Heat Transfer In Fluidized Beds Research Articles

Effects of tube configurations on the heat transfer and hydrodynamic behavior in a gas-solid fluidized bed

The study of surface-to-bed heat transfer and flow characteristics in a gas-solid fluidized bed with different immersed tube configurations were investigated using the Eulerian-Eulerian method coupled with the kinetic theory of granular flow (KTGF) computationally. Compared with cases of in-line configurations, the staggered configurations increased the heat transfer coefficients (HTC) of the center-immersed heating tube. Moreover, it was found that with an increase in the rows and columns of the configurations, the HTCs increased and decreased respectively. The surface-to-bed heat transfer process was investigated combined with the particle renewal mechanism, and it was demonstrated that the HTCs were significantly affected by the surrounding packet fraction distribution and the packet residence time on the immersed heating surface. The results could be quantitatively employed to aid the heating tube configuration in designing the external heat exchanger (EHE) of a circulating fluidized bed (CFB) boiler for the treatment of municipal solid waste (MSW).

Effect of fines addition on heat transfer performance in gas-solid fluidized bed: An integrated experimental, simulation, and theoretical study

Efficient heat transfer is crucial in industrial fluidized beds, especially for highly exothermic or endothermic processes. In this study, the influence of adding fines on the heat transfer performance in fluidized bed is investigated through experiments, simulations, and theoretical analysis. Experimental results from a hot/cold-wall fluidized bed show that fines addition enhances the heat transfer performance. Notably, the bed-wall heat transfer coefficient presents a volcano-shaped curve with increasing fines proportion, with a discernible threshold at ∼25 vol%. Our simulations and theoretical analysis reveal that the improved particle dispersion due to fines addition is responsible for such volcano-shaped phenomenon. Theoretical analysis based on kinetic theory further uncovers that fines addition leads to an increased difference between the solid volume fraction in fluidization state and that in packing state, creating more free-space for particle moving, thereby improving particle dispersion, and thus enhancing heat transfer in fluidized beds.

CFD-DEM coupled simulation of fluidized beds with improved lumped formulation for heat transfer

PurposeThis paper aims to present a novel approach for computing particle temperatures in simulations coupling computational fluid dynamics (CFD) and discrete element method (DEM) to predict flow and heat transfer in fluidized beds of thermally thick spherical particles.Design/methodology/approachAn improved lumped formulation based on Hermite-type approximations for integrals to relate surface temperature to average temperature and surface heat flux is used to overcome the limitations of classical lumped models. The model is validated through comparisons with analytical solutions for a convectively cooled sphere and experimental data for a fixed particle bed. The coupled CFD-DEM model is then applied to simulate a Geldart D bubbling fluidized bed, comparing the results to those obtained using the classical lumped model.FindingsThe validation cases demonstrate that ignoring internal thermal resistance can significantly impact the temperature in cases where the Biot number is greater than 0.1. The results for the fixed bed case clearly demonstrate that the proposed method yields significantly improved outcomes compared to the classical model. The fluidized bed results show that surface temperature can deviate considerably from the average temperature, underscoring the importance of accurately accounting for surface temperature in convective heat transfer predictions and surface processes.Originality/valueThe proposed approach offers a physically more consistent simulation without imposing a significant increase in computational cost. The improved lumped formulation can be easily and inexpensively integrated into a typical DEM solver workflow to predict heat transfer for spherical particles, with important implications for various industrial applications.

Fluidized bed gas-solid heat transfer using a CFD-DEM coarse-graining technique

Computational Fluid Dynamics - Discrete Element Method (CFD-DEM) is extensively used for modeling heat transfer in gas-solid fluidized beds. However, CFD-DEM is computationally expensive, leading to a restriction regarding the number of simulated particles. Coarse-grained CFD-DEM is a technique to circumvent this constraint, allowing one to simulate larger fluidized beds. In this work, a scaling law used for coarse-graining hydrodynamics is generalized to gas-solid heat-transfer. This approach for coarse-graining heat transfer is tested using three different superficial gas velocities where the coarse-grained particle temperatures and Nusselt numbers are obtained. The particle temperature shows good correspondence with the original system for all cases and the Nusselt number is accurately predicted by the coarse-graining scaling law.

