Fluidization characteristics of gas-fluidized beds: Air and glass beads system

Good gas-solids contacting and solids mixing are essential features, both for efficient coal combustion and desulfurization of coal in a fluidized-bed combustor. It will therefore be very useful if suitable procedures could be developed to characterize the quality of fluidization in gas-solid systems. Three different procedures are discussed in this article with reference to air and glass beads of four different average sizes:

1. (i) in one scheme, the bed voidage is measured at minimum fluidization and the variation of this quantity with Reynolds number at minimum fluidization is correlated with the powder classification scheme of Saxena and Ganzha and hence with the corresponding fluidization characteristics. A major effort is directed at substantiation and confirmation of this characterization scheme, which relates the fluidization behavior to the Reynolds number at incipient fluidization.

2. (ii) The local heat-transfer coefficient of a probe element immersed in the bed is also measured as a function of the superficial fluidizing velocity and the nature of variation is taken as representing the quality of bed fluidization.

3. (iii) A technique is developed to measure the time-history of the temperature fluctuation of a probe element and to relate this parameter to the bed-fluidization characteristics.

Additional experimental data with other solids will be generated in the future to develop these proposed three schemes further for the characterization of bed-fluidization behavior.     

Dynamic force reduction and heat transfer improvement for horizontal tubes in large-particle gas-fluidized beds

The effects of tube bank configuration on forces and heat transfer were investigated for both two-dimensional and three-dimensional gas fluidized beds. Effective dynamic forces and heat transfer coefficients were measured for several tube bank configurations, and it was found that the average forces are smaller than for a single tube. The heat transfer coefficient can be increased by providing sufficient space for particles to descend around both sides of the tube bank. The results provide useful guidelines for optimizing the configuration of tube banks to achieve high heat transfer coefficients while reducing tube erosion due to dynamic forces.     

Recent Chinese heat-transfer research on bubbling and circulating fluidized beds

Recent research work conducted in China on heat transfer to immersed surfaces in bubbling and circulating fluidized beds, including coal combustors and boilers, is reviewed. Studies include measurements of heat-transfer coefficients (local and total) from horizontal and vertical tubes in bubbling fluidized beds and their variations with operating and system parameters. Special designs developed for heat-transfer probes and theoretical models for the computation of heat-transfer coefficients are discussed. Recent work is presented on circulating fluidized beds with emphasis on hydrodynamics and heat transfer. Proposed correlations and novel designs of thin-film heat-transfer and capacitance probes are included.     

Hydrodynamic and Heat Transfer Characteristics of Magnetofluidized Beds

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Heat transfers through mechanically-stimulated particle beds

A particle bed can be effectively ‘fluidised’ by means of a vibrating diaphragm. Such beds of vibrating particles (whether in nominally-still air or even in a vacuum) exhibit high thermal-conductances, which are comparable with those of air-fludised beds. The mechanically-stimulated beds have potential applications as thermal switches.     

Analysis of particle motion and heat transfer in circulating fluidized beds

In this study, the forces affecting the motion of particle clusters near the wall of a CFB were theoretically analysed. The motion trajectory and the contact time of clusters were determined from the proposed model for two cases, steel ball having density of 6980 kg m^?3 and sand having density of 2500 kg m^?3. Computational results showed that the construction and operational parameters such as the bed equivalent diameter, the gas velocity and the bed temperature have great influence on the contact time of clusters. Based on analysis of the contact time of clusters, a theoretical model was developed for predicting the particle–gas convection heat transfer coefficient. The results were compared with experiments and were a quite agreement with the measured data in the open literature which suggests that the theoretical analysis conducted in this work can very well describe the convection heat transfer in a CFB. Copyright © 2004 John Wiley & Sons, Ltd.     

Heat transfer characteristics and particle behavior around circular tubes immersed in fluidized beds

In this study, the particle velocity, particle volume fraction (attenuation of transmission light), and heat transfer coefficient were simultaneously measured for a horizontal tube bundle immersed in a fluidized bed of 0.42 mm diameter glass beads. To improve the accuracy of the particle velocity measurements, an optical fiber probe composed of a combination of transmission and reflection types was designed. The particle velocities were calculated by using the cross-correlation method and the particle passing-time method, respectively. It was shown by comparison of particle velocities obtained by the two methods that the particle passing-time method gives more stable results. The particle velocities at the bottom and at the side of the tube were relatively large and appeared to have periodicity while those near the top of the tube were low. This particle contacting feature can be used to explain the heat transfer characteristics. © 1998 Scripta Technica. Heat Trans Jpn Res, 26(5): 306–318, 1997     

