Research On Characteristic Of The Particle Concentration Distribution And Flow At The Exit Zone In Circulating Fluidized Bed Boilers

The structure of gas-solid two-phase flow in Circulating Fluidized Bed Boilers(CFBB) is extraordinarily complexity and distinctive. A detailed and elaborativeresearch on it is important to understanding the characteristic of combustion andheat-transfer in CFBB, as well as to its correct designing, operation and controlling.However , the correlative fundamental researches lagged behind the development ofthe combustion practice. It's difficult to find some unified or complete theory todescribe the flow patterns inside a CFBB, even some achievements and viewpointsare not correct, for example, the understanding of the particle concentrationdistribution and flow at the top region and the exit zone. At present, the common viewpoints of the flow structure in CFBB is that thebottom part composed of coarse particles forms a dense phase and in condition ofbubbling fluidized state. As to the top region, most people deem it a dilute regionand in the condition of pneumatic transport. The single particle can't form clustersand inter-circulation will never appear. Meanwhile, some works focusing on the exiteffect of CFBB conclude that particles will collide will the top wall and thusconcentrate at this region, engendering a C-shaped concentration distribution in mostcircumstances. Through analyzing and investigating of the gas-solid flowcharacteristics in this region, the thesis thinks that these two kinds of prevailingknowledge on the exit zone is disparity with the practices and requires furtherdiscussion. This work surveys and focuses on the characteristic of gas-solid two-phase flowat the exit region. According to the related measurement data and practice, the authorproposed a new model of particles concentration distribution and clusters movementat the exit region in the CFBB. Conclude that the practical height of exit is higherthan the length of particle deceleration length, the gravity effect on particles will bedramatic. Due to the direction changing of airflow, the velocity difference betweendifferent size particles emerged. During the process of rising and falling, theinteractions between particles leads to local concentration, forming a downward flowof clusters at the region beneath the exit and near the back wall. The particles displaya characteristic of upper-dense and bottom-dilute distribution. The gas-solid flow pattern at the exit region is very important to understandingthe whole circulation in CFBB, but unfortunately there are few works had done onthis problem till now. In order to demonstrate the validity of the novel model on thegas-solid flow structure at this region, author applies both numerical simulation andexperimental methods. The results confirmed our conception of the particleconcentration distribution and flow characteristic at the exit zone.