Study On The Flow Characteristics Of Dense Annular Granular Jet Exposed To A Central Air Round Jet

Using high-speed camera, hot-wire anemometer, pressure measurement system and smoke flow visualization, the flow behaviors of an annular granular jet dispersed by a central air round jet have been studied systematically on the basis of pulverized coal gasification. Particle bubbles arc found to be foed periodically, and a criterion is proposed to determine whether the bubble appears. In addition, the effect of different disturbances of gas phase on the bubbling characteristics of dense gas-particle two-phase flow has been investigated experimentally. The main contents and results are summarized as follows:1. An annular granular jet surrounding an air jet at its core is studied experimentally using high-speed digital photography. The experimental results show that particle bubbles in a periodic manner are formed whether the central air is swirling or not. This flow feature is induced by the intense interaction between the central air jet and the annular granular jet. The interaction between the two phases is mainly intensified by higher superficial air jet velocity and the addition of swirl to the central air jet. In comparison with the swirl, the superficial air jet velocity seriously influences the appearance of the bubbles. The bubbling frequency, bubble size, bubble shape and bubble growth rate are obtained by analyzing a large number of images. The dimensions ratio of the initial bubble suggests that the bubble is not exactly spherical but rather shaped like a flattened ellipsoid. In addition, the dispersion angle of granular jet is found to be mainly governed by the radial growth rate of the bubble, which reveals that, in the near field, the formation of the bubble has a significant influence on particle dispersion.2. The effect of particle mass flow rate and annular channel thickness on the formation and characteristics of the bubbles is investigated experimentally in detail. The experimental results demonstrate that the particle mass flow rate and the annular channel thickness are both key factors for the bubble-formation process. A criterion, i.e., M≥0.61is proposed to determine the emergence of the bubbles. This bubbling criterion is applicable to the cases with convergent nozzles and straight nozzles with swirl and reveals that the relation between Mgand Mp is crucial to whether the bubble is formed. By analyzing a large number of images, the bubble size, bubble growth rates and bubbling frequency are obtained and well predicted by a series of dimensionless correlations. The radial growth rate of the bubble is still governed by the superficial air jet velocity. In addition, the radial velocity difference between the gas and particle phase is confirmed to be the main cause of bubbling.3. Experimental studies are undertaken on the bubbling characteristics of dense gas-particle two-phase flow with disturbance of air source and acoustic excitation. The experimental results show that the introductions of disturbance of air source and acoustic excitation can both give rise to a characteristic frequency of gas phase which is identical to the disturbance/excitation frequency, and then have an influence on the development of gas phase flow field. In addition, the above two kinds of disturbances can induce the formation of periodic bubbles at lower superficial air jet velocity (ugo), and meanwhile dominate the growth of bubbles within a certain range of ugo. In such a case, the bubbling frequency is also equal to the disturbance/excitation frequency. However, with the increase of ugo, the dominant roles of disturbances of gas phase in the formation of bubbles will be highly weakened, but their influences still exist. Comparing the experimental results for different nozzle geometry, the case with straight nozzles is found to have a biggest control range of acoustic excitation. Generally, the increase of excitation level can promote the growth of bubbles along the radial direction, and thus improve the dispersion of particles.If the effect of acoustic excitation is considered only, the increase of excitation frequency can suppress the growth of bubbles along both directions, and thus discourage the particle dispersion.