Discontinuity of particle contact with the surface and heat transfer in fluidized beds

The finite-element method has been applied to solve the unsteady-state heat transfer equation for solid particles heated at the surface. The results are given as a series of temperature profiles for these systems and are used to estimate the depth of the heat penetration region in the particles and the heat transfer coefficients between a fluidized bed and an immersed heating surface. From calculations it appears that for the typical conditions of bubbling fluidization, i.e. residence times at the heat transfer surface τ = 0.1 s, this region exceeds many times the one found experimentally. This provides theoretical support for the earlier hypothesis that particles in fluidized beds are separated from the heating surface by an air gap. The close agreement between this result and the experimental values requires the assumption of a gap thickness δ = 01d_p. Owing to the long contact times of the particles with the heating surface in the electrostatically charged fluidized bed the particle penetration theory of the heat transfer from the surface has been also verified. The heat transfer coefficients derived from this theory agree with the authors' previous experimental work. On the basis of the results obtained some assumptions concerning a generalization of the heat transfer theories have been formulated.     

Neutron imaging studies of metal-hydride storage beds

Neutron radiography and tomography were used to study the transient and steady-state distributions, respectively, of hydrogen within a prototypical, LaNi_4.78Sn_0.22-based hydrogen-storage bed during and after various absorption and desorption steps. It was shown that using deuterium instead of hydrogen enabled the imaging of thicker beds. These measurements serve to demonstrate the unique utility of neutron imaging as an important diagnostic tool for in situ, real-time characterization of hydrogen-concentration profiles in practical hydrogen-storage systems.     

Estimation of bed expansions in a freely-bubbling three-dimensional gas-fluidized bed

Knowledge of bed expansions is important in the design and operation of gas–solid fluidized beds. This paper presents a study on the estimation of expanded bed height in a large three-dimensional gas-fluidized bed with a square section of 0·61×0·61 m². All experiments were performed at the freely bubbling mode and the bed expansions were recorded by a video camera. Bed materials were used 593 μm raw perlite and 1233 μm sand falling within the categories of Geldart's Groups B and D, respectively. The bed height at minimum fluidization ranged from 0·0398 to 0·3176 m, while the excess air velocity from 0·034 m s⁻¹to 0·7453 m s⁻¹. Equations related to the bed expansion were given using a modified form of two-phase theory of fluidization. A correlation for the average bed expansion (void fraction) was also presented that has been derived from the principal form found successful in gas–liquid systems as follows: R=0·5482 d^−0·129_p(U_o−U_mf)^0·111 with an average deviation of less than 1%. The experimental findings were compared with previously reported results and were discussed in the light of available correlations. © 1998 John Wiley & Sons, Ltd.     

Effect of pore characteristics of fine lignite on particle density in separation medium

Effect of porosity on particle density was analyzed, which can be a potential factor of low efficiency. The porosity of particle with different densities was detected by surface area analyzer. In addition, Three-dimensional X-ray micro-imaging system (3D-XRM) was used to analyze the spatial distribution of internal pores. Results show that mesoporous and microporosity content increase with the increase of particle density. Three-dimensional distribution of pores shows that the pores are very rich and show a good connection. The density model including porosity and medium density was established, which shows the effect of medium filling on the change of particle density.     

Flow and heat-transfer in an internally-heated, naturally-ventilated space

This paper presents the findings of a theoretical and experimental investigation of the thermodynamics of an internally-heated control volume (CV), ventilated by natural convection. A simple bench-type experiment has been used to determine the temperature history of the air, in support of a global calculation by numerical methods. Good agreement has been achieved between the experimental results and the mathematical prediction, which has wide ranging applications in practical situations.     

The effect of stable stratification and thermophoresis on fine particle deposition in a bounded turbulent flow

Small inertial particle transfer and deposition in a thermally stratified turbulent channel flow is studied by large eddy simulation. The Lagrangian tracking approach is used to describe the dynamics of particles. The objective of this study is to examine the influences of thermal stratification and the thermophoresis on the preferential concentration and deposition of small particles with different particle relaxation time scales. The diameters of particle are ranged from 0.6 μm to 5 μm. Numerical results show that the thermophoresis strengthens the deposition of particles near the cold wall while it weakens the deposition near the hot wall. With the Richardson number increases, the particle mean velocity increases regularly in the core of the channel, and the particle fluctuation intensities decrease due to the re-laminarization tendency of stably stratified turbulent boundary layer. The magnitude of the thermophoretic force on particles decreases in the stable stratified flow, which results in smaller deposition rate with increasing the Richardson number.     

