Experimental Study On Gas-liquid Two-phase Flow In Bifurcating Microchannels

Gas-liquid two-phase flow in the microchannels is widely used in the fields of micro-electromechanical systems,electronic device cooling,biomedical engineering,such as cooling microelectronics chips with the method of flow boiling,preparation of microbubbles as ultrasound contrast agents.In the microchannel,the gas-liquid two-phase flow has a unique operating law and mechanism due to the scale effect.Therefore,exploring the mechanism of gas-liquid two-phase flow in microchannels has become a hot topic in the field of engineering thermal physics.As we all know,the bifurcation structure is the key component of microchannels for the distribution and collection of the gas-liquid two-phase flow.At the bifurcation,the gas-liquid two-phase flow and heat transfer involves bubble formation,deformation,rupture and coalescence.The static and dynamic flow instability is occurred during the process,accompanying with the two-phase flow,interface evolution as well as heat and mass transfer.Therefore,it is of practical significance to study the mechanism of gas-liquid two-phase flow in microchannels with bifurcation.At present,the available investigation on gas-liquid two-phase flow in the microchannels is mainly concentrated in the straight channel.There is a lack of research on the gas-liquid two-phase flow in the microchannels with bifurcation,especially the influence mechanism of the bifurcation structure on the evolution of flow pattern in microchannels.In this context,the experimental apparatus of microscale flow and heat transfer is designed and constructed in this paper.The gas-liquid two-phase flow and flow boiling phase change heat transfer in the microchannels with bifurcation are visually studied,with a focus on the flow boiling and bubble breakup characteristics in microchannels with T-shaped bifurcation,and the gas-liquid two-phase flow behavior in fractal tree-shaped microchannels.In summary,the main research contents and conclusions of this paper are listed as follows:A visualization experimental apparatus of flow boiling in microchannels with T-shaped bifurcation was designed and constructed.The flow boiling pattern at the T-shaped bifurcation was studied and the flow pattern map was presented.Meanwhile,the influence of heating temperature,mass flow density and microchannel size on flow pattern and temperature instability is analyzed.The results indicate that the flow pattern at the T-shaped bifurcation mainly includes bubbly flow,squeezing rupture flow,slug/intermittent annular alternating flow,and annular flow.During the flow boiling at bifurcation,the vaporization core initially appears in the branch channels.As the heating temperature increases,the vaporization core gradually extends into the main channel.The decrease of mass flow density and microchannel size cause the disappearance of the bubbly flow at the bifurcation.The increase of mass flow density and microchannel size leads to a delay appearance of annular flow.With the increase of heating temperature,the fluctuation degree of the fluid temperature decreases gradually.On the contrary,with the increase of the inlet mass flow density,the fluctuation degree of the outlet temperature increases gradually.An experiment apparatus of gas-water two-phase flow in the microchannels with single bifurcation was designed and constructed.Bubbles were generated at the T-junction to visually study the breakup behavior of the bubble in the microchannels with T-shaped bifurcation.The experimental results indicate that the size of the bubble generated at the T-junction is mainly affected by the capillary number and flow rate of dispersed phase.The equivalent size of bubbles diminishes with the increase of the capillary number,and increases with the increase of flow rate of dispersed phase.There are five flow patterns of bubble at the T-shaped bifurcation:bubble symmetry breakup without gap,bubble symmetry breakup with semi gap,bubble floating symmetry breakup,bubble asymmetry breakup and bubble non-breakup.Bubble asymmetry breakup and bubbles non-breakup mainly occurs at the low flow rate of dispersed phase.Bubble symmetry breakup mainly occurs when flow rate of dispersed phase is large enough.There is a linear relationship between the length of the bubble neck and the time at the pinch-off stage of bubble symmetry breakup without gap.Based on the gas-liquid two-phase flow in the microchannel with single bifurcation,this paper also carried out the experimental study on the gas-water two-phase flow in microchannels with multi-bifurcations.The velocity and distance of the bubble traveling in the fractal tree-shaped microchannels were studied,and the factors influencing the uniformity and symmetry of the bubble breakup were analyzed.The results indicate that the overall velocity of the bubble traveling in the fractal tree-shaped microchannel decreases as the level number of microchannel increases.The presence of bifurcation causes the localized rise in the velocity of the bubble at the bifurcation.The initial length of the bubble affects the symmetry and synchronization of bubble breakup in the fractal tree microchannel.The longer the bubble length is,the more favorable the symmetry and synchronization of bubble breakup.Moreover bubbles at high flow rate show better symmetry and synchronization of breakup.In this paper,the flow pattern evolution of flow boiling and gas-water two-phase flow in microchannels with the bifurcation and their influencing factors are studied systematically.The related research results can provide reference for understanding the mechanism of gas-liquid two-phase flow in the microstructure.