Experimental and Theoretical Investigation on the Mechanism of Transient Bubble Images in Fluidized-Bed Combustors Systematic Interpretation and Analysis

The transient bubble behavior was experimentally investigated to fundamentally understand the transient motion of gas bubbles in freely bubbling fluidized beds, simulating the FBC systems. Due to the oversimplified traditional two phase flow model concepts developed and most of the related numerous experimental works from 1960s to 1980s, the transient bubble behaviors could not have received due attention, resulting in some fundamental misconception of the bubbling phenomena particularly in coarse fluidized beds. For the improvement of the design and operation of the FBC systems, the insight into the intrinsic transient bubbling phenomena in freely bubbling fluidized beds is of vital importance. We have found several basic new bubblinc! mechanisms in this work experimentally, and been published in fluidized bed, the recorded by videos, As the results facts were found: (1) Transient some of them have not, to our best knowledge, past literature. Using the two dimensional images of transient bubbling behavior were and processed and analyzed by computers. of experiments, the following new experimental bubbles change and fluctuate their size and shape over very short time intervals ( on the order of 30 milliseconds) . (2) Bubble disappearance and reappearance occurred in the emulsion phase in addition to the known phenomena of coalescence and splitting. The bubble interaction occurred between the bubbles and adjacent emulsion phase and also among the transient bubbles. (3) Bubblers velocity fluctuated significantly, e.g., 0.6 to 3.0 m/s. (4) Under one single specific fluidization condition, two different fluidization patterns appeared to occur randomly shifting from one pattern to the other or vice verse. (5) The erosion rates of in-bed tubes at ambient and elevated temperature could be predicted using material property data and transient behavior of bubbles. By introducing a new quantitative criterion which we call a gas stress index in the emulsion phase, the comparison of the fluidization quality between two and three dimensional fluidized beds was accomplished. We found reasonable correspondence between the two beds, and concluded that the new findings of transient bubble behavior should hold true for both types of fluidized beds. The conclusions of this work are in good agreement with the conclusion of the pioneering work of DOE/METC on the transient bubbling behavior in three dimensional coarse fluidized beds using the capacitance measurement method.