Effect of riser exit geometry on bed hydrodynamics and heat transfer in a circulating fluidized bed riser column

This paper reports the variation of suspension density along the riser column and the effect of riser exit geometry on bed hydrodynamics and heat transfer in the upper region of a circulating fluidized bed (CFB) riser column. The experiments are conducted in a CFB riser column which is 102 mm × 102 mm in bed cross‐section (square), 5.25 m height, with a return leg of the same dimension. The unit is made up of interchangeable plexiglass columns. The superficial primary air velocity is varied between 4.2 and 6.4 m/s. The suspension density profile along the riser height is influenced by the exit geometry. With a 90° riser exit geometry, the suspension density profile in the upper region of the CFB riser column increases towards the riser exit. This particular trend has been observed for about 2 m length in the top region of the riser. The change in suspension density profile in the top region influences the variation of heat transfer coefficient. With a 90° riser exit geometry, the suspension density increases towards the riser exit, which in turn increases the heat transfer coefficient. The effect of riser exit geometry on hydrodynamics and heat transfer is significant for about 2 m length in the upper region of the riser column. Copyright © 2001 John Wiley & Sons, Ltd.     

Effect of dilute and dense phase operating conditions on bed-to-wall heat transfer mechanism in a circulating fluidized bed combustor

In the present paper investigations are conducted on bed-to-wall heat transfer to water-wall surfaces in the upper region of the riser column of a circulating fluidized bed (CFB) combustor under dilute and dense phase conditions. The bed-to-wall heat transfer depends on the contributions of particle convection, gas convection and radiation heat transfer components. The percentage contribution of each of these components depends on the operating conditions i.e., dilute and dense phase bed conditions and bed temperature. The variation in contribution with operating conditions is estimated using the cluster renewal mechanistic model. The present results contribute some fundamental information on the contributions of particle convection, gas convection and radiation contributions in bed-to-wall heat transfer under dilute and dense phase conditions with bed temperature. This leads to better understanding of heat transfer mechanism to water-wall surfaces in the upper region of the riser column under varying load conditions i.e., when the combustor is operated under dilute and dense phase situations. The results will further contribute to understanding of heat transfer mechanism and will aid in the efficient design of heat transfer surfaces in the CFB unit.     

Experimental investigations on heat transfer from suspension to impact separators in the riser column of a circulating fluidized bed combustor

In the present work experiments are conducted to investigate the effect of operating parameters on heat transfer from bed to U-beam impact separators located in the top region of the riser column. The effect of suspension density and bed temperature on heat transfer from bed to the impact separators (test sections) are investigated. The experimental unit consists of a circulating fluidized bed riser column, which is 0.23 m×0.23 m in bed cross-section, 6.3 m in height with a return leg and back pass. The U-beam impact separators are located in the top region of the riser column. Furnace oil # 2 is burnt in the unit and the experimental investigations are conducted. Water is circulated through the U-beam impact separators. The presence of the impact separators in the top region of riser column helps in solids separation and also to absorb certain fraction of heat liberated in the furnace. The bed to U-beam impact separator heat transfer coefficient increases with suspension density due to increased particle concentration, which results in higher cluster and particle heat transfer. The heat transfer coefficient increases with bed temperature due to increased convection and radiation.     

Numerical modeling of bed-to-wall heat transfer in a circulating fluidized bed combustor based on cluster energy balance

The bed-to-wall heat transfer in a circulating fluidized bed (CFB) combustor depends on the heat transfer contributions from particle clusters, dispersed/gas phase and radiation from both of them. From the available CFB literature, most of the theoretical investigations on cluster and bed-to-wall heat transfer are based on mechanistic models except a few based on mathematical and numerical approaches. In the current work a numerical model proposed to predict the bed-to-wall heat transfer based on thermal energy balance between the cluster/dispersed phase and the riser wall. The effect of cluster properties and the thermal boundary conditions on the cluster heat transfer coefficient are analyzed and discussed. The fully implicit finite volume method is used to solve the governing equations by generating a 2D temperature plot for the cluster and the dispersed phase control volumes. From this 2D temperature profile, space and time averaged heat transfer coefficients (for cluster, dispersed phase and radiation components) are estimated for different operating conditions. The results from the proposed numerical simulation are in general agreement with published experimental data for similar operating conditions. The results and the analysis from the current work give more information on the thermal behavior of the cluster and dispersed phases, which improves the understanding of particle and gas phase heat transfers under different operating conditions in CFB units.     

