Heat transfer characteristics in a pulsating fluidized bed in relation to bubble characteristics

A pulsating fluidized bed is operated with two sequential durations designated as an on‐period with injecting fluidization gas and an off‐period without it. The heat transfer coefficient between a vertically immersed heater and bed in a pulsating fluidized bed is measured under various pulse cycles and fluidized particles. The obtained results are compared with those in a normal fluidized bed with continuous fluidization air injection. The relationship between heat transfer coefficients and bubble characteristics, evaluated using a digital video camera, has also been investigated. For certain fluidized particles and operating pulse cycles, the fluidization of particles and the increment of heat transfer coefficients can be obtained under a mean air velocity based on a pulse cycle duration smaller than the minimum fluidization air velocity in a normal fluidized bed. Under the pulse cycles where a static bed through the whole bed is formed in the off‐period duration, the improved heat transfer rate over that in a normal fluidized bed can be measured. This may be attributed to large bubble formation. As heat transfer in the pulsating fluidized bed is obstructed with increasing time to keep a static bed due to the excessive off‐period duration, it is indicated that there is an optimum off‐period duration based on the heat transfer rate. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(4): 307–319, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10038     

鼓泡流化床气固两相流动特性的数值模拟

本文基于气固两相欧拉-欧拉双流体模型,对多孔布风鼓泡流化床内气固两相流流动特性进行了数值模拟,研究了床内压力分布,气泡的运动行为,以及气相和颗粒相速度的分布情况,并将模拟结果与相应实验数据进行比较。结果表明所用模型能较好的预测流化床内气固两相流的流动特性,模拟结果与实验结果吻合较好。     

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.     

Flow pattern identification of fluidized beds using ECT

IntroduchonPeople's understanding of the solids distribution inthe bottom zone of ~ating lluldized beds (CFB) isvery limited. There are divided assUInptions for thebottom aeon Of CFBs, e.g. turbulent flow and fastfiuldization (such as' ~ and the']) as oPPosed tobubbling fiuldized bed (e.g. Werther and Hirschberglz]).Also, different models ealst for the reactions in thisaeon, such as Plug flow model (Van Sw~['], Wen['],Fane and benal), anally dispersed Plug flow of gas(Avidant'], Edward…     

Studies on the operation of loop-seal in circulating fluidized bed boilers

Loop-seal, considered heart of a circulating fluidized bed (CFB), returns solids captured by cyclone to the base of the riser while preventing direct flow of gas from high pressure riser to the low-pressure cyclone. This non-mechanical valve is used in thousands of CFB systems yet only a limited information is available on its working. Present research studies the flow of solids through a loop-seal and the effect of several design and operating parameters on it. This experimental study was conducted in a loop-seal 110 mm × 448 mm × 400 mm high connected to a riser 152 mm diameter and 5180 mm high. Majority of the experiments was done with 171 μm sand though several other size and type of solids were studied for their flowability. It was found that for the solids to flow through the loop-seal a minimum level of aeration, in excess of that required for minimum fluidization was required. The length of the horizontal passage connecting the supply and recycle chambers of the loop-seal had an important effect on the solids flow. For example, the minimum aeration for the onset of solids flow increases with increase in this length. The pressure drop per unit length across the passage also increased with the passage length. The air fed into the supply chamber is split such that the superficial air velocity in the supply chamber (or the standpipe) remained below the minimum fluidization velocity of the particles while the remaining air conveys solids through the horizontal passage. Present study showed that the solids flowing through the horizontal passage are neither fully fluidized nor moving packed or suspended solids. It moves as a segregated flow of solids driven by hydrostatic pressure and fluid drag.     

谷物流态化干燥过程的节能研究

为了降低多层流化床干燥器的能耗,对空塔压降随气体分布板开孔率的变化规律、空塔压降随风速的变化规律、进料时的流化操作范围与各床层及总床层的压降随风速的变化规律进行了冷态试验研究。为稻谷的流化床干燥器选择了合适的开孔率、层数并得到了合适的流化操作范围。     

Heat transfer between a fluidized bed and an immersed vertical U-tube

The design details of a 0.254 m stainless steel cylindrical fluidized bed pilot plant facility, whose fabrication, installation and testing have been recently completed, are described. It primarily consists of a fluidized-bed reactor, fluidizing air-supply system, solids feeder, off-gas cleaning and exhaust system, and cooling water-supply system for heat-transfer tubes provided in the bed and in the freeboard sections. The plant is operated at ambient pressure in the temperature range 300–600 K, both in the batch and continuous modes for solids feed. Bed-pressure drop measurements as a function of fluidizing velocity for two different bed heights reveal that the quality of fluidization is good. Similar experiments have been conducted with the continuous solids feed. The heat-transfer coefficient between the bed and an immersed stainless steel U-tube is measured as a function of fluidizing air velocity at five different temperatures. The effects on bed-to-tube heat-transfer rate of solids feed rate, bed height, air-flow rate, and bed temperature are examined. All of these observed variations are interpreted in terms of the solids mixing and bubble mechanics in the bed.     

