Abstract:

During the process of solids circulation establishment in circulating fluidized beds (CFBs), the solids circulation goes through a transition from an unstable state to a stable one. In this work, the stability of solids circulation in a CFB with limited bed inventory is studied by using a pressure transducer and a digital camera. The work reveals an interesting phenomenon, termed as oscillation circulation behavior of solids transportation between the riser and the downer through a solid control valve. Two important features are noted from the experimental results. Firstly, the gas-solid flow pattern in the riser and the downer is in transition between various fluidization regimes. Secondly, the pressure drop of the riser and the downer fluctuates periodically alternating between p_d>p_r and p_d<p_r. The calculation model of oscillation cycle is deduced by force analysis of oscillation process. Further study shows that solids oscillation behavior is controlled by the gas flow and valve opening. With a constant bed inventory, the oscillatory behavior is only initiated by the critical gas flow or valve opening. The oscillation cycle decreases with the decrease in gas flow but remains constant with varying solid control valve opening. The amount of solid transfer decreases with the decrease in gas flow and valve opening, while the pressure drop fluctuation decreases.

Key words: circulating fluidized bed, oscillation circulation, flow pattern, pressure drop

摘要：

颗粒的循环特性是循环流化床研究的重点及热点之一,在建立颗粒循环时,颗粒循环由非稳态向稳态过渡。本文在循环流化床实验装置中,采用摄像法和压力检测法研究了不稳定颗粒循环条件下颗粒流动结构和各段压降的变化情况。首次发现了不稳定颗粒循环状态下的颗粒振荡循环现象及其具有的两个特征:下降段、提升段气固流动结构呈周期性变化;下降段压降p_d和提升段压降p_r周期性波动且p_d>p_r和p_d<p_r交替出现。进一步对颗粒振荡循环过程进行受力分析,建立了颗粒振荡周期的计算公式。研究还发现在颗粒藏量一定的条件下,循环气流量或颗粒循环段阀门开度大于某一临界值时才能诱发颗粒振荡循环;颗粒振荡循环周期随着循环气流量的增大而增大,与阀门开度无关。研究结果为动态颗粒循环建立过程的研究奠定了基础,有助于快速地建立颗粒的稳定循环。

关键词: 循环流化床, 颗粒振荡循环, 流动结构, 压降