气体流量和喷嘴直径对鼓泡塔内气泡大小和形状分布的影响

通过高速摄影结合图像处理算法,研究气体流量和喷嘴直径对气泡尺寸和纵横比分布的影响。结果表明,在所有喷嘴直径下,气泡当量直径的概率密度呈现双峰分布,其中第一个峰值接近直径为1.5~2 mm的小气泡区域,另一个峰值位于较大的气泡区域。将气体流量从0.1 L/min增加到0.2 L/min会导致大气泡的概率密度更高,表明气泡聚并现象普遍存在。当气体流量增加至1.2L/min时,峰值对应的气泡直径变小,并且气泡破裂占主导地位。对于直径为1~2 mm的气泡,形状受气体流量的影响较小,而3~9 mm的气泡在所有气体流量下气泡纵横比概率密度分布的峰值均位于纵横比E=0.5处。Iguchi和Chihara提出的模型能更好地预测气泡脱离直径随气体流量的增大的变化。

The pseudo-homogeneous flow regime in large-scale bubble columns:experimental benchmark and computational fluid dynamics modeling

A precise prediction of the fluid dynamics in bubble columns is of fundamental importance to correctly design“industrial-scale”reactors.It is known that the fluid dynamics in bubble columns is related to the prevailing bubble size distribution existing in the systems.In this respect,multiphase computational fluid dynamic simulations,in the Eulerian multi-fluid framework,are able to predict the local bubble size distributions and,thus,the global fluid dynamics from the fluid flow conditions and by applying modeling closured.In particular,in in“industrial-scale”reactors,owing to the large gas sparger openings,the“pseudo-homogeneous”flow regimedcharacterized by a wide spectrum of bubble sizesdis typically observed.Unfortunately,reliable predictions of the“pseudo-homogeneous”flow regime are limited up to now:one important drawback concerns the selection of appropriate models for the coalescence and break-up.A set of closure relations was collected at the Helmholtz-Zentrum Dresden-Rossendorf that represents the best available knowledge.Recently,the authors have extended the validation of this set of closure relations to the“pseudo-homogeneous”flow regime,by comparing the numerical predictions to a comprehensive experimental dataset(gas holdup,bubble size distributions and local flow measurements).Unfortunately,the previous study suffers from some limitations;in particular,in the previous experimental dataset,the bubble size distributions concerned only one axial position and a detailed characterization of the gas sparger was missing.This study contributes to the existing discussion and proposed a step ahead in the study of the“pseudo-homogenous”flow regime.To this end,we propose an experimental study,to improve the comprehensive dataset previously obtained.The novel datasetdobtained for two gas velocitiesdconcerns bubble size distributions at different axial and radial positions and a precise characterization of the gas sparger.The comprehensive bubble size distribution dataset may serve as basis to improve the coalescence and break-up closures;conversely,the precise characterization of the gas sparger served as an improved input to the numerical simulations.The numerical results,with two different lift force implementations,have been compared with the whole dataset and have been critically analyzed.Reasons for the discrepancies between the numerical results and the experimental data have been identified and may serve as basis for future studies.