Effect of pressure on thermal aspects in the riser column of a pressurized circulating fluidized bed

In the present paper the effect of pressure on bed‐to‐wall heat transfer in the riser column of a pressurized circulating fluidized bed (PCFB) unit is estimated through a modified mechanistic model. Gas–solid flow structure and average cross‐sectional solids concentration play a dominant role in better understanding of bed‐to‐wall heat transfer mechanism in the riser column of a PCFB. The effect of pressure on average solids concentration fraction ‘c’ in the riser column is analysed from the experimental investigations. The basic cluster renewal model of an atmospheric circulating fluidized bed has been modified to consider the effect of pressure on different model parameters such as cluster properties, gas layer thickness, cluster, particle, gas phase, radiation and bed‐to‐wall heat transfer coefficients, respectively. The cluster thermal conductivity increases with system pressure as well as with bed temperature due to higher cluster thermal properties. The increased operating pressure enhances the particle and dispersed phase heat transfer components. The bed‐to‐wall heat transfer coefficient increases with operating pressure, because of increased particle concentration. The predicted results from the model are compared with the experimentally measured values as well as with the published literature, and a good agreement has been observed. The bed‐to‐wall heat transfer coefficient variation along the riser height is also reported for different operating pressures. Copyright © 2005 John Wiley & Sons, Ltd.     

A study on the effects of operational parameters on bed-to-wall heat transfer

In this study the effects of operational parameters on bed-to-wall heat transfer in CFBs are investigated such as solids volume fraction, particle diameter, suspension density, solid circulation rate. Based on a previously developed 2D CFB model, a modified cluster renewal model is used in this investigation. The model uses the particle-based approach (PBA) and integrates the hydrodynamics and combustion aspects. The study is also validated with experimental data. As a result of this study, it is observed that the bed-to-wall heat transfer coefficients are strongly dependent on particle diameter and solids concentration at the riser wall. The smaller particles result in higher heat transfer coefficients than larger particles for the same solids volume fraction values. The heat transfer coefficient increases with suspension density. However, at a constant suspension density, the superficial velocity does not have a significant influence on the heat transfer coefficient. PBA is satisfactorily adapted to cluster renewal model so that to define the bed-to-wall heat transfer mechanisms for the upper zone.     

Bed-to-wall heat transfer behavior in a pressurized circulating fluidized bed

An experimental investigation has been made to study the effect of pressure and other relevant operating parameters on bed hydrodynamics and bed-to-wall heat transfer in a pressurized circulating fluidized bed (PCFB) riser column of 37.5 mm internal diameter and 1940 mm height. The experiments have been conducted with and without bed material for the consideration of frictional pressure drop due to gas density at elevated pressures. The pressure drop measured without sand particles is assumed as the pressure drop due to gas density for the calculation of bed voidage and suspension density profiles. The specially designed heat transfer probe is used to measure the bed-to-wall heat transfer coefficient. The experimental results have been compared with the published literature and good agreement has been observed. The axial bed voidage is less in the bottom zone of the riser column and is increasing along the height of the bed. With the increase in system pressure, the bed voidage is found to be increasing in the bottom zone and decreasing in the top zone. The heat transfer coefficient increases with the increase in system pressure as well as with the gas superficial velocity. The heat transfer coefficient is also observed to be increasing with the increase in average suspension density.     

3D modelling of radiative heat transfer in circulating fluidized bed combustors: influence of the particulate composition

A three-dimensional model is developed to predict the bed-to-wall radiative heat transfer coefficient in the upper dilute zone of circulating fluidized bed (CFB) combustors. The radiative transfer equation is solved by the discrete ordinates method and Mie scattering theory is applied to calculate the absorption and scattering efficiency factors of particles existing in CFB combustors. Empirical correlations calculate both spacial variation of solid volume fraction and temperature distribution at the wall. The model considers the influences of the particle properties (including particle size distribution, particle optical constants and solid composition) on the radiative heat transfer coefficient. Simulation results show that the particle properties have significant influences on the bed-to-wall radiative heat transfer coefficient in CFB combustors. A very good agreement of predicted results is shown with experimental data.     

