Study On The Mechanism Of Coaxial Air-blast Atomization

Using high speed camera, laser particle size analyzer and image processing technology, the breakup morphology and mechanism of Newtonian fluid and coal water slurry (CWS) have been studied theoretically and experimentally on the basis of CWS gasification. The main contents and results were summarized as follows:1. The effect of coaxial air-blast nozzle exit size on primary atomization has been investigated with water-air system. Correlations for the breakup regime limit of air-blast cylindrical jet at different nozzle exit area ratios were obtained, and the breakup regime similarity of liquid jet in coaxial air flow, liquid jet in cross air flow and secondary atomization was studied. The air-blast annular sheet breakup could be classified into three regimes, which were shell breakup, cellular breakup and fiber breakup. The morphological property, atomization regime range and mechanism were studied, respectively. In order to have an overview of the different breakup models taking place over the wide range, these breakup regimes were categorized in nondimensional numbers map.2. In secondary atomization, the correlation between bag number and Rayleigh-Taylor (R-T) wave number was studied. Based on morphological classification, breakup modes of secondary atomization were identified at present, which was somewhat arbitrary. So R-T wave number in the region of maximum cross stream dimension (NRT) was suggested as the new criterion instead of Weber number. The atomization regime ranges and prediction expressions were obtained. Bag breakup, bag-stamen breakup, and dual-bag breakup could all be described as general bag breakup based on the same breakup mechanism. A simplified theoretical formula based on the R-T instability was studied to estimate the critical Weber number at different Ohnesorge number. In bag breakup, correlations on mean fragment size, ring fragment mean size and ring fragment number were obtained. Fragment size distribution based on number formed from ring was a gamma distribution. Fragment size distribution based on number formed from whole drop was an exponential distribution. In bag-stamen breakup, the deformation and breakup characters were investigated. And the corresponding correlations were also obtained.3. The non-dimensional numbers in CWS atomization were suggested. In primary atomization based on morphology, the breakup regimes of CWS cylindrical jet can be termed different modes:Rayleigh-type breakup, fiber-type breakup and atomization. The dimensionless CWS jet breakup length was studied and correlated. A simplified formula was studied to estimate the oscillation frequency of CWS jet. In secondary atomization, deformation and breakup regimes of CWS drops could be termed some different modes: deformation, multimode breakup (including two sub-modes:hole breakup and. tensile breakup), and shear breakup. Correlations on the ranges of breakup modes were also obtained. Total breakup time was studied and correlated, which increased with Ohnesorge number. The breakup time of CWS was smaller than viscous liquid.4. Using laser particle size analyzer, the drop size character of air-blast coaxial nozzle (2 channels and 3 channels) CWS atomization was studied. Influence of fluid velocity on CWS atomization performance was analyzed. Water was selected as the comparison; experimental data showed that air velocity had greater influence on CWS than water; liquid velocity had greater influence on water than CWS. Photograph of CWS atomization were also obtained using high speed camera. Irregular big fragments were often found in high viscous CWS atomization, which would great increase fragment average diameter and make the measured value of laser particle size analyzer small.