Numerical Study On The Two-Phase Flow And Heat Transfer In Droplet-Based Microfluidics

Droplet-based microfluidics has been widely used in chemical reaction,biochip,material fabrication and heat transfer enhancement.It is of great engineering significance and academic value to reveal the droplet generation mechanism,flow and heat transfer characteristics.In this thesis,the effects of three-dimensional flow characteristics of two phases,microchannel geometry and wall velocity slip boundary conditions on the droplet generation in the microfluidic T-junctions are numerically studied,and the heat transfer enhancement characteristics of the droplet in the microchannel are also investigated.This thesis will undoubtedly help to deepen the understandings of droplet generation and transport in droplet-based microfluidics,and lay a good theoretical foundation for the better application of this technology.The comparition between two-dimensional(2D)and three-dimensional(3D)numerical studies of the droplet formation in microfluidic T-junctions was performed.The results show that the droplet generation can be divided into 2D and 3D collapse stages,and the latter process is faster.The 2D simulation can approximately reproduce the 2D collapse.However,it cannot capture the 3D collapse.The 3D simulation is better in agreement with the experiment data.It is also found that,with increasing the depth to width ratio of the microchannel,the 3D characteristic of two-phase flow enhances while the effect of the channel wall on the droplet generation has been fatally weakened.Meanwhile,the dispersed phase is converted to 3D collapse stage earlier with an acceleration of droplet formation.The detailed analysis reveals that the critical capillary number for droplet generation is exponentially related to the depth to width ratio.With the increasing of depth to width ratio,the critical capillary number decreases and the exponential formulas are different when the ratio is below or above 1.In this thesis,a modified T-junction with rectangular rib is proposed,which can significantly improve the droplet generation performance,including expanding the dripping regime and reducing the jetting regime.Therefore,the droplet size is reduced with a better monodispersity.The study indecades that the rectangular rib not only increases the driving force of droplet formation(the shear stress exerted by the continuous phase and the pressure drop along the thread),but also reduces the resistance between dispersed phase and wall.The size of droplet produced in the modified T-junction still meets the scaling law of capillary number and flow rate ratio.However,the scaling law should be adjusted based on the rib height.The results showed that the rib width has weak effect on the droplet generation characteristics.The droplet size increases with the increasing of viscosity ratio or surface tension,and decreases with contact angle.The rectangular rib could effectively depress the adverse effect of the wetting property between wall and dispersed phase.The effects of the wall velocity slip on the droplet generation is also numerically studied.In dripping regime,both the shear stress exerted by the continuous phase and the pressure drop along the thread decrease with the slip length of continuous phase,which makes the droplet diameter larger.In squeezing regime,the shear stress exerted by the continuous phase on the thread increases with the slip length of continuous phase while the resistance between wall and dispersed phase decreases with the slip length of dispersed phase.As consequence,the droplet size decreases with the increasing of both slip lengths.The increase of slip length of each phase can depress the elongation rate of the thread,which can significantly avoid the double collapse state of droplet generation,and improve the monodispersity.Adding droplets into the single-phase flow in microchannel can significantly enhance the heat transfer with a small pressure loss.The Nu number at the droplet location is greatly increased,and the average Nu number can be promoted by more than 50%.With the increasing of inlet velocity,the vorticity and mixing in droplet increase.However,the thickness of the liquid film also increases,making it harder for the circulation in droplet to approach the heated wall.Comprehensive effect of the two factors makes the Nu number increase firstly and then decrease with the increasing of inlet velocity.Larger surface tension makes the deformation of droplet more difficult,leading to the reduction of liquid film thickness.Thus,the heat transfer is enhanced.Increasing the viscosities of both dispersed and continuous phases weakens the effect of droplet on the heat transfer.The former restrains the circulation in the droplet.The latter enhances the shear stress exerted by continuous phase,which increases the liquid film thickness and restrains the circulation in the continuous phase.Increasing the slip length of continuous phase can enhance the heat transfer.Compared with the single-phase flow,however,the promotion percentage of Nu number in the droplet two-phase flow is decreased.It is due to the decrease of shear stress exerted by continuous phase on the droplet.Increasing the droplet diameter can enhance the heat transfer.If the liquid film thickness is constant,the heat transfer enhancement of single droplet keeps unchanged.When the droplet size is increased,the average Nu number increases firstly,and then decreases after the liquid film thickness is reduced to the minimum.The numerical results show that the Nu number is a linear function of the number of droplets per unit channel length.When the droplet equivalent diameter is smaller than the feature size of microchannel,the droplet size affects the slope of function.The simulation results are in good agreement with the empirical criteria.