Biomass solar fast pyrolysis with a biomimetic mini heliostat field and thermal receiver for nitrogen heating

Biofuels may significantly influence the world's transition from hydrocarbons to renewable energies. Pyrolysis uses thermal energy in an anaerobic ambiance to crack biomass and create biofuels (bio-oil and synthesis gas). This work reports a detailed computational study of a novel system to develop fast biomass pyrolysis with concentrated solar energy. A fluidized bed reactor was coupled to a solar thermal receiver to heat the inert fluid (nitrogen) using a biomimetic mini heliostat field which provides the required thermal energy. A detailed kinetic model for the pyrolysis of lignocellulosic biomass was solved simultaneously with a set of differential equations based on mass, momentum, and energy conservation principles. An Eulerian-Eulerian approach was considered with three phases. The computational model was compared with data reported in the literature for the fluidized bed reactor and heat transfer in the solar thermal receiver with good agreement. Concentrated solar fluxes on the receiver, nitrogen outlet temperatures, temperature contours inside the reactor, concentration contours of relevant gases, and global product percentages (Tar, Char, and Gas) are reported. The products reach an almost constant composition of Gas, Tar, and Char after 6 s of reaction, indicating the annual operational stability of the solar pyrolysis plant in terms of biofuel production.

Liquid air energy storage system based on fluidized bed heat transfer

Liquid air energy storage (LAES) is a large-scale energy storage technology that has gained wide popularity due to its ability to integrate renewable energy into the power grid. Efficient cold/heat energy storage, which currently mainly includes solid-phase packed beds and liquid-phase fluids, is essential for the LAES system. However, the current heat/cold energy storage methods have limitations, such as the risk of flammability and explosion, the dynamic effect of the thermocline, and expensive investment. To address these limitations, a novel LAES based on fluidized bed heat transfer (FB-LAES) is proposed, and quartz sand is adopted as the heat/cold energy storage material. A thermodynamic model is developed, and the effects of heat exchanger design conditions and fluidization parameters on system performance are investigated, which indicates that the lower air-side/particle-side pressure drop and higher particle transport coefficient (PTC) favor the efficiency improvement of the FB-LAES system. The comparison demonstrates that the round-trip efficiency (RTE) of the FB-LAES system can reach 58.76% without stringent design conditions, while the RTE of the LAES system with traditional solid-phase (packed bed) and liquid-phase (methanol and propane) cold energy storage are 48.76% and 57.46%, respectively.

Numerical investigation of bed-to-tube heat transfer in a shallow fluidized bed containing mixed-size particles

Mixed-size particles with low processing cost have great potential to act as the absorbing media in the next-generation concentrated solar power plants. However, the research on the heat transfer from the particles to heated surfaces in gas–solid fluidized beds is limited. In this paper, the bed-to-tube heat transfer coefficient (HTC) in a shallow fluidized bed with immersed tubes is determined numerically. The Eulerian–Eulerian approach that incorporated the kinetic theory of granular flow has been implemented in a two-dimensional model. The distribution of the local HTC around the tube is investigated for various particle systems. The results show that the HTC reaches a maximum at the upper right (45°) and upper left (135°) of the tube, respectively, indicating a strong relation between the local HTC and solid volume fraction (SVF). The average HTC increases as the mean particle size gets smaller. For monosized particles, the HTC increases from 925 W/(m2·K) to 3033 W/(m2·K) as the particle size decreases from 300 to 100 µm. For binary particles, the growth in the HTC is nonlinear, with an increase in growth rate being observed as the small-size component becomes dominant. For mixed-size particles with the same mean size, a higher HTC can be achieved by enlarging the size difference between the components. A modified correlation for predicting the bed-to-tube HTC is established. Besides, the impacts of gas velocity and temperature, and heat radiation on the bed-to-tube HTC are discussed. It is found that an optimal gas velocity exists to maximize the average HTC and SVF simultaneously, while the gas temperature has a minor contribution. Due to the small heat transfer temperature difference, the effect of thermal radiation on the HTC can be neglected even though the bed temperature reaches 800 °C.

