The measurement of instantaneous local heat transfer coefficients in a circulating fluidized bed

Instantaneous and time-averaged local bed-to-wall heat transfer coefficients were measured in a 9.3 m tall, 152 mm ID cold model circulating fluidized bed riser at three different heights for 171 μm Ottawa sand at a superficial gas velocity of 7 m/s, and for solids circulation fluxes up to about 70 kg/m².s. All data were obtained with an instantaneous heat transfer probe consisting of a thin platinum film deposited on a 1 cm² piece of glass protected by a thin plastic film. Instantaneous heat transfer coefficients in the riser showed sudden and dramatic peaks caused by strands or sheets of particles sweeping past the probe. Consistent with previous work, time-averaged coefficients varied nearly linearly with suspension density. The heat transfer coefficient decreased from the bottom to the middle of the column, and then increased near the top due to an increase in suspension density for the exit geometry employed.     

Heat transfer to the ceiling of the riser of a circulating fluidized bed

Very little information on the heat transfer to the ceiling of a circulating fluidized bed (CFB) boiler is available in the published literature though it constitutes a significant part of the furnace heat absorption. So, to explore this less-known heat transfer process a series of experiments were conducted at four different superficial gas velocities and three external solids circulation rates in a CFB pilot plant with a riser having a height of 5 m and a cross section of . The experimental results suggest that both solids circulation rates and superficial gas velocities had a significant influence on the local heat transfer to the ceiling close to the riser exit to the gas solids separator. However, on the ceiling, opposite of the exit, solids circulation rates and superficial gas velocities had only a minor influence on the local heat transfer coefficients.     

Bed-to-wall heat transfer characteristics in a circulating fluidized bed

The bed-to-wall heat transfer coefficients were measured in a circulating fluidized bed of FCC particles (d_p = 65 μm). The effects of gas velocity (1.0–4.0 m/s), solid circulation rate (10–50 kg/m²s) and particle suspension density (15–100 kg/m³) on the bed-to-wall heat transfer coefficient have been determined in a circulating fluidized bed (0.1 m-ID x 5.3 rn-high). The heat transfer coefficient strongly depends on particle suspension density, solid circulation rate, and gas velocity. The axial variation of heat transfer coefficients is a strong function of the axial solid holdup profile in the riser. The obtained heat transfer coefficient in terms of Nusselt number has been correlated with the pertinent dimensionless groups     

Olive cake combustion in a circulating fluidized bed

In this study, a circulating fluidized bed of 125 mm diameter and 1800 mm height was used to find the combustion characteristics of olive cake (OC) produced in Turkey. A lignite coal that is most widely used in Turkey was also burned in the same combustor. The combustion experiments were carried out with various excess air ratios. The excess air ratio, λ, has been changed between 1.1 and 2.16. Temperature distribution along the bed was measured with thermocouples. On-line concentrations of O₂, SO₂, CO₂, CO, NO_x and total hydrocarbons were measured in the flue gas. Combustion efficiencies of OC and lignite coal are calculated, and the optimum conditions for operating parameters are discussed. The combustion efficiency of OC changes between 82.25 and 98.66% depending on the excess air ratio. There is a sharp decrease observed in the combustion losses due to hydrocarbons and CO as the excess air ratio increases. The minimum emissions are observed at λ=1.35. Combustion losses due to unburned carbon in the bed material do not exceed 1.4 wt% for OC and 1.85 wt% for coal. The combustion efficiency for coal changes between 82.25 and 98.66% for various excess air ratios used in the study. The ash analysis for OC is carried out to find the suitability of OC ash to be used as fertilizer. The ash does not contain any hazardous metal.     

Temperature fluctuations and heat transfer in a circulating fluidized bed

Characteristics of temperature fluctuations and heat transfer coefficient have been investigated in the riser of a circulating fluidized bed (0.102 m ID and 4.0 m in height). Effects of gas velocity and solid circulation rate on the temperature fluctuations, suspension density and heat transfer coefficient between the immersed heater and the bed have been considered in the riser. To analyze the characteristics of temperature fluctuations at the wall of the riser, the phase space portrait and Kolmogorov entropy of the fluctuations have been obtained, and the relation between the temperature fluctuations and the heat transfer coefficient has been examined. It has been found that the heat transfer system becomes more complicated and irregular with decreasing gas velocity and increasing solid circulation rate or suspension density in the riser. The heat transfer coefficient and Kolmogorov entropy of the temperature fluctuations have decreased with increasing the superficial gas velocity, while they have increased with increasing the solid circulation rate or suspension density in the bed. The heat transfer coefficient has been well correlated in terms of the Kolmogorov entropy, suspension density as well as operating variables in the riser. This paper is dedicated to Professor Dong Sup Doh on the occasion of his retirement from Korea University.     

