Fluid Flow And Boiling Heat Transfer Characteristics In Microchannels With Complex Structure

Recently,with the rapid development of micromachining technology,micro devices and systems which are mainly composed of microchannels appear continually,such as micro heat sink,micromixer,microseparator,microvalve,micropump,et al.The fluid flow and heat transfer in microscale have caused extensive concern of international academia.In the field of microelectronic technology,the heat dissipating capacity of micro devices with high heat flux is close to l0~7W/m~2,the microchannels with simple structure are not able to meet the cooling requirement.In the field of biochemical engineering,microfluidic system plays a more and more significant role as an important branch of micro-electromechanical system.Microfluidic technology focuses on achieving complex manipulated function based on building microchannel systems.Microchannel structure modification is a popular method to improve the fluid mixing performance and control the microdroplet generation process in microscale,which is important to the design and application of the microfluidic chip.In this paper,aiming at the fluid flow,heat transfer and mixing in microchannel,on the one hand,a novel microchannel heat sink with complex structure is proposed.The single-phase fluid flow and heat transfer characteristics in the microchannel heat sink have been investigated and the geometric structure of the heat sink has been optimized.On the other hand,the flow boiling characteristics in the microchannel heat sinks have been studied experimentally.The boiling pattern,heat transfer,pressure drop and wall temperature performances for different microchannel heat sinks are analyzed.The mechanism of boiling heat transfer enhancement and the suppression effect on flow boiling instability of microchannel heat sink with complex structure are analyzed.Furthermore,the effects of the microchannel structure on mixing performance and microdroplet generation process are studied.This paper provides guidance to the optimization design of the microchannel heat sink and microfluidic chip.The main contents include the following aspects:Firstly,in order to improve the heat transfer performance of microchannel with cavities at low Reynolds number,microchannel with cavities and fins is designed by adding fins in the center of microchannel with cavities.The effects of the combination of cavities and fins on the flow and heat transfer are analyzed by comparing the rectangular microchannel,microchannel with triangular cavities,microchannel with rectangular fins,microchannel with triangular cavities and rectangular fins.The method of entropy generation minimization is adopted to analyze the mechanism of heat transfer enhancement.The simulation results show that the combination of cavities and fins increases the heat transfer area,disturbs the mainstream,induces the local chaotic advection and decreases the laminar stagnation zones in cavities,which can enhance heat transfer obviously.The combination of cavities and fins decreases the heat transfer irreversibility and improves the heat transfer efficiency of the heat sink.Secondly,the influence of geometric dimension and shape of the cavity and fin on the fluid flow and heat transfer characteristics is investigated,and the comprehensive performance of different microchannel heat sinks is evaluated.The simulation results show that:the separation,disturbance,and blocking effects of the fin on the mainstream relate to the fin width;while the jetting and throttling effects of the cavity on the fluid relate to the cavity width;the fin shape has an effect on the mainstream separation point,thus leading to the variation of vortex area in the downstream of the fin and the pressure drop of the microchannel;the variation of cavity shape results in the variation of intersection angle between the contraction segment of the cavity and the mainstream direction,the vortex in the cavity and the friction loss of the microchannel are affected accordingly.The heat transfer performance can be improved further by optimizing the structure parameters of the microchannel.Thirdly,a visualization experimental system is set up to investigate the flow boiling heat transfer in microscale,which is able to obtain the microscale flow image and the heat transfer characteristics synchronously.Three types of silicon parallel microchannel heat sinks including rectangular microchannel heat sink,microchannel heat sink with triangular cavities,microchannel heat sink with triangular cavities and rectangular fins are designed and fabricated.The experiments are performed to study the flow boiling characteristics in three microchannel heat sinks selecting pure acetone liquid as the working fluid.The bubble nucleation characteristic,boiling curve,heat transfer coefficient,pressure drop and wall temperature performances of different microchannel heat sinks are compared.Moreover,the effects of mass flux and heat flux on the flow boiling characteristics are also studied.The experimental results show that:the mass flux and heat flux play significant roles in heat transfer performance,two-phase pressure drop and flow boiling instability;the wall superheat of the rectangular microchannel at the onset of boiling is relative high,and the phenomena of vapor flow reversal and early dry-out are serious;the microchannel with triangular cavities can enhance boiling heat transfer significantly,reduce the pressure drop and suppress the flow boiling instability simultaneously;for microchannel with triangular cavities and rectangular fins,the liquid films attached to the cavities and fins can delay the partial dry-out and improve the critical heat flux;the cavities and fins increase bubble nucleation density and improve the bubble departure frequency.In addition,at high mass flux,the microchannel heat sink with cavities and fins is able to reduce the flow boiling instability.Lastly,in order to enhance the mixing performance in microscale,a micromixer with gaps and rectangular fins is proposed.The fluid flow and mixing characteristics in the novel micromixer are investigated numerically and experimentally.The mixing enhancement mechanism of the gap and fin is analyzed based on the field synergy principle.And the effects of geometric parameters of the gap and fin on the comprehensive performance of micromixer are studied.The results show that the fluid is squeezed,accelerated,separated and disturbed by the gaps and fins in the novel micromixer.The vortexes are generated and the contact area between two fluids is increased,thus the mixing efficiency is enhanced.According to the field synergy principle,the mixing enhancement can be attributed to the good synergy between the velocity field and the concentration field.The comprehensive performance of the micromixers is analyzed by field synergy principle which provides an alternative way to evaluate the performance of micromixers.In addition,a three dimensional numerical simulation of droplet generation process in the cross-flow microchannel is performed by the volume of fluid(VOF)method.The effects of microchannel inlet angle,surface tension coefficient and viscosity of continuous phase on the microdroplet generation process are investigated.New correlation of the dimensionless droplet length for the squeezing regime is proposed.The results show that the two-phase pressure difference and the surface tension play important roles in the droplet generation process during the squeezing regime.The microchannel inlet angle and fluid physical properties have an obvious influence on the time period and size of the microdroplet.