Effects of particle type on the particle fluidization and distribution in a liquid–solid circulating fluidized bed boiler

• Circulating fluidized bed technology is combined with boiler. • Effects of particle type on particle fluidization and distribution are investigated. • Particles accumulation exists at the bottom of the drum for all types of particles. • Particle properties obviously affect the fluidization and distribution of particles. • The non-uniform degree decreases with increased feedwater flow rate. A liquid–solid circulating fluidized bed boiler is designed and built for visualization research by applying the fluidized bed heat transfer and fouling prevention technology to the water side of the boiler. Four types of engineering plastic particles with different physical properties are selected as the solid working media. The effect of particle types on the fluidization and distribution of particles in the boiler is investigated under different feedwater flow rates and amount of added particles by using the charge couple device image measurement and acquisition system. The results show that all kinds of particles can’t be normally fluidized and accumulate in the drum at low amount of added particles and feedwater flow rate. The particles with great density and low sphericity are more likely to accumulate. The average solid holdup in the riser tubes increases with the increase in feedwater flow rate and the amount of added particles. The non-uniform degree of particle distribution in the riser tubes generally decreases with the increase in feedwater flow rate and the amount of added particles. The particles with small density and settling velocity have high average solid holdup in the riser tubes under close sphericity. In generally, the smaller the density and settling velocity, the more uniform the particle distribution in the riser tubes. Three-dimensional diagrams of the non-uniform degree of particle distribution in the riser tubes of the boiler are established.

Experimental study on heat transfer and hydrodynamics of a concentric double-pipe pulsed fluidized bed using microencapsulated phase change material and sand

This paper presents an experimental study on the heat transfer and hydrodynamic of a concentric double-pipe gas-solid pulsed fluidized bed filled in outer tube with microencapsulated phase change material and inner tube with sand. The effect of flow velocity, frequency and bed height are investigated on the heat transfer. Experiments are carried out at frequency in the range of 1 to 5Hz with the bed height of 15, 18.5and 22 cm and the ratio of air velocity to minimum fluidization velocity ( U / U mf ) of 1, 1.1, 1.2, 1.3, 1.4, 1.5. Results show that heat transfer enhances as the frequency, bed height and U / U mf increase. Besides, it is found that pulsed flow has higher heat transfer enhancement than continuous flow. For example, at U / U mf of 1.5, the heat transfer coefficient of the pulsed flow increases by 7.5% than that of the continuous flow at the bed height of 22 cm. • Hydrodynamics and heat transfer in a concentric double-pipe pulsed bed are studied. • Pulsed flow in bed has higher heat transfer coefficient than continuous flow. • Heat transfer coefficient increases as U / U mf , frequency, and bed height increase. • Heat transfer becomes higher as the fluidity increases.

Experimental investigation on heat transfer in a gas-solid fluidized bed with a bundle of heat exchanging tubes

A fluidized bed reactor is commonly used for highly exothermic reactions for different chemical industrial processes. However, inefficient removal of the generated heat due to the exothermic reaction can seriously influence reactor performance. Hence, quantifying and understanding the heat transfer phenomena in this reactor is essential to enhance the performance of the reactor and consequently the chemical process. To achieve a better quantification and understanding of the heat transport in this reactor, an advanced heat transfer technique has been used in this study to quantify the impact of the presence of the cooling tubes on the local heat transfer coefficient under different operating conditions for this reactor. It has been found that the local heat transfer coefficient in the fluidized bed reactor equipped with a bundle of vertical tubes increases significantly as superficial gas velocity increases at the wall region, while different behavior was noticed at the center of the reactor. Moreover, the results show that the local heat transfer significantly decreases at the reactor's core region for all studied superficial gas velocities. Furthermore, the new tube arrangement offers a uniform local heat transfer profile for all studied operating conditions. The obtained new high-quality experimental data for the local heat transfer coefficient in a fluidized bed reactor equipped with a bundle of tubes can be used for validation CFD simulations or mathematical models, facilitating the design, scale up, and operation of this reactor.

