Study On Gas-liquid Two-phase Dispersion And Mass Transfer In Microchannels

Micro-chemical technology is a new promising direction for chemical engineering emerged in recent years. And the characteristics of gas-liquid two-phase flow are of key importance for the design and application of micro-chemical devices.The bubble formation in viscous fluid was studied using online high-speed camera imaging systems. The effects of operating conditions and viscosities of continuous phase for the bubble size were estimated and microbubles with the dimensionless bubble volume ranging from 0.8 to 20 were successfully prepared. The shearing-squeezing mechanism was proposed for the bubble formation in viscous fluid according to the estimation of the superficial forces on the system. Based on the mechanism, a mathematical model was developed to predict bubble sizes by considering the effects of operating condition, viscosity of continuous phase and the predicted values were well accordant with experimental result.The pinch-off mechanism for Taylor bubble formation was investigated in a microfluidic flow-focusing device. And results show that the gaseous neck collapses nonlinearly with time and the minimum width of the gaseous thread r₀ can be scaled with the remaining time τ as a power-law relationship: r₀∝t^α. Two distinct collapse processes are observed: the liquid squeezing collapse stage and the free pinch-off stage. In the liquid squeezing collapse stage, the collapse is induced by the liquid constriction. The exponent α increases with the liquid flow rate and guadually approaches up to 0.33. While in the free pinch-off stage, the value of α is close to 0.5, and is independent of the liquid flow rate. The transition of the exponent between the two stages is triggered by the interface rearrangement resulted from the reversal of the liquid due to the fading of liquid constriction. The analyses of the governing forces exerted on the gaseous thread during the bubble formation process show that the final pinch-off is dominated by the liquid inertia, and the surface tension is still important. A neck shape dependent mechanism was proposed for the bubble breakup in low viscosity fluid according to the estimation of the local forces on the break region. The mechanism was confirmed by theory analyse and further experiemt under low liquid viscosity. Based on the mechanism, a generalized model was proposed for the formation of Taylor bubble in a flow-focusing device, consisting of four stages: expanding stage, linear collapse stage, liquid squeezing collapse stage, and the free pinch-off stage.The pinch-off mechanism for Taylor bubble breakup was investigated in a microfluidic T-junction divergence. A critical minimum width of the neck δc is observed: when δ<δc, a fast unsteady breakup stage was observed; and a quasi-steady slow collapse stage was observed, when δ>δc. During the fast breakup stage, the process was driven by surface tension and independent of the external flow. The minimum width of the neck can be scaled with the remaining time τ as a power-law relationship: atd μ, with a=22.0 in the beginning and a=5.0 just before the pinch-off. During the slow collapse stage, the surface of neck was quasi-steady. Before the formation of the tunnel, the width of the depression region can be scaled with the collapse time t as a power-law relationship: w_d/w∝t^0.75. After the formation of the tunnel, the width of the depression region can be scaled with the collapse time t as a logarithmic function: w_d/w∝alnt+b.The evolution of profile of the neck and characteristic of self-similar were investigated. Due to the disproportional action of surface tension, the shrink of the neck in radius dimension was faster than that in axial dimension. The self-similar characteristic was observed by normalized the profile with minimum radius of the neck r0 and r0α. The deformation parameter α decreases with the increase of liquid viscosity, due to the enhance of viscous tension and the declining of the surface tension. The confinement of the wall and the liquid squeesing also play important roles on the surface evolution, and these roles are enhanced with the increasing of the liquid viscosity. In addition, the surface curvature is also self-similar analogous to the surface profile. It is indicated that the evolution of the surface profile is dominated by surface tension.The initial bubble size, bubble forming frequency, void fraction and pressure drop for gas-liquid two-phase flow accompanying with mass transfer were investigated experimentally by a high-speed camera and a pressure transducer system. Results show that the role of gas absorption on the initial bubble sizes and bubble forming frequency was unnoted, but significant on the void fraction and pressure drop. The pressure drop could be enlarged due to the gas adsorption under certain conditions, and the critical situation for the increase of pressure drop could be identified by the theory analysis. A phenomenon of the dependence of pressure drop on void fraction was observed. Several mathematical models were developed to predict initial bubble sizes, bubble forming frequency, void fraction and pressure drop bubble by considering the effects of operating condition and the rate of absorption.The volumetric mass transfer coefficient and liquid side mass transfer coefficient were investigated for the processes of physical adsorption, weak chemical adsorption and strong chemical adsorption, respectively. The differences of mass transfer characteristic among the whole adsortion stage, bubble formation stage and bubble flow stage were also deeply analyzed. The mass transfer efficiency in the inlet stage was significantly larger than the main channel: the liquid side mass transfer coefficient in inlet stage was 1.5-3 times of magnitude than that in main channel during strong chemical adsorption, 1.8-4 times during weak chemical adsorption and 4-15 times during physical adsorption. Meanwhile, the mass transfer was significantly strengthened during the flow pattern transition from Taylor flow to bubble flow. In addition, several dimensional correlations were developed to predict volumetric mass transfer coefficient and liquid side mass transfer coefficient of main channel for the processes of physical adsorption, weak chemical adsorption and strong chemical adsorption, respectively, by considering the effects of operating condition and the rate of absorption.Finally, the nonlinear dynamic principles of gas-liquid two-phase flow and mass transfer were investigated. The effects of operating conditions and the rates of absorption on the dynamics of bubble size, liquid slug size, void fraction and bubble velocity were examined. The roles of operating condition, transition of flow pattern and liquid film around the bubble on the mass transfer were revealed by the nonlinear dynamics of liquid side mass transfer coefficient. The results enriched the gas-liquid mass transfer theory in microchannels.