Browsing by Subject "Bubble"

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    Bubble size effect on effervescent atomization.
    (2011-07) Shepard, Thomas G.
    This paper presents the results from a number of studies conducted in an effort to gain insight into how to control bubble size during gas injection through a porous media into a liquid cross-flow, what effect bubble size has on the spray characteristics from an effervescent atomizer, and to provide input for future effervescent atomizer designs and studies. Experiments were performed in a specially designed atomizer which allowed for manipulation of the air injector geometry in order to vary bubble size from sizes much smaller than the nozzle exit diameter to much larger than the exit diameter. A parametric study was conducted to examine how three different bubble control mechanisms affect the average bubble size, bubble size standard deviation, and gas to liquid mass flow ratio (GLR) at the transition point between bubbly flow and slug flow. It was found that changing the channel hydraulic diameter at the air injection site had the largest effect while pore size and electrolyte concentration had smaller, though still significant, effects. A dimensional analysis was performed which arrived at a similarity parameter which correlates to bubble size for air injected through a porous media into a cross-flow. Bubble size was seen to have an effect on the stability, spray half cone angle, and Sauter mean diameter of the liquid droplets produced by an effervescent atomizer. The effect of bubble size on improving spray characteristics was shown to be optimal for bubble sizes on the order of the exit diameter. The mechanism by which bubble size has an affect is suggested to be due to liquid velocity fluctuations at the exit of the nozzle as opposed to the often cited mechanism of causing the flow to choke thus allowing gas to expand beyond the exit. A comparison of bubbly and annular flows at identical conditions further suggests that effervescent atomizer design and operation may benefit from trying to produce annular flow conditions at low GLRs rather than bubbly flow conditions at high GLRs.
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    Numerical and Theoretical Studies of Air Entrainment and Bubble Acoustics under Breaking Waves
    (2021-12) Gao, Qiang
    Bubbles and breaking waves play a critical role in many physical processes. However, bubble formation mechanism, trajectories, and their acoustic signatures are still poorly understood due to the complex process of breaking waves. To study the bubble transport dynamics and their formation mechanism, a technique for Lagrangian tracking of bubbles and detecting their time-evolution behaviors is developed. Five possible behaviors are considered: formation, extinction, continuity, binary fragmentation, and binary coalescence. The technique is based on establishing a network of mappings between bubbles identified at adjacent time instants. The accuracies for continuity, binary fragmentation, and binary coalescence are estimated to be 99.5%, 90%, and 95%, respectively. The algorithm is proved to be accurate and robust by extensive validations using the breaking wave cases. Bubble entrainment mechanism and bubble trajectory are investigated. Air filaments and cavities in plunging breaking waves, generically cylinders, produce bubbles through an interface instability. A generalized dispersion relation is obtained that spans the Rayleigh–Taylor and Plateau–Rayleigh instabilities as cylinder radius varies. The analysis provides insight into the role of surface tension in the formation of bubbles from filaments and cavities. Small filaments break up into bubbles through a Plateau–Rayleigh instability driven through the action of surface tension. Large air cavities produce bubbles through a Rayleigh–Taylor instability driven by gravity and moderated by surface tension, which has a stabilizing effect. Surface tension, interface curvature, and gravity are all important for cases between these two extremes. Bubble trajectories and their interaction with breaking wave flow fields are also studied here. A simulation framework for bubbly flow and the sound radiated by breaking waves is presented. It consists of a two-phase flow solver, an algorithm to track bubbles and bubble creation rates, and a module to compute the sound generated by newly-formed bubbles. The sounds from breaking, third-order Stokes waves of 0.25m wavelength and two slopes are calculated. The results show encouraging agreement with existing laboratory observations and identify the importance of air cylinder breakup in bubble creation.