Effect of inclination angle on the thermal performance of a three-phase closed thermosyphon containing SiC particles

Three-phase closed thermosyphon (THPCT) with variable inclination angle (0°–30°) is designed and built to investigate thermal performance by applying fluidized bed heat transfer technology to the two-phase closed thermosyphon (TPCT). Visual experiments were conducted to help explain the flow in THPCT. Water and silicon carbide (SiC) particles are selected as working media. The effect of solid holdup (0–20%) and input power (100–300 W) is also discussed. Results show that the addition of SiC particles can improve the thermal performance of the thermosyphon and reduce the overall thermal resistance under different inclination angles. The reduction rate of the overall thermal resistance fluctuates with the increase in inclination angle. The maximum reduction rates of the overall thermal resistance for the inclination angles of 0°, 10°, 20°, and 30° are 32.9%, 26.7%, 37.0%, and 30.5%, respectively. The maximum reduction rates of the overall thermal resistance exist at low input power, depending on the solid holdup. The overall thermal resistances of the TPCT and THPCT generally decrease with the increase in input power but fluctuate with the increase in inclination angle. In most cases, the minimum overall thermal resistance is obtained at the inclination angle of 0°. The convective heat transfer coefficient of the condensation section is obviously lower than that of the evaporation section under different inclination angles. The input power and particle addition have a more obvious influence on the heat transfer of the condensation section compared with the evaporation section. The results are beneficial to the application of the THPCT in some industrial fields, such as solar collector and frozen earth.

Heat transfer to a sphere immersed in a fluidized bed of coarse particles with transition from bubbling to turbulent flow regime

The present work concerns an experimental study on heat transfer in gas-solid fluidized bed of coarse (Geldart D) particles to a larger immersed sphere at high superficial velocities from 2 to 5.5 U mf. The heat transfer coefficient was determined by measuring the temperature of the test sphere during its heating in a fluidized bed in the temperature range of 65–175 °C. The test spheres of different sizes and different materials were utilized. For the given gas-particles system the flow regime changes from rapidly growing bubbles to turbulent fluidization at superficial velocity U c ≈ 3Umf. It has been found that in rapidly growing bubbles regime, the heat transfer coefficient is higher for smaller test spheres while it is almost independent of the superficial gas velocity. In turbulent regime, the heat transfer coefficient increases with increase of gas velocity while the size of the test sphere exhibits less influence. In the rapidly growing bubbles regime, experimental data for heat transfer coefficient can be predicted adequately with correlation of Scott et al.. For the turbulent flow regime a new correlation for prediction of the heat transfer coefficient has been proposed.

Experimental Investigation of the Heat Transfer between Finned Tubes and a Bubbling Fluidized Bed with Horizontal Sand Mass Flow

The sandTES technology utilizes a fluidized bed counter current heat exchanger for thermal energy storage applications. Its main feature is an imposed horizontal flow of sand (SiO2) particles fluidized by a vertical air flow across a heat exchanger consisting of several horizontal rows of tubes. Past international research on heat transfer in dense fluidized beds has focused on stationary (stirred tank) systems, and there is little to no information available on the impact of longitudinal or helical fins. Previous pilot plant scale experiments at TU Wien led to the conclusion that the currently available correlations for predicting the heat transfer coefficient between the tube surface and the surrounding fluidized bed are insufficient for the horizontal sand flow imposed by the sandTES technology. Therefore, several smaller test rigs were designed in this study to investigate the influence of different tube arrangements and flow conditions on the external convective heat transfer coefficient and possible improvements by using finned tubes. It could be shown that helically finned tubes in a transversal arrangement, where the horizontal sand flow is perpendicular to the tube axes, allows an increase in the heat transfer coefficient per tube length (i.e., the virtual heat transfer coefficient) by a factor of 3.5 to about 1250 W/m2K at ambient temperature. Based on the literature, this heat transfer coefficient is expected to increase at higher temperatures. The new design criteria allow the design of compact, low-cost heat exchangers for thermal energy storage applications, in particular electro-thermal energy storage.

Effects of gas composition and operating pressure on the heat transfer in an oxy-fuel fluidized bed: A CFD–DEM study

CFD-DEM modeling was conducted to study the particle heat transfer in an oxy-fuel fluidized bed under different gas compositions and operating pressures. In particular, to determine the appropriate operating fluidization velocity for the pressurized oxy-fuel fluidized bed, three fluidizing parameters, the excess velocity Uex, fluidization number FN, and Froude particle number Frp, were evaluated by examining the similarities of the flow patterns and the particle heat transfer. The results revealed that the particle heat transfer is positively correlated with the gas density and specific heat capacity, as well as the fluidization velocity. Increasing the operating pressure augments the particle heat transfer, with the rate of increase being the highest at a constant Uex while being less significant at a constant Frp. Moreover, using the constant Frp under pressurized conditions makes possible an almost constant bed expansion and a more desirable particle temperature distribution, as well as a similar thermal input.

