Gas-solid Flow Characteristics Research Articles

Investigation of gas–solids flow characteristics in a circulating fluidized bed with annular combustion chamber by pressure measurements and CPFD simulation

For large scale circulating fluidized bed (CFB) boilers, a new type of annular combustion chamber was proposed to improve the secondary air penetration and provide adequate space to arrange the heat transfer surface. To investigate the gas–solids flow characteristics and optimize the structure of the annular combustion chamber, a “cold” test rig was built with a cross sectional area of 1.18m2 and a height of 8.0m. High frequency pressure transducers were used to obtain the pressure fluctuations characteristics. CFD simulation based on computational particle fluid dynamic (CPFD) was used to obtain pressure fluctuations and particles concentration. The time domain analysis, power spectrum analysis and Hilbert–Huang transform were applied to the experimental and simulation data. Different flow regimes including single bubble regime, exploding bubble regime and turbulent fluidization regime were identified. The flow characteristics were investigated in terms of standard deviation and higher-order moments, auto-correlation, power spectrum, and EMD energy entropy. The results show that the gas–solids flow in the bottom region of the annular combustion chamber was relatively uniform. The gas–solids flow in annular combustion chamber was proved similar to that in a conventional CFB riser.

High-fidelity simulation of the 3-D full-loop gas–solid flow characteristics in the circulating fluidized bed

The full-loop gas–solid motions in a three-dimensional circulating fluidized bed are numerically modeled using the computational fluid dynamics combined with the discrete element method. The time-averaged flow characteristics and the particle-scale details related to solid motion are discussed. The results show that solid motion shows an ‘S’ rising path in the riser but a spirally falling behavior in the cyclone. Typical core-annulus flow structure and non-uniform distribution of solid holdup appear in the riser. The inside and outside swirling flows of gas motion as well as the fishtailing phenomenon are captured in the cyclone. The solid residence time in the riser shows an early-occurred peak with a long tail. In the riser, details related to solid motion at the particle-scale level are obtained. The sudden changes of these particle-scale details when increasing the superficial velocity may be considered to be a new criterion of identifying the flow regime change.

CFD–DEM simulation of the filtration performance for fibrous media based on the mimic structure

In order to obtain the comprehensive filtration information, such as the deposition morphology of particles, evolutions of the instantaneous pressure drop and efficiency, 3-D mimic models of fibrous media had been established using a stochastic algorithm and computational fluid dynamics (CFD) - discrete element method (DEM) model was adopted to simulate the gas-solid flow characteristics in fibrous media exposed to particle loading in this work. Meanwhile, the simulation results had been validated by the macro-empirical model proposed in the previous literature. In addition, the influence of various parameters including porosity, face velocity and particle size on the filtration performance of fibrous media had also been investigated. It is concluded that simulation results of the CFD–DEM model are good agreement with predictive values of the macro-empirical model. Moreover, the porosity of fibrous media, face velocity and particle size have an important effect on the filtration performance of the fibrous media.

Numerical Simulation of the Flow Characteristics of 35t/h Internally Circulating Fluidized Bed

The gas-solid flow characteristics of the 35t/h oil shale-combustion circulating fluidized bed boiler (Developed by Wangqing Longteng Energy Development Co., Ltd) was simulated using Eulerian-Eulerian model (EEM), which was based on the kinetic theory of granular. The distribution of particle volume fraction and the distribution of particle velocity revealed the mechanism of the internal recirculation flow of particles in the furnace. The simulation results provided a reference for the flow structure optimization of the inner circulating fluidized bed and the enlargement of the inner circulating fluidized bed boiler.

Numerical simulation of particle plume resident time affected by horizontal crosswise wind speed

This article explores the resident time of free-falling particle plume with the characteristics of gas-solid two-phase flow affected by horizontal crosswise wind speed. The computational fluid dynamic software FLUENT was adopted in this study. The numerical simulation was carried out to study the influence of horizontal environmental wind speed to the velocity of particle plume. In the simulation, the ambient wind speeds were assumed as 1, 1.2, 1.4, 1.6, and 1.8 m/s. The particle diameter was set as 567 μm and the mass flow was given as 5.2 g/s. The Euler and Lagrangian models were used to calculate the coupling pressure and velocities of the particles and the entrained air in the simulation. The simulation results showed that the greater the horizontal wind speed was the shorter the particle resident time and the shorter the time of particle velocity reached the maximum value.

