Bubble Dynamics And Behaviors On Micro-orifices In Mini/micro Channels

As a basic problem of gas-liquid two-phase flow, the phenomenon of bubbling fromrigid orifices wide exists in many industrial applications, especially in energy, chemistryand environmental fields, such as direct methanol fuel cell, wastwater treatment, frictionreduction, gas-liquid contactors etc. The behaviors of bubbling and gas-liquid two-phaseflow will affect the heat and mass transfer even the safty of the equipment. Taking directmethanol fuel cell as an example, the gas diffusion layer plays an important role in fuelcell operation, which is a porous medium with pore size of several micrometers toseveral hundred micrometers. The bubbles are forming on the surface of thesemicro-sized pores in anode channels. There are direct connections between bubblingbehaviors in anode channel and gas-liquid two-phase transmission in gas diffusion layer,which affect fuel cell performances dramatically. Therefore, the understanding ofbubbling on micro-orifices and gas-liquid transmission in porous medium can help theoptimization of the fuel cell.However, most of the past works on bubbling dynamics are conducted on theorifices with millimeter-scale. The bubbling dynamics on micro-orifices should bestudied further. Therefore, in the present work, the behaviors of bubbling onmicro-orifices and gas-liquid two-phase flow in micro-channels are studied based onhigh-speed photography and digital image processing techniques. The main studies andconclusions are as follows.①The dynamic behaviors of bubbling from micro-orifices submerged in stagnantliquid are studied by visual experiments. The bubbles motion are tracked andtheir parameters are obtained, such as bubble volume, deformation, velocityand bubbling periodicity, etc. The effects of orifice size and gas flow rate onbubbling behaviors are discussed. The bubbling periodic and bubbling flowpatterns are analyzed. The results show that the bubble departure volume,departure time and waiting time decreased with decreasing of orifice diameter.The steady growth period dominants the whole bubble growth process atmicro-orifices, which is nucleation period at macro-orifice. The phenomenonof micro-bubbles inrush has been captured, which is relevant to the differentmanners of bubble rupture at different orifices. Based on the Young-Laplaceequation, a non-spherical bubble growth model has been constructed, and the variations of bubble shape and characteristic parameters with time have beenobtained. The influences of orifice sizes and wettabilities have been discussed.A good agreement between simulation and experiments has been presented.②The bubble formation and gas-liquid two-phase flow in mini-channel havebeen studied by visual experiments. The bubble motion is tracked, and thebubble size, deformation and velocity are obtained. The effects of gas velocity,liquid velocity and the incline angle of the channel on bubbling behaviors arestudied. The results show that bubbling in mini-channel will experience fourstages: formation, inrush, damping oscillation and steady movement. Withincrease of gas flow rate, the coalescence between bubbles intensifies, thebubble size distribution concentrates, and the bubble velocity increases. Withincrease of liquid flow rate, the location of bubble turns into steady movementstage advances. A phenomenon of micro-bubbles accumulation at the corner ofthe channel is presented, which affects by gas flow rate, liquid flow rate andchannel incline angel.③Bubbling on single orifice/multiple orifices in micro-channel with liquid crossflow is studied by visual experiments. The bubbles characteristic parametersare obtained, the bubbling tracing are achieved, and bubbling periodic isanalyzed. The results show that, with increase of gas flow rate, the flow patternin micro-channel transforms from bubbly to parallel flow gradually. Undersingle orifice condition, the bubble size increases firstly and then remains withincrease of gas flow rate. With increase of liquid flow rate, the bubble sizereduces and their distributions concentrate. The distinct periodic bubbling stagehas been found at low gas flow rate and at medium liquid flow rate conditions.Under multiple orifices condition, the selectivity of gas flow path has beenfound, which is determined by gas flow rate and arrangement of orifices.④The bubbling behaviors in tapered channel are studied by visual experiments.In the experiments, the variation of bubble size, deformation, velocity and thebubbling periodic are studied. The effects of gas and liquid velocity onbubbling behaviors are discussed. The results show that the bubble is stretchedwhen it moves along a divergent channel. Finally, it pinches off from one sideof the channel, and coalescence with following bubbles will happen. Withincrease of gas flow rate, the flow pattern transforms from bubbly to parallelflow gradually. With increase of liquid flow rate, the bubble size decreased, its distribution concentrates and bubbling frequency increases. Base on thegas-liquid interface’s shape and stability, a model of force analysis ongas-liquid interface in tapered channels is constructed. And the processes ofliquid imbibitions and drainage in tapered channel have been simulated. A goodagreement between simulation and experiments has been performed. Then theinfluence of the channel tapered angel and wettabilities on the behaviors ofdroplet migration has been studied. And a condition that the droplet balancepoint exists in tapered channel is proposed.⑤The gas-liquid two-phase flow in micro-channel with permeable wall, consistsof a homogeneous and a heterogeneous network structured micro-channels, hasbeen studies by visual experiments. The transmission of gas and liquid innetwork structured channels is also studied. A network adaption method hasbeen used in image processing to obtain the phase distribution in networkstructured micro-channels. The results show that with increase of gas flow rate,the liquid saturation reduces and the bubbles can form at multiple locations.With increase of liquid flow rate, the liquid invades into the network structuredchannels faster, and the final liquid saturation increases. The gas flow rate, thenumber of gas supply orifices and its location all have significant influence onphase distribution in network channels and behaviors of two-phase flow inmain channel.