A priori error estimates for finite element approximations to transient convection-diffusion-reaction equations in fluidized beds

Abstract In this work, a priori error estimates for finite element approximations to the governing equations of heat and mass transfer in fluidized beds are derived. These equations are time dependent strongly coupled system of five semilinear convection-diffusion-reaction equations. The a priori error estimates for all the five variables are obtained for the error measured in L ∞(L 2) and L 2 ( E ) ${L}^{2}\left(\mathcal{E}\right)$ , E $\mathcal{E}$ is the energy norm.

Effect of particle diameter and heating position of evaporation section on thermal performance of a vapor–liquid–solid three-phase closed thermosyphon

A vapor-liquid-solid three-phase closed thermosyphon (THPCT) with variable heating position at evaporation section is built by combining the fluidized bed heat transfer technology with the two-phase closed thermosyphon (TPCT). The effect of particle diameter and heating position on the thermal performance is investigated by varying the heating power (100–300 W) and solid holdup (5–15%). Results show that the addition of the glass beads with different diameters can enhance the heat transfer under certain conditions. The maximum reduction rates of the overall thermal resistance are 25.4% and 27.7% for Le = 200 mm and Le = 200 mm − up40, respectively. The upward movement of the heating position is beneficial to reducing the overall thermal resistances of the TPCT and the THPCT with small particles, but is not conducive to the heat transfer of the THPCT with large particles. A medium particle diameter and a low solid holdup are suggested for the THPCT with different heating positions.

Resource-saving equipment for drying of dispersed materials in a fluidized bed

The design of dryers for bulk products with continuous operation is proposed. The regulation of the speed of the supplied air allows to increase the productivity and reliability of the equipment and to reduce its overall dimensions. Keywords: fluidized bed drying, heat and mass transfer, productivity, reliability. fln-84@mail.ru

Investigation of the heat transfer characteristics of a novel thermosyphon with different particle sizes

In this study, the fluidized bed heat transfer technology is introduced into two-phase closed thermosyphon (TPCT) to build a novel thermosyphon system, which aims to improve the heat transfer performance of TPCT. A series of experiments are conducted to investigate the joint effects of particle size, amount of added particles and heating power on heat transfer characteristics of the novel thermosyphon. Results show that the heat transfer performance of TPCT is significantly improved in the evaporator section because of the addition of silicon carbide particles. However, in the condenser section, the existence of particles leads to the deterioration of heat transfer. In general, the heat transfer enhancement effect of the large size particles is superior to that of the small size particles. The overall thermal resistance is reduced because of the addition of large size particles. The 3D diagrams are established to assess the joint effects of various parameters.

A posteriori error estimates and an adaptive finite element solution for the system of unsteady convection-diffusion-reaction equations in fluidized beds

The a posteriori error estimates for finite element approximations to the governing equations of heat and mass transfer in fluidized beds are derived in this work. These are a system of five time dependent coupled nonlinear convection-diffusion-reaction equations. Based on the variational formulation, computable residual based a posteriori error estimates are obtained. The time discretization has been done using the implicit Euler method. The a posteriori error estimates for all the five variables are derived using the total residual and error indicators due to spatial discretization, time discretization and linearization. An adaptive finite element solution of these model equations is computed and the performance is illustrated.

Bed-to-wall heat transfer in a gas-solid fluidized bed with external solids circulation:Modified packet renewal model

A pilot scale experimental setup was constructed to investigate the effect of hydrodynamics on bed-to-wall heat transfer in a gas-solid fluidized bed with external solids circulation. The bubble size and velocity are increased rapidly with gas velocity at bubbling fluidization. The increased gas velocity promotes the increase in bubble frequency at turbulent fluidization. The external solids circulation can break up bubbles, attributing to the decrease in bubble size and velocity and increase in bubble frequency. The bubble properties are employed to calculate the packet parameters in the packet renewal model. And the packet residence time plays a dominated role in the heat transfer process. The heat transfer bypass occurs due to poor solids radial mixing and external solids circulation. Two correction factors are introduced to modify the Mickley and Fairbanks' model, and the results show that the modified model can predict the heat transfer coefficients successfully.