Investigation of radiative heat transfer in fixed bed biomass furnaces

This paper presents an investigation of the radiative heat transfer process in two fixed bed furnaces firing biomass fuels and the performance of several widely used models for calculation of radiative heat transfer in the free-room of fixed bed furnaces. The simple optically thin (OT) model, the spherical harmonic P₁-approximation model, the grey gas model based on finite volume discretization (FGG), and the more accurate but time consuming spectral line weighted-sum-of-grey-gases (SLW) model are investigated. The effective mean grey gas absorption coefficients are calculated using an optimised version of the exponential wide band model (EWBM) based on an optical mean beam length. Fly-ash and char particles are taken into account using Mie scattering. In the investigated updraft small-scale fixed bed furnace radiative transfer carries heat from the bed to the free-room, whereas in the cross-current bed large-scale industry furnace, radiative transfer brings heat from the hot zones in the free-room to the drying zone of the bed. Not all the investigated models can predict these heat transfer trends, and the sensitivity of results to model parameters is fairly different in the two furnaces. In the small-scale furnace, the gas absorption coefficient predicted by using different optical lengths has great impact on the predicted temperature field. In the large-scale furnaces, the predicted temperature field is less sensitive to the optical length. In both furnaces, with the same radiative properties, the low-computational-cost P₁ model predicts a temperature field in the free-room similar to that by the more time consuming SLW model. In general, the radiative heat transfer rates to the fuel bed are not very sensitive to the radiative properties, but they are sensitive to the different radiative heat transfer models. For a realistic prediction of the radiative heat transfer rate to the fuel bed or to the walls, more computationally demanding models such as the FGG or SLW models should be used.     

An investigation of the heat transfer behavior of longitudinal finned membrane water wall tubes in circulating fluidized bed boilers

Experiments were conducted in a cold model circulating fluidized bed (CFB) having riser cross sectional area of 100 mm × 100 mm, height of 4.8 m, bed temperature of 75°C and superficial velocity of 8 ms^− 1. Local sand having average diameter of 231 μm, particle density of 2774 kg m^− 3 and bulk density of 1515 kg m^− 3 was used as bed material. The experiments were conducted for three tube configurations: a membrane tube, a membrane tube with a longitudinal fin at the tube crest and a membrane tube with two longitudinal fins at 45° on both sides of the tube crest. Heat transfer, heat transfer coefficient ratio and heat transfer ratio of the three tubes are presented as a function of cross sectional average suspension density. The results show that membrane tubes with one and two longitudinal fins have higher heat transfer than membrane tubes although they have lower heat transfer coefficient. In addition, numerical simulations were conducted to investigate the heat transfer behavior of the three tubes under the normal operating conditions of CFB boilers. It was found that the membrane tube with two longitudinal fins at 45° on both sides of the tube crest had the flattest inner wall tube heat flux profile. Moreover, in terms of temperature distribution in the tube material, it has the lowest profile.     

垂直管内三相流化床沸腾传热特性

研究了三相流化床沸腾传热的特性和影响传热系数的诸因素。在传热过程中，由于固体粒子的存在，强化了传热。以玻璃球粒子为固相的三相流化床沸腾传热系数，是相同条件下汽液两相流沸腾传热的二倍。以铜粒子为固相的三相流化床沸腾传热系数，是相同条件下汽液两相流沸腾传热系数的３倍。     

Heat transfer mechanism due to particle convection in circulating fluidized bed

The nature of heat transfer in circulating fluidized beds (CFBs) by particle convection was revealed from the experimental results of the particle flux in the lateral direction. As particles contact and rebound from the heat transfer surface because of their lateral motion, they establish a large local temperature gradient. Heat from particles is then transferred primarily through the gas to the heat transfer surface. The data of the lateral particle flux provide a basis to establish a model of heat transfer in CFBs.     

Particle collision behavior and heat transfer performance in a Na2SO4 circulating fluidized bed evaporator

The particle collision behavior and heat transfer performance are investigated to reveal the heat transfer enhancement and fouling prevention mechanism in a Na₂SO₄ circulating fluidized bed evaporator. The particle collision signals are analyzed with standard deviation by varying the amount of added particles ε (1%–3%), circulation flow velocity u (0.37–1.78 m·s^-1), and heat flux q (7.29–12.14 kW·m^-2). The results show that the enhancement factor reach up to 14.6% by adding polytetrafluoroethylene particles at ε = 3%, u = 1.78 m·s^-1, and q = 7.29 kW·m^-2. Both the standard deviation of the particle collision signal and enhancement factor increase with the increase in the amount of added particles. The standard deviation increases with the increase in circulation flow velocity; however, the enhancement factor initially decreases and then increases. The standard deviation slightly decreases with the increase in heat flux at low circulation flow velocity, but initially increases and then decreases at high circulation flow velocity. The enhancement factor decreases with the increase in heat flux. The enhancement factor in Na₂SO₄ solution is superior to that in water at high amount of added particles. The empirical correlation for heat transfer is established, and the model results agree well with the experimental data.     

