Study On Liquid Multiscale Circulation Structure In A Bubble Column

As one of the most fundamental and important fluid dynamics properties of a bubble column, liquid circulation has been considered only as a phenomenon caused by the nonuniform gas holdup radial profile. It is primarily responsible for liquid phase mixing, heat and mass transfer. During the last 40 years, a large number of investigations on liquid circulation in bubble columns have been reported which involve mostly tall columns under both bubbly and churn turbulent flow regimes. These studies have mainly considered the average flow pattern practically in the form of a global circulation cell which comprises an upward flow in the column core and a downward flow near the wall. Unfortunately, the precise circulation flow structure of liquid in a bubble column is still not fully understood, mainly due to the complexity of flow characteristic in the column and the difficulties encountered in performing of experimental investigations.The liquid circulation flow structures developed so far are mostly based on liquid velocity profiles, very few studies have been reported based upon tracing the liquid flowing trajectory. The present study attempts to identify the liquid multiscale circulation structure in a bubble column of 600 cm in height and 50 cm in diameter by tracing the liquid flowing trajectory, which has not been reported up to now. Since circulation flow means periodicity, so if there is only a macroscale circulation vortex which encompasses nearly the whole column or only mesoscale circulation vortexes with the size of each circulation vortex being equal to the column diameter, the liquid tracer injected into the system at some position along the column will be detected next time in a periodic manner. According to this train of thought, we develop a multiple-point tracer injection and detection device which can help us to inject the liquid tracer into the system at the various positions along the axis of the column and to track the tracer flowing trajectory at different positions along the column. With the aid of this equipment, a series of experiments will be conducted in detail to investigate the liquid flow behavior with the tracer injected at the top, middle and bottom of the column. Four small gas-liquid separators are used to eliminate the bad effect of gas bubbles in the sampling liquid on the measuring concentration signals.In order to interpret the multiscale characteristic of liquid circulation structure from the reverse side, we introduce several draft tubes with different length into the column center whose diameter is about 0.7 times of the main column diameter. In consideration of the local periodic and oscillating characteristics of response signals obtained in the experiments, a wavelet multiresolution decomposition on the signals will be employed to help us to further understand the liquid multiscale circulation structure in the column from time and frequency domains.The experimental results indicate that the liquid has multiscale circulation vortexes structure in the column. The vortexes structure comprises of macroscale circulation vortex which encompasses the whole column, microscale circulation vortex in which fluid mixing occurs on the microscopic level, and mesoscale circulation vortex whose size distribution is between the above two. In other words, the bubble column is filled with various circulation vortexes large and small. The macroscale circulation vortex corresponding to the bulk motion of liquid phase may demonstrate certain regularity over a period of time. From the viewpoint of time averaging, such macroscale circulation flow structure generally comprises an upward flow in the core and a downward flow near the walls of the column where the driving force for the flow is the density difference caused by maldistribution of gas holdups. This kind of vortex structure is significant in the global convective motion of liquid phase and is responsible for the high level of backmixing in the liquid phase. The microscale circulation vortexes are everywhere in the column, with the size up to microscopic level. They are the major cause of long tail of response signals in tracer experiments. They have the characteristics of certain randomness and fluctuation due to the agitation of gas bubbles and entrainment of liquid in the wakes of bubbles. In addition, these microscale circulation vortexes swing and rotate all the time in the column in a chaotic manner and are responsible for energy dissipation of the system. Moreover, these high frequency microscale fluctuation circulation vortexes make a great contribution to the micro-mixing process and also play a vital part in interphase mass and heat transfer processes. The structures in the form of mesoscale circulation vortexes have no constant shape and dimensions but have been in a state of change. Furthermore, the infinite microscale fluctuation vortexes and the irregular mesoscale circulation vortexes are dynamically superimposed on the macroscale regular circulation vortex structure. The large scale circulation vortexes contain small scale circulation vortexes which also include smaller scale vortexes, and there are interactions between these circulation vortexes of different scales which make the gas liquid flow structure in the bubble column rather complex.