| The phenomenon of jet flow caused by bubble collapse at free surface is widely existed in industry and daily life,such as the droplet entrainment of steam generator,aroma diffusion of sparkling wine,etc.Therefore,it is necessary to realize controllable jet flow.The present researches mainly focus on jet phenomenon after bubble collapse at free surface with enough high liquid level.However,when the liquid level is lower enough,bubble collapse may occur before leaving the orifice.Under such limited conditions,the influence factors,mechanisms and characterizations of bubble jet flow is rarely reported.This paper just focuses on this issue.In this paper,a visualization experimental system was built.A high speed camera was used to observe the phenomenon of jet flow produced by bubble collapse at different liquid level,orifice diameter and inlet gas flow rate.The ranges of liquid level,orifice diameter and inlet gas flow rate were 0.5-12.0 mm,0.15-1.20 mm and 2.5-20.0 mL/min,respectively.In order to record the complete experiment phenomenon of bubble growth and jet flow,the high-speed camera was placed at several inclination angles and the frequency was adjusted to 4000 fps.The experimental results show that:(1)Whether the bubble collapse can produce jet droplets is closely related to the liquid level and orifice diameter,but not to the inlet gas flow rate.(2)Under the same orifice diameter and inlet gas velocity,with the increase of liquid level height,bubble collapse leads to three cases in turn:no jet droplets,stable jet droplets and possible jet droplets.(3)For the same liquid level height and inlet gas flow rate,the smaller the orifice diameter,the easier to generate jet droplets.(4)Inlet gas velocity does not affect the generation of jet flow,but only the frequency of bubble formation.In order to reveal the influence mechanism of liquid level and orifice diameter on jet flow,the shooting frequency of high-speed camera increased to 10000 fps to observe the changes of bubble shape and spread of capillary.Based on the bubble shape parameters and cavity depth,a capillary wave attenuation model was established.The analysis results show that at the same liquid level,the bubbles with smaller orifice diameter behave a thinner and taller type,the cavity depth accounts for a larger proportion,the capillary wave amplitude attenuation is more complete,so the jet is fast and thin,and the droplets are easier to be generated;Under the same pore diameter,bubbles with smaller liquid level height behave pudgy shape,the cavity depth accounts for a smaller proportion,the capillary wave amplitude attenuation is not complete,so the jet is slow and strong,making it difficult to produce droplets.The results of model analysis match well with the experimental results.In industrial applications,attention should be paid not only to the conditions of bubble jet,but also to the size distribution of jet droplets.For example,the jet droplet size affects the separation efficiency of gas-liquid separator.In this paper,the jet droplet size under different working conditions were measured,it is found that the jet droplet size presented a single peak distribution under limited conditions mentioned above.The average jet droplet size increases with the increase of liquid level height and orifice diameter.However,the inlet gas flow rate has no significant effect on the droplet size distribution.There are two ways to produce droplets by jet breaking:(1)Droplets tear successively from the top of jet;(2)Droplet breaks from the top of the jet firstly,then the remaining water column breaks completely from the bottom of the jet and continue breaking to droplets.Due to the different formation modes of jet droplets,the particle size distribution is different:the size distribution of jet droplets generated by former mode is relatively concentrated,while it is relatively dispersed by latter one. |
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