Simulation of Gas-Solid Flow Characteristics in Three-Dimensional Rotational Spouted-Fluidized Bed

In order to study the gas-solid flow characteristics in a rotational spouted-fluidized bed dryer, the eulerian multi-phase model was applied in three-dimensional numerical simulation of a rotational spouted-fluidized bed to analyze the effect of different velocity ratios between bottom and tangential wind on gas and particle velocity distribution characteristics, and the change rule of gas-solid flow state with the time at the velocity ratio of 30 m·s-1/30 m·s-1 was derived. The results show that the increase of tangential wind velocity is propitious to enhance the gas flow rate in the region near the wall and make the gas-solid phase mix sufficiently as well as augment of the contact area of gas and particle phase, and decrease of the gas flow dead zones and the adhesion of viscous materials to cylinder wall. However, the negative pressure formed by the entrainment effect of tangential wind goes against the development of gas flow along the axial direction reducing the penetration effect of axial wind to the granular layer.

Application of CPFD method in the simulation of a circulating fluidized bed with a loop seal Part II—Investigation of solids circulation

The Computational Particle Fluid Dynamics (CPFD) numerical method was used to study the gas solid flow characteristics in a circulating fluidized bed (CFB) with a loop seal in this work. The influences of operating parameters, including loop seal aeration rate (Ql), fluidized air velocity in the riser (Ur) and total bed inventory (Mp) on the solid circulation characteristics were mainly investigated in this part. The solid circulating rate (Gs), total pressure drop in the riser (ΔPr), particle packed height (Hs), pressure gradient (ΔPs/Hs) and gas flow rate (Gg) in the standpipe, as well as the particle concentration and pressure distributions in the loop were carefully analyzed under different operating conditions. The simulation results were compared with the experimental results also reported in this work and good agreements were obtained. The results showed that the increase of Ql resulted in a decrease of Hs and increases of Gs, ΔPr and ΔPs/Hs. By increasing Ur, the increase of Gs was limited and the maximum value was approaching. An increase of Mp also had a positive effect on Gs while an elevated Hs was obtained as a result of mass and pressure balance. No gas bypass flow phenomenon occurred in the operating cases of this work from the simulation results. The pressure drop in the riser could be properly predicted while the gas–solid interactions in the standpipe and loop seal were slightly underestimated by the CPFD method, where a relatively lower ΔPs/Hs was obtained for CFB modeling.

Numerical Investigation of the Gas‐Solid Flow Characteristics in a Three‐Dimensional Spouted Bed with a Draft Tube

AbstractGas‐solid motions in a three‐dimensional conical spouted bed with a draft tube are investigated based on a simulation carried out by the coupling approach of computational fluid dynamics combined with the discrete‐element method. The distribution properties of the velocity, the concentration, and the flux of the solid phase are discussed. The vertical solid velocity in the central region initially increases, diminishes gradually, and finally decreases sharply in the region above the draft tube. Vigorous lateral solid motion occurs in the periphery of the fountain and the spout‐annulus interface. In addition, the vertical solid flux shows a large value in the spout. A larger vertical velocity but a more dilute solid concentration can be detected along the axial direction when enlarging the gas flow rate.

Gas-solid flow characteristics in high-density CFB

The gas-solid flow characteristics in the riser of a high density CFB of square (0.27 m×0.27 m×10.4 m) or circular (ϕ 0.187m×10.4 m) cross section, using Geldart B particles (quartz sand), was investigated experimentally. The influence of riser structure on the hydrodynamic behaviors of a high-density circulating fluidized bed was investigated. The solid circulation rate was up to 321 kg/(m2s) with the circular cross-section under the operating conditions of the main bed air velocity 12.1 m/s and loosen wind and back-feed wind flow 25.1 m3/h. Different operating conditions on realizing high density circulation was analyzed, while both solids circulation rate and particle holdup depended highly on operating conditions. The circulating gas-solid flow was accompanied by an evidently-dense character in the riser’s bottom zone and became fully developed in the middle and upper zones.