The impact of bed temperature on heat transfer characteristic between fluidized bed and vertical rifled tubes

In the present work, the heat transfer study focuses on assessment of the impact of bed temperature on the local heat transfer characteristic between a fluidized bed and vertical rifled tubes (38mm-O.D.) in a commercial circulating fluidized bed (CFB) boiler. Heat transfer behavior in a 1296t/h supercritical CFB furnace has been analyzed for Geldart B particle with Sauter mean diameter of 0.219 and 0.246mm. The heat transfer experiments were conducted for the active heat transfer surface in the form of membrane tube with a longitudinal fin at the tube crest under the normal operating conditions of CFB boiler. A heat transfer analysis of CFB boiler with detailed consideration of the bed-to-wall heat transfer coefficient and the contribution of heat transfer mechanisms inside furnace chamber were investigated using mechanistic heat transfer model based on cluster renewal approach. The predicted values of heat transfer coefficient are compared with empirical correlation for CFB units in large-scale.     

Numerical modeling of axial bed-to-wall heat transfer in a circulating fluidized bed combustor

The water-wall surfaces located above the secondary air inlet within the circulating fluidized bed (CFB) combustor are exposed to the axial bed-to-wall heat transfer process. In the current work, the axial bed-to-wall heat transfer coefficients are estimated for three different axial voidage profiles (covering three widely occurring average particle concentrations) in order to investigate the effect of voidage, time, initial and fixed temperature of the bed and annulus, and gas gap between wall and solid particles; on the axial heat transfer process. A 2D thermal energy balance model is developed to estimate the axial heat transfer values for the gas–solid suspension along the height of the riser column with horizontally changing mass distribution. The gas–solid mass distribution is fixed with time thus providing a spectrum of changes in axial bed-to-wall heat transfer profile with time. The current work provides an opportunity to understand the axial heat transfer relationship with particle concentration and instantaneous behaviour. The results from the work show that: (i) first few seconds of the suspension temperature near the wall has maximum energy thus providing a small time frame to transfer more heat to the surface (CFB wall); (ii) both axial and horizontal particle concentrations (influenced by the operating conditions) affect the axial heat transfer locally; (iii) initial temperature of the bed between average and maximum values provide end limits for the axial heat transfer; (iv) annulus region has higher thermal energy than the core due to increased particle presence; and (v) a particle-free zone near the wall (gas gap) having a maximum thickness of 1 mm, tends to reduce up to 25% of axial heat transfer value. The model trends have close agreement with experimental trends from published literature; but the model values differ when correlating with real values due to inconsistencies in riser diameter and nature of variation in parameters.     

Fundamental heat transfer mechanism between bed‐to‐membrane water‐walls in circulating fluidized bed combustors

In the present work, the fundamental mechanism between bed‐to‐membrane water‐walls in the riser column of a circulating fluidized bed (CFB) combustor is presented. The bed‐to‐membrane water‐wall heat transfer depends on the contributions of particle heat transfer, dispersed phase heat transfer and radiation heat transfer. The fundamental mechanism of particle heat transfer and the effect of fraction of wall exposed to clusters and gas gap thickness between cluster and wall on particle heat transfer coefficient and bed‐to‐wall heat transfer coefficient are investigated. The influence of operating parameters like cross‐sectional average volumetric solids concentration and bed temperature on particle and bed‐to‐wall heat transfer are also reported. The present work contributes some fundamental information on particle heat transfer mechanism, which is responsible for increasing the bed‐to‐wall heat transfer coefficient (apart from dispersed phase convection and radiation heat transfer). The details on particle heat transfer mechanism will enable to understand the basic heat transfer phenomena between bed‐to‐membrane water‐walls in circulating fluidized bed combustors in a detailed way, which in turn will aid for better design of CFB combustor units. The particle heat transfer mechanism is significantly influenced by the fraction of wall exposed to clusters and gas gap thickness between clusters and wall. Copyright © 2003 John Wiley & Sons, Ltd.     