Effect of air staging on fine particle,dust and gaseous emissions from masonry heaters

There is a need to decrease the detrimental particle and gaseous emissions from residential wood combustion appliances. One encouraging alternative is to stage the air supply which improves the combustion conditions in small appliances. In this study, two types of combustion technologies were studied in conventional masonry heaters (CMH) and modern masonry heaters (MMH). Air staging in the MMHs considerably reduced the particle and gas emissions. The greatest reduction was observed in gaseous and particulate organic emissions. Methane emissions were reduced by 74%–91% and carbon monoxide by 26%–81%. The reduction of fine particle mass (PM₁) was 14%–58%. Elemental carbon (EC, i.e. soot) emission increased in small combustion appliances but declined in large appliances. In addition, dust (TSP, Total Suspended Particulate matter i.e. Dust) emissions from hot flue gas were compared with the fine particle mass emissions from diluted sample. PM₁ emissions measured from diluted flue gas were 1.1–4.4-fold as compared to TSP collected from hot flue gas. This may be attributable to the fact that organic vapors partially had penetrated into the TSP filter in a gaseous form whereas when they were diluted, semivolatile species condensed on the particles. It can be concluded that air staging is an effective way to reduce gaseous and organic emissions from batch combustion appliances. Particle emission measured from diluted flue gas represents a more realistic results than TSP (hot sampling), because in dilution, also the organic fraction of the particle emissions is taken into account.     

Enhancement of convection heat-transfer in a rectangular duct

Characteristics of wall-to-air heat transfer for a fully developed forced convection have been studied in a large rectangular packed duct with 160 cm heated length, 40 cm width, and for low bed equivalent diameter to particle diameter ratio. The separation distance between the top and bottom walls is 10 cm. A uniform heat flux is supplied at the top wall, while the bottom wall is insulated. Raschig rings in two and spherical packing in three sizes have been used in the air flow passage to investigate the enhancement of heat transfer due to packing. Temperature profiles for the steady and unsteady states have been measured. In modeling, the Ergun equation and energy equations are solved to calculate the temperature profile for the steady-state only. It has been found that the introduction of the packing into the air flow passage increases the wall-to-fluid heat transfer approximately three times compared with that of empty bed. This finding can enhance the rational use of energy from solar air heaters, chemical reactors, electronic cooling and many other engineering applications.     

Heat transfer to a bayonet heat exchanger immersed in a gas-fluidized bed

Bayonet tubes are frequently used for heat removal in fluidized bed coal combustors and gasifiers. They consist of two coaxial tubes, through which the cooling fluid flows; first through the inner tube, and then back through the annulus, or vice versa. Such a flow arrangement creates obvious difficulties in defining a characteristic temperature difference to which the heat transfer rate can be related, and on the basis of which the heat transfer coefficient between the bed and the tube can be evaluated. A bayonet tube-fluidized bed system is mathematically modelled here, and an analytical scheme is presented for the calculation of the bed-tube heat transfer coefficient. A characteristic temperature difference (CTD) is defined which reduces to the familiar logarithmic mean temperature difference (LMTD) for a particular case. A set of curves is presented relating the CTD with the LMTD.     

Combustion in crater beds

A preliminary study of a new type of heat-recirculating particulate bed combustor has been carried out with a view to overcoming some limitations arising in the scaling up of spouted bed combustors. A fountain of particles, somewhat similar to that produced in spouted beds, is raised by a jet of gaseous reactants flowing downward into a crater formed below the bed surface. Reactants are preheated prior to combustion both by heat transfer from the products across the walls of the inlet tube and by the interaction of reactants with the inert particles heated by the flame. Combustion characteristics are studied for single and multiple inlet tubes in small laboratory burners and in a much larger 50 KW system. Crater beds are compared with other two-phase combustors, especially spouted beds. They readily lend themselves to scaling up by the use of multiple jets, from one or from several inlets, and recirculate heat between products and reactants both within the craters and by heat transfer to the inlet tubes. Tests show that they can be used either with gaseous reactants entering through the jet or with only oxidant introduced in that way, fuel being included within the bed. Stability limits established for the laboratory burners show a leaner burning capability than is achievable in spouted or fluidized beds and the device offers several advantages over other circulating two-phase systems.