Study Of Gas-liquid Two-phase Flow And Bubble Behaviors In Microchannels

Microfluidics is a cutting-edge technology emerged in recent years. And gas-liquid two-phase flow is an important research project in microfluidics. The gas-liquid two-phase flow behaviors, the bubble formation and moving behaviors in microchannels were investigated in the present thesis, by using a home-made laser image system and a high speed digital camera. The velocity distributions of the microfluidics were investigated with the help of micro Particle Image Velocimetry (micro-PIV). The specific studies were included as follows:The gas-liquid two-phase flow in vertical rectangular microchannels was investigated and five flow patterns were observed: bubbly, slug, annular, annular-stratified and stratified. A flow pattern map was constructed corresponding to the experimental data. A model was proposed to predict the film thickness. The predicted values agreed well with the experimental data.The bubble formation in both Newtonian and non-Newtonian fluids in a cross-flowing microfluidic T-junction was investigated. Various bubbles were generated in three different flow regimes: the squeezing, the dripping and the transition (sqeezing-to-dripping) regimes. Models for each regime were proposed to predict the bubble size. The shear thinning property of the PAAm solutions plays an important role on the shape and evolution of the gaseous thread, and the ultimate size of bubbles.The velocity profile for single phase in square microchannels was measured by a micro-PIV system, and the experimental results were in good agreement with the values calculated by the classical theory. The measured velocity profile for non-Newtonian fluids could provide a new method for predicting the rheology property of non-Newtonian fluids.The mechanism for bubble formation in both Newtonian and non-Newtonian fluids in microfluidic flow-focusing devices was investigated. The influences of the parameters on the formation process and the size of slug bubbles were studied. The models for predicting the collapse speed and the bubble size were proposed, respectively. The results showed that the bubble formation process could be divided into three stages: expansion, collapse and pinch-off stages. The collapse speed of the gaseous thread in the second stage is controlled by the squeezing pressure, and is proportional to the liquid flow rates; while the minimum width of the neck of the gaseous thread in the third stage could be scaled with the remaining time to the ultimate pinch-off as a power law with an exponent of 1/3. The experimental results showed that the PAAm solutions prolong the gaseous thread in the tangential direction of the neck. A model was proposed to predict the bubble size generated in the PAAm solutions, on the basis of the ratio of gas/liquid flow rates and that of the Reynolds number of both phases. A novel approach to predict the rheology property of non-Newtonian fluids qualitatively, based on the relationship between measured velocity profile and the bubble shape, was brought forward.Bubble coalescence in a microchannel with an expansion section was studied. For Newtonian fluids, the diminishing of the surface tension can reduce the probability of the coalescence. For non-Newtonian fluids, the coalescence time is prolonged with the increase of distance for the position of coalescence, and also with the increase of the distance between two aligned bubbles. Two different mechanisms for bubble coalescence were analyzed.The bubble behavior in a microchannel with a loop was investigated. Six different flow regimes were observed in the T-junction divergence. A simple model was proposed for predicting the transitions between various breaking and non-breaking regimes. A new method for bubble management was provided for generating pair bubbles, based on the recombination behavior of dual bubbles at the outlet of the loop.