| Gas-liquid Taylor flow is one of most exploited flow patterns in microchannel.The stable flow structure ensures the enhanced radial mixing,large specific surface area,short transmission distance,and uniform reaction conditions,etc.,which enables the dual control of flow and reaction in micro-space conditions.Moreover,due to its modular mode of operation.it is easy to realize the coupling and superposition in multiple processes.The reaction in microchannel can be amplified through multi-stage parallel connection,which provides a new solution for efficient and safe production.Now,Taylor flow has been widely used in the processes of chemical separation and reaction as a multi-phase strengthening strategy.With the increasing of complexity in the application environment,there is an urgent need for a new type of controllable high-performance microreactor.However,in microscale,surface tension and viscous forces dominate the dispersion of gas phase and mass transfer in microchannels,which is different from the tubes.In addition,the interaction of two-phase flow and mass transfer is complex and changeable with the increases of residence time,which is easily affected by the channel structure and operating conditions.Therefore,to clarify the flow characteristics and local mass transfer mechanism of Taylor bubbles as well as their internal relationship is the basis to further enhancing mass transfer performance.Due to the low velocity and the stable flow state,the liquid film is more likely to be completely saturated in mass transfer process and becomes inactive.In view of this,a new hydrophobic microchannel with the gas cavities in side wall is constructed.The alternative arranged gas cavity and channel wall provides the partial slip boundary during the Taylor bubble transport and mass transfer.The velocity slip at the cavity interface enhanced the flowability of the liquid film,and further improves the mass transfer efficiency of Taylor flow.This dissertation focuses on the effect of gas cavity on the hydrodynamics of Taylor bubble and liquid film transport.Furthermore,the mechanism of mass transfer enhancement and the interaction of flow and local mass transfer are clarified.Above results provide a theoretical basis for the design of air cavity structure.The enhancement of gas cavities on the liquid transport around the Taylor bubble is investigated by comparison experiment.Different from straight microchannel,the gas-liquid interface reduces the adhesion between the liquid and channel wall.The accelerated liquid increases the bubble deformation under the same operating conditions with a sharper bubble head and more flatten bubble tail,resulting in an increase on specific surface area by 10%.At the same time,such bubble deformation increases the thickness of the liquid film and the average liquid phase ratio of the cross-section,and reduced the resistance in liquid film;the combined effect of Taylor bubble and gas cavity induces periodic fluctuation of velocity slip over time which intensified the turbulence in the liquid film;the instantaneous velocity slip at the cavity interface promotes the liquid transport in the liquid film,and the overall leakage is increased by 32.2-81.1%.The characteristics of Taylor bubble in Microchannel with Gas Cavities(MGC)are systematically investigated,including the Taylor flow map,bubble hydrodynamics and pressure drop in which the cavity width,cavity spacing as well as gas liquid ratio are considered.Compared to straight microchannel,the gas cavity expands the flow pattern of the liquid phase inertial control zone,with a smaller range of unstable flow patterns(stirring flow and annular-Taylor flow),and larger region of Taylor flow.Compared with the cavity width,the region of Taylor flow is more sensitive to the cavity spacing,and it increases as the cavity spacing decreases.Moreover,the increased inertia effect enables the smaller bubble in MGC with a 7-22%reduction of bubble length,which further increased the Taylor unit.Therefore,the corresponding volumetric gas holdup is increased by 8.3-29.7%.Above dispersion of gas bubble is positively correlated with the proportion of gas phase at the boundary.Specifically,the gas holdup increases with the cavity spacing decreases and the cavity width increases.Such bubble distribution in MGC leads to a higher the interface pressure drop under the same conditions,and the overall pressure drop increases by 5.3-24.1%.It should be mentioned that,according to theoretical analysis,the frictional pressure drop in the liquid film is the main component of the overall pressure drop,and it can be significantly reduced by the gas cavity.The local oscillation movement at the bubble tail is observed by tracking a single Taylor bubble in MGC.The oscillation behaviors and hydrodynamics of the Taylor bubble are experimentally studied in the rectangular microchannel with the gas cavity that has various width and spacing.The oscillation amplitude is highly dependent on the cavity width and liquid film thickness.According to the Laplace equation,the smaller cavity width can induce larger amplitude due to the larger damping force from the cavity interface,while the larger capillary number and cavity spacing tend to thicken the liquid film for a small oscillation amplitude.The above-mentioned oscillation changes the total local resistance of the transportation process,resulting in the bubble velocity fluctuation.The fluctuation frequency is positively correlated with the number of gas cavities.Numerical simulation reveals that the oscillation is determined by the force balance at the cavity interface;the interaction of the deformation at the cavity interface and the pressure fluctuation is highlighted.A criterion based on the pressure balance at the cavity interface is proposed to predict the oscillation movement,and the oscillation is more sensitive to the cavity width in current experiment.Based on the unit cell model,the mass transfer performance and enhancement mechanism in MGC is illustrated.Overall mass transfer coefficient and local mass transfer performance are also quantified during the CO2 bubble dissolution in the channel.In the range of the experiment,the liquid film contribution in mass transfer is significantly improved by gas cavity.In particular,the saturation time of the absorption in liquid film is delayed with a lower CO2 concentration.This enhancement in liquid film mass transfer is enlarged at higher liquid flow rate and larger liquid viscosity due to the thicker liquid film around the bubble.Besides,the importance of the mass exchange between the liquid film and the bulk slug is highlighted.A mass transfer model that considers the separate contributions of liquid film,bubble caps and liquid film to bulk slug under the partial slip boundary conditions is proposed to describe the mass transfer in MGC.Under experiment conditions,the cavity configuration has little effect on over mass transfer coefficient.However,the liquid film contribution is enhanced at higher gas proportion at the boundary through influencing the leakage flow rate.In current experiment,the precise control of the concentration of the liquid film side of 0-90%(saturated concentration)is realized by adjusting the cavity width and cavity spacing in the range of 0.1-0.3 mm and 0.1-0.4 mm,respectively. |
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