuations maximal at 2.5 Hz.     

循环流化床锅炉用旋风分离器性能的实验研究

结合水煤浆流化-悬浮燃烧的特点,通过全面测定循环流化床锅炉用旋风分离器在不同操作参数下的分离效率,研究了入口气速、入口颗粒浓度、入口颗粒物性等对旋风分离器的压降和分离性能的影响规律。实验结果表明,影响旋风分离器分离性能的主要物性参数是颗粒的中位粒径、密度,在入口颗粒的中位粒径相差较大时分离性能主要受粒径的影响,而当入口颗粒粒径相差较小时密度对分离器分离性能的影响则更为显著。     

Experimental investigation of bubble and particle motion behaviors in a gas-solid fluidized bed with side wall liquid spray

Bubble and particle motion behaviors are investigated experimentally in a gas solid fluidized bed with liquid spray on the side wall. The particles used in the experiment are classified as Geldart B particles. The results reveal that when the fluid drag force is less than the liquid bridge force between particles, liquid distribute all over the bed. Bubble size increases as the increase of inter-particle force, then decreases owing to the increase of particle weight with increasing liquid flow rate. When the fluid drag force is greater than the liquid bridge force, liquid mainly distribute in the upper part of the bed. And it is difficult for the wet particles to form agglomerates. Bubble size decreases with increasing liquid flow rate due to the increasing of minimum fluidization velocity. Besides, the acoustic emission (AE) measurements illustrate that the liquid adhesion and evaporation on particles could enhance the particles motion intensity. Consequently, the bubble and particle behaviors change due to the variation in fluidized gas velocity and liquid flow rate should be seriously considered when attempting to successfully design and operate the side wall liquid spray gas solid fluidized bed.     

循环流化床锅炉内颗粒速度分布的实验研究

为了研究操作参数对循环流化床锅炉内颗粒速度分布的影响,在冷态模型装置上使用PV6A型颗粒速度光纤测量仪、毕托管对循环流化床锅炉炉膛内的颗粒速度进行了测量与分析。结果表明:在流化风速为2.57 m/s、循环质量流率为0.58 kg/(m2.s)、一次风量为2 300 m3/h、二次风量为1 500 m3/h时,循环流化床锅炉炉膛内颗粒浓度分布、压降、流化状态、质量循环都达到一个较稳定的水平,呈现典型的环核分布特征。     

Estimation of agglomerate size of multi-walled carbon nanotubes in fluidized beds

The objective of this study was to determine hydrodynamic characteristics of multi-walled carbon nanotubes (MWCNTs) agglomerates and examine their sizes. The bed collapsing process of MWCNTs agglomerates was found to be closer to that of Geldart group C particles than group A particles. Median diameters of MWCNTs agglomerates determined by sedimentation method at initial superficial gas velocity of 0.120 and 0.190 m/s were 157 and 221 μm, respectively. The size of these MWCNTs agglomerates in fluidization state was measured by image analysis using a high speed camera. Median diameters of these MWCNTs agglomerates in freeboard were increased from 138 to 189 μm as superficial gas velocity was increased from 0.088 to 0.190 m/s at static bed height of 0.16 m. Median diameter and size distribution determined by sedimentation method fitted well with those measured using image analysis. Results were reasonable at superficial gas velocity up to 0.190 m/s.     

Numerical investigation of gas-particle hydrodynamics in a vortex chamber fluidized bed

Gas-particle hydrodynamic behaviour inside a vortex chamber fluidized bed is studied numerically with respect to different design and operating conditions. A three-dimensional computational fluid dynamics (CFD) model of a cylindrical vortex chamber is developed. Simulations are carried out with particles and without particles. In order to understand the gas-particle flow behavior velocity distribution, particle volume fraction distribution, radial pressure distribution and axial pressure distribution inside the vortex chamber are analyzed in detail. Particles of different diameters are used and its effect on the gas-particle flow behaviour is studied. Design parameters like the number of gas inlet slots and slot width are varied and their impact on the hydrodynamics of the vortex chamber is investigated. The numerical model is validated by comparing the numerical results with experimental results reported in literature.     