Axial and radial heat transfer studies in a circulating fluidized bed

The axial and radial variation of the heat transfer coefficient in a circulating fluidized bed riser column, and the effect of operating parameters thereon, are investigated. The experimental set-up consists of a riser column of 102 mm×102 mm in bed cross-section, 5·25 m in height with a return leg of the same dimensions. The unit is fabricated with plexiglass columns of 0·6 m in length which are interchangeable with one another. Two axial heat transfer test sections of 102 mm×102 mm in cross-section, 500 mm in height, and made of mild steel, are employed for the axial heat transfer study and one horizontal tube section of 22·5 mm OD made of mild steel is employed for the radial heat transfer study. The primary air velocity is varied between 4·21 and 7·30 m s⁻¹. Local sand of mean size (d_p) 248 μm is used as the bed material. One empirical model with the help of dimensional analysis has been proposed to predict the heat transfer coefficient to a bare horizontal tube in a CFB riser column and the model results are validated with the experimental data; good agreement has been observed. © 1997 John Wiley & Sons, Ltd.     

Effect of operating parameters on heat transfer in the lean phase region of a CFB riser column

The radial variation of the heat transfer coefficient across the bed width, including the effect of fins and fin shapes (surface roughness), on the heat transfer characteristics in the lean phase region of a circulating fluidized bed riser column are investigated. Three test sections (bare horizontal tube, V-fin tube and square fin tube) are employed for the investigation. The experimental unit consists of a riser column of 102×102 mm in bed cross-section, 5·25 m in height with a return leg of the same dimensions, and both made up of plexiglas columns. For the same operating conditions, the provision of fins results in a drop in heat transfer coefficient, but increases total heat transfer owing to increased surface area. The present experimental data are compared with published literature and good agreement has been observed. The experimental data also corresponds to the two-zone hydrodynamics (i.e. the core–annulus structure) of a circulating fluidized bed. © 1997 John Wiley & Sons, Ltd.     

The simulation and experimental validation on gas-solid two phase flow in the riser of a dense fluidized bed

Gas-solid flow in dense CFB (circulating fluidized bed)) riser under the operating condition, superficial gas 15.5 m/s and solid flux 140 kg/m2s using Geldart B particles (sand) was investigated by experiments and CFD (computational fluid dynamics) simulation. The overall and local flow characteristics are determined using the axial pressure profiles and solid concentration profiles. The cold experimental results indicate that the axial solid concentration distribution contains a dilute region towards the up-middle zone and a dense region near the bottom and the top exit zones. The typical core-annulus structure and the back-mixing phenomenon near the wall of the riser can be observed. In addition, owing to the key role of the drag force of gas-solid phase, a revised drag force coefficient, based on the EMMS (energy-minimization multi-scale) model which can depict the heterogeneous character of gas-solid two phase flow, was proposed and coupled into the CFD control equations. In order to find an appropriate drag force model for the simulation of dense CFB riser, not only the revised drag force model but some other kinds of drag force model were used in the CFD. The flow structure, solid concentration, clusters phenomenon, fluctuation of two phases and axial pressure drop were analyzed. By comparing the experiment with the simulation, the results predicted by the EMMS drag model showed a better agreement with the experimental axial average pressure drop and apparent solid volume fraction, which proves that the revised drag force based on the EMMS model is an appropriate model for the dense CFB simulation.     

Modelling heat transfer in a circulating fluidized bed

A model for the bed-to-wall heat transfer under low temperature condition in a circulating fluidized bed (CFB) was developed based upon a simplified cluster renewal concept. The age of clusters in contact with the wall at different locations along the height of the CFB was estimated as the weighted average age considering their formation and disintegration. One set of experimental data on heat transfer in a 4.5-metre high, 0.15-metre diameter CFB under low temperature condition (67–77°C) was chosen for comparison with prediction of local heat transfer coefficient. The experimental observation and prediction have shown a qualitative agreement.     