双支腿循环流化床炉膛翻床条件与防止控制试验研究

在双支腿循环流化床冷态试验台上进行了床料平衡试验,得到不同流化参数下风量和床压的动态曲线,基于并行流道阻力特性分析,提出双支腿炉膛翻床条件的预测方法,并模拟了控制逻辑对翻床的控制效果.结果表明:当流化风量大于临界翻床风量时,双支腿炉膛处于自平衡区,两侧炉膛的流化风量和床压的瞬时偏差能够相互纠正,两侧床料处于动态平衡;当流化风量小于临界翻床风量时,将发生翻床现象;临界翻床风量随着静止床料高度的增加而减小;采用调整风门开度的方法解决翻床问题存在局限性,床料平衡的首要策略是使双支腿系统处于自平衡状态运行.     

隔板式内循环流化床的流动特性研究

以石英砂和稻壳为实验床料,在隔板式内循环流化床气化炉冷态实验装置上对颗粒的内循环流动特性进行了研究,考察了高速区和低速区的流化速度、结构尺寸和侧风量等对颗粒内循环流动的影响.结果表明:在保持低速区流化速度一定的条件下,随着高速区风速增大,颗粒循环量先增大后减小;流化速度不变的条件下,颗粒循环量随孔口和侧风量的增大而增加,但增加趋势逐渐变缓.实验给出了合理的运行设计参数.通过实验数据回归,得到了石英砂和稻壳通过隔板式内循环流化床孔口的颗粒循环量关联式,计算结果与实验值误差分别小于6%和14%,能较好地预测孔口颗粒流动.     

循环流化床脱硫塔直_旋流复合流化下的两相流场试验研究

循环流化床脱硫装置的文丘里管直流流化速度随锅炉负荷的变化而变化,这会影响脱硫效率。本文提出了适应锅炉负荷变化的直／旋流复合流化方式,并用PDA测量系统对这种流化方式的气固两相流场进行测试,得到了循环流化床内旋流风率和假想切圆半径改变时气固切向速度和浓度分布。试验表明,复合流化循环流化床的切向速度随着半径增大而升高,气固切向滑移速度比直流流化增大,脱硫塔内的浓度增加,内循环增强,脱硫效率随之提高。     

双循环流化床冷态实验研究

由主循环流化床(主床)和副循环流化床(副床)组成的双循环流化床冷态实验系统中,测量了不同工况下的压力分布以及主床和副床的物料循环率,分析了装料量、主床一次风流化速度和副床二次风流化速度对压力分布以及物料循环率的影响,为双循环流化床的工艺设计提供实验依据.     

非均匀布风流化床的DEM模拟

对二维非均匀布风流化床内的颗粒运动进行了数值模拟,用欧拉方法处理气相场的同时用拉格朗日方法处理离散颗粒场,直接跟踪颗粒场中的每个颗粒。模拟结果表明,非均匀布风流化床内存在颗粒的内循环运动,因此颗粒的混合特性优于均匀布风流化床。     

流态化浓相输粉及计量的试验研究

本项试验研究工作实现了流态化浓相的稳定输送,试验测定了在常压及增压情况下流化床给粉流量随床层压降变化的规律,成功地解决了煤粉难以正常流化的问题。叙述了采用流化方法进行计量的基本原理,介绍了自动计量系统,并对计量精度进行了检验。     

Effect of operating parameters on sulphur capture in a pressurized circulating fluidized bed combustor

Experiments are conducted to investigate the effect of system pressure, Ca/S ratio and primary air velocity on sulphur capture in a pressurized circulating fluidized bed (PCFB) combustor. The pressure inside the PCFB combustor is varied from 200 to 700 kPa. The Ca/S ratio is varied from 1.6 to 3.0. The primary air velocity ranges from 3 to 7 ms^?1. The bed temperature is maintained at 750°C. The sulphur capture increases with system pressure in the present range of experimental investigations. The sulphur capture also increases with Ca/S ratio up to a certain ratio and then shows a decreasing trend for the given operating conditions. A semi‐empirical model is developed for explaining the sulphur capture mechanism in the pressurized circulating fluidized bed combustor under batch combustion conditions. The experimental data are validated with the model predictions and a reasonable agreement has been observed. Copyright © 2002 John Wiley & Sons, Ltd.     