Evaluation of sorption-enhanced reforming over catalyst-sorbent bi-functional particles in an internally circulating fluidized bed

An internally circulating fluidized bed (ICFB) has been applied in various industrial processes owing to its potential in the reduction of heat loss and compact size. In this work, the sorption-enhanced reforming process in the ICFB is investigated. The dense discrete phase model (DDPM) is employed to evaluate the performance of catalyst-sorbent bi-functional particles, considering the particle size distribution. The results demonstrate that the utilization of bi-functional particles can significantly increase hydrogen production. The impacts of operating parameters including solid loading and regenerator velocity on solid circulation rate and gas leakage are examined. It is found that the gas leakage between reactors is increased by 46.6% when the regenerator gas velocity varies from 1.8 m/s to 2.4 m/s so as to weaken the hydrogen yield.     

Bed pressure drop of a bubbling fluidized-bed with overflow solid discharge

The pressure drop of a bubbling fluidized-bed that employed an in-bed inlet and an overflow outlet for continuous flow of solid particles was investigated with variation in the particle size and density, the solid flow rate and the gas velocity. The bed pressure drop decreased with increasing the gas velocity, but increased with the solid flow rate. The characteristics in lifting the solid particles vertically to the level of the overflow outlet by bubbles appeared different from the ones of particle entrainment and bed expansion. Regardless of size and density of particles, bed height in minimum fluidizing condition (pressure head by solid bed weight, H_mf,f) decreased with increasing the volume flow rate of bubble but increased with the mass flow rate of solid particles. The nominal vertical height from H_mf,f to the level of the overflow outlet that the particles should overcome in the course of discharging out of the fluidized-bed with the aid of bubbles increased as either the volume flow rate of bubble increased or the mass flow rate of solid particles decreased. The power consumed while bubbles lifted particles to be discharged appeared to be same at the fixed volume flow rate of bubble. A correlation was proposed successful even for predicting the bed pressure drop of the recycle chamber of the loop seal and the external solid circulation rate in the circulating fluidized-bed system.     

Experimental study of the gas flow behavior in the inlet of a granular bed filter

The Moving Granular Bed Filter (MGBF) is an important apparatus being developed for filtration of the hot gas. Our research group demonstrated a good solution to diminish stagnant zones in MGBF during the filtration process. However, there still remain some systematic problems that have to be worked out prior to commercializing a whole facility.The design of the gas inlet component of the granular bed filter is important for achieving a uniform gas distribution and higher usage rate of the filter media. Non-uniformity may lead to a lower usage rate. The new gas inlet component design uses baffle devices in order to achieve a more uniform gas velocity distribution. Fixed bed and moving bed conditions were studied. The uniformity of the gas velocity distribution can be characterized by the standard deviation definitions and the differences in the mean velocities between the two filtration surfaces. The baffle lengths and angles affected the uniformity of gas velocity in inlet and filtration surfaces. The optimal experimental parameters were found by using different baffle lengths, angles and mass flow rate of filter media. The uniform gas velocity distributions were obtained by a series experiments. Furthermore, the results give important information about IGCC system that will be helpful for designing better models of moving granular bed filters in the future.     

Enhancement of air purification by unique W-plate structure in two-stage electrostatic precipitator: A novel design for efficient capture of fine particles

In this paper, a novel W-plate two-stage ESP was developed and investigated systematically through the experimental and simulated process. Numerical models and available calculation procedure of solving coupling electrostatic field, fluid field, and particle dynamics were established, whose accuracy was validated by experiments. The relationship among collection efficiency, gas velocity, and particle diameter was studied, and the distribution of electrostatic field, the evolution of EHD flow and fluid field, and particle dynamics, including particle charging, particle trajectory, transverse velocity, and particle concentration, were also investigated thoroughly. Results showed that W-plate two-stage ESP exhibited excellent number-based collection efficiency for fine particles which benefited from the reasonable structure design and the exceeding weak influence of EHD flow. Besides, the particle charging process suggested that the diameter decided the dominant charging mechanism, and the trajectory also played an important role in controlling the charging action. Compared with the behavior of each particle injected at different inlet positions, fine particles injected near the discharge wire got more charging number and quicker capture. Importantly, W-plate structure could exert its crucial role in capturing particles with the help of fluid field and inertial effect when inlet gas velocity increased rapidly. W-plate two-stage ESP had more than 90% number-based collection efficiency for >3 μm diameter particles and more than 75% number-based collection efficiency for 0.3–1 μm diameter submicron particles at 2 m/s gas velocity in both experimental and simulated investigations.     