Segregation and mixing effects in the riser of a circulating fluidized bed

Segregation and mixing effects of binary mixtures of particles having difference in sizes and densities were studied in 0.1016 m-diameter riser of a circulating fluidized bed at gas velocities between 2.01 and 4.681 m/s and solids circulation rate between 12.5 and 50 kg/m² s. Two groups of bed materials (three quartz sand-spent fcc catalyst mixtures with different initial mass % of sand and two coal-iron mixtures, one with almost same sizes but with different densities and the other having both different sizes and densities) were used. Using local axial mass % of heavier/coarser particles and their mean sizes the extent of segregation was evaluated. The influence of operating conditions like superficial gas velocity and solids circulation rate on segregation was examined and found that with their increase segregation effects generally tend to decrease and a uniform mixture conforming to initial composition of the mixture results. Using the data available in the literature and those of the present authors an empirical correlation to obtain the extent of segregation in CFBs has been proposed.     

Particle scale study of heat transfer in packed and bubbling fluidized beds

The approach of combined discrete particle simulation (DPS) and computational fluid dynamics (CFD), which has been increasingly applied to the modeling of particle‐fluid flow, is extended to study particle‐particle and particle‐fluid heat transfer in packed and bubbling fluidized beds at an individual particle scale. The development of this model is described first, involving three heat transfer mechanisms: fluid‐particle convection, particle‐particle conduction and particle radiation. The model is then validated by comparing the predicted results with those measured in the literature in terms of bed effective thermal conductivity and individual particle heat transfer characteristics. The contribution of each of the three heat transfer mechanisms is quantified and analyzed. The results confirm that under certain conditions, individual particle heat transfer coefficient (HTC) can be constant in a fluidized bed, independent of gas superficial velocities. However, the relationship between HTC and gas superficial velocity varies with flow conditions and material properties such as thermal conductivities. The effectiveness and possible limitation of the hot sphere approach recently used in the experimental studies of heat transfer in fluidized beds are discussed. The results show that the proposed model offers an effective method to elucidate the mechanisms governing the heat transfer in packed and bubbling fluidized beds at a particle scale. The need for further development in this area is also discussed. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Prediction of heat transfer to liquid-solid fluidized beds

Heat transfer coefficients to a liquid-solid fluidized bed in a cylindrical tube have been measured using water as liquid phase and three types of cylindrical steel particles, as well as glass, nickel, copper and lead spheres of different sizes as solid phase. The independent varaibles included heat flux, liquid velocity and particle physical properties. The experimental results as well as a data bank containing a large number of measured heat transfer coefficients for solid-liquid fluidization over a wide range of operational parameters have been compared with the predictions of most published correlations. A model for the prediction of heat transfer coefficients is proposed which predicts the present experimental data and the data of other investigators with good accuracy.     

水平液固循环流化床换热器传热性能评价

对管内插入Kenics静态混合器的水平液固循环流化床换热器进行实验研究,实验考察了静态混合器扭率、静态混合器安装方式、液体流速、颗粒体积分数对传热性能和流阻性能的影响,并运用综合强化传热性能评价指标（PEC）对其进行分析。实验发现,传热性能和阻力系数均随扭率增加而减小。当雷诺数在10000~45000之间时,扭率为1.5、2、2.5、3.5的Kenics静态混合器的PEC指标均大于1,说明了水平流化床换热器插入Kenics静态混合器能够改善传热。在雷诺数达到25000左右、Kenics静态混合器扭率为2.5、颗粒体积分数为4%时,水平流化床换热器的PEC指标最高达到1.18。当两个扭率均为2.5的Kenics静态混合器安装间距为200mm时,水平流化床换热器的PEC指标最高。     

振动流化床浸没水平管传热特性研究

为了深入研究振动流化床浸没水平管的传热特性,分别以沙子和玉米细颗粒作为实验物料,用水平探头测定了振动流化床中这2种床层颗粒与浸没水平管间的传热系数,分析了操作气速、振动频率、空气进口温度等因素对传热过程的影响。结果表明:在低气速下,振动是影响振动流化床中传热的主要因素,振动的引入可以明显改善流化作用,可以在低气速下得到较好的传热效果,同时达到节能的效果。通过分析实验结果,建立了振动流化床的传热关联式,模型计算值与实测值能较好吻合。研究结果可为干燥膏状物料时确定适宜的操作参数提供参考。     