下行床内颗粒浓度的电容层析成像测量的研究

利用电容层析成像(electrical capacitance tomography,ECT),以非侵入的方式,对下行床内气固流动情况进行测量研究。针对下行床内固体颗粒浓度低的特点,采用了SIRT图像重建算法,得到了高质量的ECT图像。在颗粒流量和流化风量变化的情况下测量了不同截面固体颗粒体积份额平均值的变化,得到了下行床内气固流动规律。实现了快速的下行床内粉粒体的ECT在线可视化测量,具有较大的工程和学术价值。     

Thermal aspects of a circulating fluidized bed with air staging

In this study, effects of air staging on wall‐to‐bed heat transfer were investigated in a laboratory scale circulating fluidized bed (0.23 m ID, 7.6 m high). The bed was operated under ambient conditions with silica sand particles (d_p=89 µm, ρ_p=2650 kg m^?3). Two different designs of secondary air (SA) injectors were used for air staging: radial and tangential. Bed‐to‐wall heat transfer measurements were carried out at three elevations above the SA injection port. The results indicate that similar to non‐SA operation, the heat transfer with air staging depends strongly on the cross‐sectional average suspension density. Tangential secondary injection was found to increase the bed‐to‐wall heat transfer above the SA injection port significantly due to increased suspension density compared with non‐SA operation. Copyright © 2005 John Wiley & Sons, Ltd.     

Wall-to-bed heat transfer in circulating fluidized beds

Heat Transfer from the wall of a circulating fluidized bed to the fast bed suspension has been investigated for several materials. The range of investigation includes dense and dilute phase fast fluidization and pneumatic transport. The overall heat transfer coefficient was found to be a function mainly of cross-sectional average suspension density. Effects of superficial velocity and solids mass flux were obscured by their interrelationship to the suspension density. Two models from the literature are evaluated using present and published data.     

Mixing characteristics of light paper pellets in a sand fluidized bed

The mixing behavior of light paper pellets in a sand fluidized bed is investigated for varying proportions of paper pellets (0–20 mass%) over a range of superficial air velocities (0–1.5 m/s) at ambient conditions. It is found that a fluidization number greater than 2 is essential to achieve good mixing as long as the proportion of paper pellets in the bed is up to about 10 mass%. For 20 mass% of paper pellets in the bed, mixing did not occur and paper pellets preferentially collected in layers. Bed voidage and minimum fluidization velocities for mixed beds of sand and paper pellets are reported. These studies provide the supporting data needed for the co-combustion of paper pellets in a hot-sand, fluidized bed.     

Energy analysis in fluidized‐bed drying of large wet particles

Energy analysis of a fluidized‐bed drying system is undertaken to optimize the fluidized‐bed drying conditions for large wet particles (Group D) using energy models. Three critical factors; the inlet air temperature, the fluidization velocity, and the initial moisture contents of the material (e.g., wheat) are studied to determine their effects on the overall energy efficiency to optimize the fluidized bed drying process. In order to verify the model, different experimental data sets for wheat material taken from the literature are used. The results show that the energy efficiencies of the fluidized‐bed dryer decrease with increasing drying time and become the lowest at the end of the drying process. It is observed that the inlet air temperature has an important effect on energy efficiency for the material where the diffusion coefficient depends on both the temperature and the moisture content of the particle. Furthermore, the energy efficiencies showed higher values for particles with high initial moisture content while the effect of gas velocity varied depending on the material properties. A good agreement is achieved between the model predictions and the available experimental results. Copyright © 2002 John Wiley & Sons, Ltd.     

关于循环流化床锅炉冷态空气动力场试验的研究

以某480 t/h循环流化床锅炉的空气动力场试验为例,详细介绍循环流化床锅炉空气动力场试验目的、试验项目和试验方法,并得出结论和建议。通过测定循环流化床锅炉锅炉布风板阻力特性和流化特性,得出锅炉的临界流化风量,为锅炉的热态运行提供必要的依据,保证循环流化床锅炉的安全、经济运行。