Direct tensile tests on particulate agglomerates for the determination of tensile strength and interparticle bond forces

The importance of direct tensile tests on solid and capillary bonded particulate agglomerates is investigated and compared to compression test measurements. The properties of wet agglomerates are varied by changing the contact angle by means of functionalization of the particle surface. Process conditions are considered by variation of ambient humidity. A qualitative evaluation of the results is performed by analyzing the measured force distance curves of different tensile tests. The results are quantitatively evaluated by calculating the breakage strength, mass related breakage energy and breakage probability showing that the ratio between tensile and compressive tests is highly dependent on the adjusted parameters. Next to the process parameter effect, also the influence of agglomerate size is considered. Tensile strength data are used to estimate the single bond forces between the primary particles of the agglomerates. Tensile and compressive test results are compared to numerical results (DEM) of agglomerate breakage using an elastic stiff bond model.     

Simulation of dispersion and collection process of agglomerated particles in collision with fibers using discrete element method

The behavior of agglomerates in collision with fibers was simulated using the three-dimensional modified discrete element method and the influences of several factors on the fraction of collected particles were examined. Furthermore the single fiber collection efficiency for agglomerated particles was also investigated. In the case where gas velocity is quite low, agglomerates are only deformed but barely dispersed and thus collected as a single deformable particle. By contrast above some critical gas velocity, constituent particles are dispersed and at the same time partly collected on fibers. The fraction of collected particles first increases then decreases as the van der Waals attractive force between particle and fiber increases. The reason for the decrease in fraction of collected particles in strong adhesion region is that the smooth deformation of agglomerates along the fiber surface is inhibited by too strong adhesion. It was also suggested that there exists an optimum size ratio between the agglomerate size and fiber radius for the collection fraction. This is also closely related to the deformation of agglomerate along the fiber surface. In case of non-agglomerated particle collision, all the particles entering within the collision region are collected by fiber. By contrast in case of agglomerate collision, the dispersion of agglomerates as well as collection occurs in the same process and all the particles colliding with the fiber are not necessarily collected. Consequently the single fiber collection efficiency considerably decreases comparing to that for non-agglomerated particle collision.     

Simulation of dispersion of agglomerates in gas phase – acceleration field and impact on cylindrical obstacle

The dispersion of agglomerates by acceleration field and impact onto cylindrical obstacles in gas phase was successfully simulated by the three-dimensional modified discrete element method qualitatively. In case of gas-phase acceleration fields, agglomerates are dispersed into much smaller fragments comparing to the dispersion in liquid-phase shear or elongation flows. The microscopic structure of agglomerates much affects the dispersibility of agglomerates and the agglomerates containing particle of single-point contact with other particles are extremely easy to be dispersed. The impact onto obstacles is more effective dispersion mechanism than acceleration fields, since rigid agglomerates, which cannot be dispersed by acceleration fields, can be dispersed at rather low gas velocity. Although when the gas velocity is very low, agglomerates are only deformed and stuck to the obstacle surface. Then the dispersion becomes significant and size of fragments decreases with the increase in gas velocity.     

3D printing of tuneable agglomerates: Strain distribution and effect of internal flaws

The current study presents a novel and reliable method for producing 3D printed agglomerates with different colour distributions and material properties with 2-fold aims: providing feasible and accurate control on compression of agglomerates under different compression angles, and better tracking of individual particle position after agglomerate breakage. Multi-coloured agglomerates in cubic tetrahedral and random sphere shapes were printed with both rigid and soft bonds. The printed agglomerates were analysed thoroughly of their surface and structural properties including surface roughness and printing accuracy. The agglomerate breakage behaviours under static compression were analysed as a function of bond strength, loading rate and loading directions, with strain distribution plotted over the random sphere agglomerate structure. In addition, agglomerate structures with designed internal macro-voids in different positions and sizes were also created for breakage study, in an effort to better understand parameters governing the mechanical properties of agglomerates with cavities and voids which is inevitable in particle industry but poorly understood at present.     