Mean and fluctuating gas phase velocities inside a circulating fluidized bed

Gas phase velocities is an area in circulating fluidized beds (CFB) that has traditionally received little attention. The dynamics and motion of particles or clusters inside the bed has been the main focus of research. This is because particles dominate the fluid mechanics and heat transfer inside a CFB. However, gas phase motions also effect particle motion. Gas eddies or fluctuations can play an important role in transporting particles to and from the wall. They also help in providing a uniform temperature throughout the bed by promoting mixing. This paper deals with how particles effect the mean and fluctuating gas velocities throughout the cross-section of a riser.Gas velocities were measured inside a cold scale model CFB using a shielded hot wire anemometer. At the centerline, typical mean gas velocities were measured which were approximately twice the superficial gas velocity. These high velocities are likely caused by the negligible net gas upflow in the annulus region. The presence of many dense, downward flowing clusters in the annulus makes this a reasonable assumption.Previous work on gas phase turbulence in two phase flows has typically used either laser measurement techniques in very small diameter risers or in larger risers with very low particle concentration. The general results have shown that smaller particles, on the same order of magnitude as those typically used in CFB and FCC reactors, tend to damp out the gas phase fluctuations. This implies that gas phase motion behaves close to a laminar fashion. This present research measures gas phase fluctuations with typical particle concentrations inside a CFB (∼1-5%). The results indicate that at larger particle concentrations where clusters are formed, the gas phase fluctuations increase dramatically. This suggests that length scales based on cluster size, as opposed to particle size, should be used in estimating the increased levels of gas fluctuations caused by the solid phase. Hence, models which ignore the effect of clusters on the gas or which treat the gas phase as laminar like flow, yield a misleading picture of the flow dynamics inside a CFB riser.     

Numerical and experimental analyses of anti‐fouling and heat transfer in the heat exchanger with circulating fluidized bed

Fluidized bed type heat exchangers are known to increase the heat transfer and prevent the fouling. For proper design of circulating fluidized bed heat exchanger it is important to know the effect of design and operating parameters on the bed to the wall heat transfer coefficient. The numerical analysis by using CFX 11.0 commercial code was done for proper design of the heat exchanger. The present experimental studies were also conducted to investigate the effects of circulating solid particles on the characteristics of fluid flow, heat transfer, and cleaning effect in the fluidized bed vertical shell and tube type heat exchanger with counterflow, at which a variety of solid particles such as glass (3 mmØ), aluminum (2–3 mmØ), steel (2–2.5 mmØ), copper (2.5 mmØ), and sand (2–4 mmØ) were used in the fluidized bed with a smooth tube. Seven different solid particles have the same volume, and the effects of various parameters such as water flow rates, particle diameter, materials, and geometry were investigated. The present experimental and numerical results showed that the flow velocity range for collision of particles to the tube wall was higher with heavier density solid particles, and the increase in heat transfer was in the order of sand, copper, steel, aluminum, and glass. This behaviour might be attributed to the parameters such as surface roughness or particle heat capacity. Fouling examination using 25,500 ppm of ferric oxide (Fe₂O₃) revealed that the tube inside wall is cleaned by a mild and continuous scouring action of fluidized solid particles. The fluidized solid particles not only keep the surface clean, but they also breakup the boundary layer improving the heat transfer coefficient even at low‐fluid velocities.     

Wall-to-bed heat transfer in three-phase fluidized beds of low density particles

Bed-to-wall heat transfer was measured in three-phase fluidized beds under conditions typical of biochemical process applications. The thermal resistance of the fluidized bed, which was significant in the absence of gas, became negligible when gas was introduced. Decreasing the particle density at constant gas and liquid velocity increased the bed-to-wall heat transfer coefficient. Previously published heat transfer correlations were used and gave poor predictions of our data. A new correlation was developed which predicted very well all the heat transfer coefficient results in this paper.     

Fractal characteristics of gas-solids flow in a circulating fluidized bed

A fractal approach is adopted to describe the dynamic behavior of a circulating fluidized bed. Two times series, differential pressure fluctuations along the riser height and solids momentum fluctuations along the radial direction, are measured and analyzed in terms of fractal dimensions. The influences of operating conditions and axial/radial positions on the fractal dimension are discussed. Attempts are also made to interpret the flow structure in the bed in terms of the fractal dimension. It is found that fractal analysis can provide a useful tool for understanding the characteristics of gas-solids flow in circulating fluidized beds.     

Modeling of grid region heat transfer in a shallow gas-solid fluidized bed

Heat transfer coefficients between the bed and an immersed horizontal tube in the grid-region of a shallow gas-solid fluidized bed were experimentally and theoretically studied. Experiments were carried out in two fluidized bed columns with inside diameters of 88 and 137 mm, respectively. The fluidized particles tested were sand, limestone and glass beads. Experimental parameters also included particle size, superficial gas velocity, tube diameter, tube location and distributor design. A mechanistic model considering the contributions of jet phase, emulsion phase and dead phase was derived for estimating the grid-region heat transfer coefficients. Most of the model predictions were found to be within 25% of the experimentally observed data.