A study of heat transfer in a circulating fluidized bed

An experimental investigation was carried out to study the effects of operating parameters on the local bed-to-wall heat transfer coefficient in a 4.5 m tall, 0.150 m diameter circulating fluidized bed with a bed temperature in the range of 65°C to 80°C, riser flow rate varying from 1400 litres/min to 2000 litres/min, bed inventory in the range of 15 kg to 25 kg of sand, and average sand sizes of 200 μm, 400 μm and 500 μm. A heat flux probe was attached to the riser wall at five different vertical locations for measuring the heat flux from the bed to the wall surface. From the present work, the heat transfer coefficient in the dilute phase was found to be in the range of 62 to 83 W/m²K, 51 to 74 W/m²K, and 50 to 59 W/m² K for sand sizes of 200 μm, 400 μm and 500 μm, respectively. Relevant mathematical correlations were developed to predict local heat transfer coefficient based on the results of the practical work.     

Modeling gas oil spray coalescence and vaporization in gas solid riser reactor

The sprayed feed droplet behavior, including coalescence and vaporization into gas–solid flow, is complex especially near the atomizer region in fluid catalytic cracking (FCC) riser reactor. A three dimensional CFD model of the riser reactor has been developed, which takes into the account three phase hydrodynamics, heat transfer and evaporation of the liquid droplets into a gas–solid flow as well as phase interactions. A hybrid Eulerian–Lagrangian approach was applied to numerically simulate the collision and vaporization of gas oil droplets in the gas–solid fluidized bed. This numerical simulation accounts the possibility of coalescence of feed spray droplets in computing the trajectories and its impact on droplet penetration in the reactor. The modeling result shows that droplet coalescence mainly occurs at the initial part of the atomizing region and where three phase flow hits the reactor wall and bounces back. The model has the ability of inspecting the effects of feed injector geometry on the overall reactor hydrodynamic and heat transfer. The CFD simulation results showed that the evaporated droplet gas caused higher local velocities of the gas and solid particles and gas–solid flow temperature reduction.     

循环流化床传热系数的计算模型

本文在循环流化床流动模型的基础上建立了传热模型，流动模型根据实际运行情况考虑了颗粒的宽筛分，并把床层在轴向上分为密相床和稀相床两部分。在密相床内，传热按照鼓泡床传热微型进行计算；在稀相床内，传热模型建立在颗粒团更新的假设基础上，根据假设，床层由颗粒浓度很低的上升稀相和相对颗粒浓度较大的颗粒团两部分组成，两部分交替地与床壁面接触，床层和受热面间局部换热系数和颗粒浓度及两部分接触壁面的份额有关。模化结     

Numerical investigation of gas-to-particle cluster convective heat transfer in circulating fluidized beds

Convective heat transfer between gas and cluster in a circulating fluidized bed (CFB) riser is numerically studied using a three-dimensional computational fluid dynamic (CFD) model. Distributions of gas velocity and temperature as well as the gas-cluster heat flux and convective heat transfer coefficient are obtained. Variations of heat flux of each particle in the cluster are predicted. The heat flux of the individual particle inside the cluster is smaller than that of an isolated particle. The convective heat transfer coefficients increase with the increase of cluster porosity and Reynolds number. The convective heat transfer coefficients of the downward moving cluster are larger than that of upward moving cluster. Numerical results of an isolated particle are in agreement with data from previously published correlations.     

Method of calculation of heat transfer coefficient of the heater in a circulating fluidized bed furnace

Knowledge of heat transfer coefficients is important in the design and operation of CFB boilers. It is the key to determining the area and the layout of the heat transfer surfaces in a CFB furnace. Local bulk density has a close relationship to the local heat transfer coefficient. Using a heat flux probe and bulk density sampling probe, the local bed to wall heat transfer coefficient in the furnace of a 75 t/h CFB boiler was measured. According to the experimental results and theoretical analysis of the facts that influence the heat transfer, the heat transfer coefficient calculation method for the CFB furnace was developed. The heat transfer surface configuration, heating condition, and the material density are considered in this method. The calculation method has been used in the design of CFB boilers with a capacity from 130 t/h to 420 t/h. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 540–550, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10056