【作者】 叶科；

【导师】 岳光溪；

【作者基本信息】 清华大学 ， 动力工程及工程热物理， 2006， 硕士

【摘要】 受我国燃用煤种和现有煤粉制备系统的限制,风水联合冷渣器经常遇到一风室流化恶化结焦的问题。将风水联合冷渣器传统的板式布风装置改为管式布风,可以有效避免风室结焦。风水联合冷渣器风室处于鼓泡流化状态,本文将对鼓泡流态化下混合颗粒的流化特性进行研究。本文搭建截面尺寸125×250mm,高1200mm的冷态实验台。实验选取粒径分别为150-250μm和1-2mm的两种粒径电厂灰颗粒,研究混合颗粒的初始流化风速和大小颗粒的混合与分层特性。实验结果表明,双组分混合颗粒的初始流化风速介于大小颗粒初始流化风速之间,更接近于小粒径颗粒,并随大颗粒质量分数的增加而增大,增大的趋势先慢后快;混合物料床层膨胀比随流化风速的提高而增大,增大的趋势先快后慢,实验结果与经验公式计算结果有较大误差;二元组分颗粒之间的混合程度随大颗粒组分和流化风速的增大而增强;颗粒的分层受时间的影响,当加入物料大小颗粒完全分离时,颗粒之间的混合主要发生在前10s,30s后颗粒之间的混合情况与充分流化时基本相同。实验条件有限,本文利用Fluent软件对二元组分颗粒的流化状态数值模拟,弥补某些工况的不足,同时也是对实验结果的验证。数值模拟发现,提高流化风速有利于颗粒的混合,二元组分颗粒之间的混合与分层速度很快,前10s内已基本完成,模拟结果与实验结果吻合。本文还对同一流化风速不同粒径二元组分颗粒的混合情况进行了比较,计算发现两种粒径颗粒的粒径越接近,混合情况越好。最后,通过分析床内颗粒速度分布,发现鼓泡床内颗粒的运动方式在轴向为中心处向上、贴壁处向下的内循环流动,而颗粒在横截面是中心处向壁面运动的。更多还原

【Abstract】 Fluidization deterioration and agglomeration often occur in the first wind box of the wind-water cooler used in the circulating fluidized bed (CFB) boilers in China, because of the limitations of coal quality and coal preparation systems. Replacing the traditional plate type distributor of the cooler with the pipe type one is deemed as an effective way to improve the fluidization and avoid agglomeration. Fluidization characteristics, especially the particle mixing in the new type cooler that operating in the bubbling bed condition is important for the cooler design and operation, and hereby has been studied in the thesis.A cold experimental rig was built with a section area of 125×250 mm and 1200mm in height. In the experiments, ash particles with diameters of 150-250μm and 1-2mm are chosen as the bed material to study the minimum fluidization velocity, mixing and segregation characteristics of particles.The experimental results showed that the minimum fluidization velocity of mixed particles was between those of the large and small particles, closer to that of smaller particles. The velocity increased with the increasing of mass faction of the larger particles in the bed material. As the fraction of larger particles increased, the velocity increased gradually as the fraction was of a small percentage, and then increased rapidly as the fraction exceeded a certain value. It was also found that the bed expansion ratio of bed material increased with the superficial velocity, first rapidly and then gradually. There is a noticeable discrepancy between the results obtained from present experiments and the empirical formula given in the literatures. In addition, the mixing intensity between two-component particles increased with the superficial velocity and the mass fraction of larger particles. Furthermore, the segregation of particles showed a spatial-dependent behavior. When larger and smaller particles were completely separated in the beginning of the experiment, the mixing of particles mainly completed in the first 10s, and segregation phenomena was hardly detected after 30s and the bed turned into full fluidization.Numerical simulation of fluidization with the two-component particles was conducted by using Fluent software to verify the experimental results and to make up for deficiency of some operating modes. The results of numerical simulation show that the increase of air velocity is in favor of the mixing of particles. The mixing and segregation of particles complete in a short period, e. g., in the first 10s. The results are in accordance with experimental data. For a fixed superficial velocity, simulation results showed that the uniformity of the mixing increases as the size deviation of two-component particles decreases. Moreover, the simulation found that the internal recirculation exists in the bubbling bed, with particles move upwards in center and downwards near the wall. In the cross section of the bed, particles move toward wall from the center region.更多还原