Flow Characteristics of Gas-Solid Two-Phase Flow in Annular Pipe of Gas Drilling

Air drilling technology has been widely used in the oil and gas exploration, coal, geothermal, geological exploration, nuclear industry and other fields due to its high drilling rate and low cost. However, the design of the pneumatic conveying system for the mineral detritus is still largely based on empiricism. The paper was set in the background of gas drilling, mainly studied the gas-solids two-phase flow characteristics in 90 degree bent annular pipe and backward-facing step of an annular pipe, which are very important parts of air drilling. They refer to the bent part and backward-facing step of an annular channel formed by the drill pipe and the borehole wall. A detailed numerical simulation and experimental studies were carried out for the flow structure and pressure losses of gas-solid two-phase in the annular pipe of gas drilling. Since a unified theory has not been developed for the two-phase flow in annular pipe, a lot of experimental work should be conducted. In the experimental research, the paper independently designed and built an annular pipe pneumatic conveying system with 90 degree bend and backward-facing step, including designing material screw feeder, material receiving hopper, pipeline, control system, data acquisition system, and etc. As known, many parameters, such as gas velocity, diameter and density of the particle, and solids loading ratio, can influence the conveying process. How these primordial influence factors act on the pressure losses of two-phase flow in annular pipe was analyzed in this paper. In the numerical simulation research, turbulent two-phase flow calculations were performed with a commercial CFD computer code referred to as FLUENT to study the gas-solid two phase flow in the sections of backward-facing step and 90 degree bent pipe respectively by using Euler-Lagrange method. The RNG κ-ε model and stochastic tracking were involved in the calculation of turbulence dispersion of two phases. The discrete phase model was performed for the solid phase. In the end, the numerical study 3-D results were translated to 1-D results using the standard averaging transformation to compare with experimental results. Predicted results obtained for pressure drop and velocity variations in full developed flows in the cases examined are in good qualitative agreement and are not in quantitative agreement with experimental data. The deviations between the simulations and experimental data lie in the range of 20%-30%. These results suggest commercial CFD codes such as FLUENT can be used productively for investigations into gas-solid two-phase flow phenomena and as an aid in pneumatic conveying design. The studies of the two-phase flow characteristics in the paper will contribute to reliable determination of the optimal condition of pneumatic conveying in gas drilling.

Downward Gas–Solids Flow Characteristics in a High-Density Downer Reactor

A specially designed high-density downward gas–solids flow fluidized bed, consisting of a solids reservoir tank, a solid distributor, an acrylic downer (3.0 m high), a gas–solid separator and a solid receiver tank, has been developed to study the fundamentals of high-density downward gas–solids flow characteristics in the downer reactor. Experiments have been carried out using two types of sand particles (ρp=2600 kg/m3, dp=80 and 320 µm) as bed materials with superficial downward gas velocities (Ugd) ranging from 0 to 5 m/s. Particle diameter has a great effect on the solids mass flux (Gs) and solids hold-up (εs) in the downer. It is found that Gs in a 50 mm diameter downer reached up to 1876 and 815 kg/m2·s, with average εs up to 0.16 and 0.07 in the developed region, for 80 and 320 µm sand, respectively. The solids hold-up decreased with increasing Ugd at a fixed solids mass flux in the developed region. By considering the effects of downer diameters, operating conditions, and solids properties, a correlation for the prediction of dense solids hold-up (εs*>0.07) in the developed region of the downer is proposed based on experimental data from the literature and this study.

Development of a novel fluidized bed ash cooler for circulating fluidized bed boilers: Experimental study and application

A novel bottom ash cooler (BAC) called compound fluidized bed ash cooler (CFBAC) was developed in this paper. The CFBAC combined the major technical features of spouted bed and bubbling bed, and could achieve the selective discharge on the bottom ash. Experiments about the gas–solid flow characteristics of the CFBAC were conducted in a visible cold test rig. The experimental results indicated that the separation chamber working in the spouted bed state had a good particle separation effect on the boiler bottom ash. A small quantity of fluidizing air was needed for the cooling chambers to work in the bubbling bed state. The particle separation effect could be controlled by the fluidizing air flow and physical dimensions such as the height of separation partition. The CFBAC had also been industrially applied in a 300 MW circulating fluidized bed unit. The application showed that the CFBAC had a well separation effect, an excellent adaptability on the bottom ash, a good cooling effect and a large ash discharge capacity over 30 t/h. Compared with the water-cooled BAC, the CFBAC had a better energy conservation performance.

Flow Distribution Prediction of Gas-Solid Two-Phase Flow in Y-Shaped Branch Pipeline

This paper reported characteristics of gas-solid two-phase flow for a Y-shaped branch pipe. Y-shaped branch pipeline with one fixed and one adjustable branch angle was built up. Micro-glass bead and millet particles with similar diameter and different solid density were conveyed in this experimental system. As a result, trend of solids mass ratio flowing in changeable branch was used to predict flow distribution characteristics experimentally. It was found that the solids flow distribution of the two materials had similar trend and were significantly affected by the branch angle and gas velocity. Finally, a calculating equation to predict the solids flow distribution of branch pipe was achieved based on experimental data, which had good accuracy.