Agglomerating fluidization of group C particles: major factors of coalescence and breakup of agglomerates

An experimental study was carried out to determine the influence of different superficial gas velocities on the agglomerates of cohesive particles. The probability of agglomerate coalescence and breakup is proposed on the basis of the principle of force balance. Theoretical analysis shows that the higher superficial gas velocity and fluid density, the lower the particle cohesion, and that collisions between small and large agglomerates are advantageous for the agglomerating fluidization of cohesive particles. The average agglomerate size estimated by the model of force balance decreases with increasing superficial gas velocity, which is in agreement with experimental data.     

Circulating particle flow and air bubble behavior at various superficial air velocities in two-dimensional gas–solid fluidized beds

Circulating particle flow and behavior of air bubbles in a two-dimensional gas-solid fluidized bed of various superficial air velocities are investigated by recording videos of movement of a plastic pellet put into the fluidized bed and rising air bubbles using a video camera. The movement velocity of the plastic pellet and properties of the air bubbles such as the bubble rising velocity and the bubble distribution coefficient, which shows the proportion of the bubbles erupting at the center of the bed surface, are measured by analyzing the videos. It is found that the plastic pellet moves following the circulating particle flow; the particles rise up at the center of a column and fall down near the side walls, and that the movement velocity increases with the superficial air velocity. The bubble rising velocity, the apparent erupting bubble size and the bubble distribution coefficient increase, and the bubble eruption frequency slightly decreases, with the superficial air velocity. These results indicate that the circulating particle flow is generated by the rising air bubbles. In particular, the fact that the air bubbles rise at the center of the column and coalesce with other bubbles is closely related to the generation of the circulating particle flow.     

A parametric study of dispersed laminar gas-particle flows through vertical and horizontal channels

Particle diameter, particle phase material density and inlet particle volume fraction are three important parameters governing the flow physics of dispersed gas-particle flows. In this work, an inhouse numerical solver is developed to investigate the effects of particle diameter (Stokes number), particle phase material density, inlet particle volume fraction and inlet phase velocities in the flow characteristics of gas-particle flows through vertical and horizontal channels and also in open domains. It is found that, for a constant inlet particle volume fraction, lower diameter particles attain a higher steady state velocity at any section inside the channel than the higher diameter particles; while the corresponding steady state gas velocity at any section increases with increase in particle diameter. On the other hand, for a constant particle diameter, the steady state gas phase velocity at any section decreases with increase in inlet particle volume fraction. Significant changes in both gas and particle velocity and volume fraction profiles have also been observed with inlet slip, i.e., when the velocities of both the phases at inlet are distinct as opposed to being equal, keeping all other flow and physical parameters invariant.     

The yield stress of jammed magnetofluidized beds

By means of a magnetic field externally imposed, fluidized beds of magnetizable particles may experience a transition from a fluidlike unstable to a solidlike stable state. In our work, measurements have been taken of the gas velocity and particle volume fraction at the jamming transition as well as of the tensile yield stress of the stabilized bed subjected to a small consolidation. The influence of diverse physical parameters such as initialization mode, magnetic field orientation, average particle size and size polydispersion, are analyzed. Noninvasive visualization of the bed structure has revealed that magnetic stabilization is determined by the formation of particle chains. Due to the enhancement of the interparticle attractive force with field strength and particle size, the transition to stability takes place at higher gas velocities as the magnitude of these parameters is increased. The magnetic yield stress of magnetofluidized beds of naturally aggregated particles because of a large presence of fines is significantly larger than that corresponding to naturally nonaggregated particles. Moreover, the jamming transition occurs at larger gas velocities (or smaller field strengths) in the former case since the agglomerates behave magnetically as large effective particles. The effect of the magnetic field on the yield stress ia only relevant when it is applied during initialization of the bed in the bubbling regime and particles are free to move and restructure in chains. Measurements of the yield stress are presented when the applied magnetic field is oriented either in the vertical or horizontal direction (B co-flow and B cross-flow modes, respectively). The variation of the magnetic yield stress with particle size was found to be dependent on the field direction. This can be justified by the dependence of the interparticle magnetic force on the chain average angle with the field, which is affected by particle size as the stabilized bed is subjected to small consolidations.     

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.