Effect of interparticle forces on the conical spouted bed behavior of wet particles with size distribution

This paper aims to analyze air–solid flow behavior in conical spouted beds composed of glass bead mixtures coated by glycerol. Four mixtures of glass beads are used as the solid phase. Although these mixtures have the same mean Sauter diameter, each one is characterized by a different size distribution function (mono-sized; flat, Gaussian or binary size distribution). When glycerol is added to the bed of these particles, which are spouted by air, the gas–solid flow characteristics are changed due to the growth of interparticle forces; however, the trends of these changes are affected by the glass bead mixture type as well as by the concentration of glycerol. For beds of mono-sized particles, the minimum spouting velocity is maintained almost unchanged as the glycerol concentration rises; while, for beds of inert particle mixtures, this velocity increases, becoming greater for flat and binary size distribution particles. Conversely, the minimum spouting pressure drop decreases as the glycerol concentration rises for all beds of particles used. Based on theoretical prediction of interparticle forces, it is shown that these changes in the minimum spouting conditions can be explained by the magnitude of these forces.

Effect of Solids Concentration Distribution on the Flue Gas Desulfurization Process

A dry flue gas desulfurization (FGD) process at 600-800 degrees C was studied in a pilot-scale circulating fluidized bed (CFB) experimental facility. Various fresh sorbent distribution types and internal structures were modeled numerically to investigate their effect on the gas-solid flow and sulfate reaction characteristics. Experimental results show that, after the fresh sorbent supply was stopped, the desulfurization efficiency declined rapidly even though the sorbent recirculation was maintained. Therefore, the fresh sorbent is the main contributor to the desulfurization process and the primary effect of the recirculated sorbent was to evenly distribute the fresh sorbent and to prolong the sorbent particle residence time. The numerical results demonstrate thatthe desulfurization efficiency varied greatly for the various fresh sorbent bottom injection methods. The desulfurization efficiency of the bottom-even injection method was 1.5 times that of the bottom two-sided injection method. Internal structures effectively improved the fresh sorbent solids concentration distribution and the desulfurization efficiency. Optimized internal structures increased the desulfurization efficiency of the bottom two-sided injection method by 46%, so that it was very close to that of the bottom-even injection method with only a 4.6% difference.

Experimental investigation on flow structures and mixing mechanisms of a gas–solid burner jet

The objective of this study was to investigate the flow structures and mixing mechanisms of a gas–solid two-phase jet from a burner employed in the utility boiler. Laboratory experiments were conducted in a 0.15 m internal diameter test facility. Gas–solid jet flow downstream of the burner exit was measured by a fiber optic probe. Local solid concentration and particle size distribution were obtained using the probe that was traversed over the cross-section of the jet. The effects of the side flow on the primary air gas–solid flow characteristics were also studied. The measurements showed the availability of fiber optic probe to investigate the gas–solid jet flow downstream the industrial burner nozzle with high solid concentration. Obvious slip between the gas phase and the solid phase in the gas–solid jet flow was found in the experiments. The side flow disturbed the solid concentration distribution in the jet, a more smooth solid concentration profile across the jet can be achieved after the side flow was introduced. The particle size distributions in the jet were similar with the distributions of the solid concentration, the particles with large diameters always appeared in the zones where the solid concentrations were high.

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.

Gas–solid two‐phase flow in FCC riser

AbstractFCC riser performance was examined in a large cold‐riser model made of 0.186‐m‐ID plexiglass. In the prelift zone, distributions of particle velocity and local density in six cross sections were investigated, and the experimental results indicated that the whole prelift zone can be divided into three subzones in the axial direction. In the feedstock injection zone, some new flow parameters were introduced and five flow parameters in five different cross sections were measured. When a nozzle jet was injected, secondary flow of the jet exited in the vicinity of the riser wall due to vortices, and this flow moved forth at first and then merged into the main stream. Based on the gas–solid two‐phase flow characteristics in each zone, the whole feedstock injection zone of the riser can be divided into four subzones. A new axial particle backmixing model and a simplified method for computing the radial distribution of the nozzle jet concentration in a different riser cross section can predict flow parameters of particles and the jet.

Effect of elevated carrier-gas pressure on hydraulic characteristics of gas-solid particle two-phase flow

The effect of elevated gas pressure on the hydraulic resistance and critical velocity of a two-phase flow is studied on a pneumatic-transport bench. It is established that for each working-pressure level there exists a limiting solid-phase concentration, the exceeding of which causes an abrupt rise in hydraulic resistance. 16 refs., 3 figs.

Steady-state and transient gas—solids flow characteristics in vertical transport lines

The generalized application of the drift velocity approach to gas—solids flow is given, together with supporting data and suggested parameter evaluation techniques, to allow the prediction of the phase densities, phase flow rates and allowable operating conditions related to gas—solids flow in vertical pipe systems. Both steady-state and transient flow conditions are considered in co-current and countercurrent flow. Continuity wave velocities are employed to determine the stability of the flow and the presence of flooding. The method of analysis outlined here, previously used successfully in gas—liquid systems, is shown to also hold for gas—solid systems.