| Micromixer is the functional element to realize rapid and uniform mixing between fluids in microfluidics.It has been widely used in biology,medicine,chemical engineering and other fields.The fluids in micromixers are usually in laminar flow states with low Reynolds number,and the mixing process dominated by molecular diffusion is slow and inefficient.The size effect is significant in microchannel since the characteristic length is shortened to microscale,and the sidewall roughness has significant effects on fluids states and mixing process.Thus,it is valuable to study the structure design,flow states,roughness effects and machining technology of micromixers.The research of design,processing and applications has been carried out around the chaotic advection type micromixer induced by Dean vortices(serpentine ones and spiral ones).Based on the real manufactured sidewall morphology of micromixers,the effects of sidewall roughness on flow states and mixing performance were revealed.The micromixer structure was optimized and redesigned,varied flow states morphologies were analyzed,the chaotic advection mixing process induced by Dean vortices was quantitatively evaluated.Through the micromixers,Cu2O nanoparticles with high photocatalytic performance were synthesized and high efficiency conversion of castor oil to biodiesel was achieved.Firstly,the microchannel sidewall morphology models based on micromilling process were established.The mixing performance and flow states characteristics including flow velocity,friction loss and concentration distribution of micromixers with different wall roughness were analyzed through the finite element simulation and mixing experiments.It was shown that the sidewall roughness hindered the flowing of fluids and led to the increase of friction loss.The increase of friction loss became significant with the increase of sidewall roughness and Reynolds number.The relationship between friction factor and Reynolds number under various sidewall roughness was established and revealed,similar to the Moody diagram.On the other hand,the sidewall roughness had the effects of characteristic length shortening,resulting in the narrower flow regions.For the chaotic advection type micromixer induced by Dean vortices,when the sidewall roughness was greater than 1.208%,separation vortices were generated in the roughness valley and the fluids were pushed to the central region of microchannels.As a result,the flow velocity of the central region was increased,which enhanced the chaotic advection and improved the mixing performance.Secondly,the structure of serpentine micromixers was optimized in order to achieve the rapid and uniform mixing with low pressure drop.An ellipse curve serpentine micromixer was proposed by using ellipse curve with continuous curvature variation to induce strong Dean vortices and chaotic advection.Derived from the ellipse curves with continuously changed curvature that could induce intense Dean vortices and chaotic advection,the ellipse curve serpentine micromixer was proposed.The effects of ellipse geometry parameters on mixing performance,pressure drop and Dean vortices morphology were analyzed.The results showed that the larger eccentricity of the ellipse brought about the stronger chaotic advection and the better mixing performance but also the higher pressure drop.For ellipses with the same eccentricity,the ellipses with focus on the micromixer main flow direction had better mixing performance and lower pressure drop than those with focus on the micromixer vertical flow direction.With the same circumference,ellipses with micromixer main flow direction and eccentricity of 0.77 and above were the optimum ellipse structure.Combining mixing performance,input power and residence time,the mixing power consumption parameter was proposed to evaluate the mixing cost-performance of micromixers.The continuous varied curvature of ellipse curve avoids the generation of separation vortices and flow dead zones.Therefore,the ellipse curve that could realize efficient mixing without producing excessive pressure drop was the micromixer structure with excellent cost-performance.Then,the chaotic advection mixing process induced by Dean vortices has been investigated,and the quantitative evaluation parameter and evaluation method have been presented.Based on the spiral micromixers with centrosymmetric structure and single spiral structure,the mixing performance and the variation of flow state characteristics including streamlines,concentration distribution morphology and Dean vortices morphology were compared.In the centrosymmetric structure,the concentration distribution vortices were degraded since the rotating direction of Dean vortices reversed.The interface area between fluids were reduced,which hindered the mixing process.In the single spiral structure,the concentration distribution vortices varied periodically and the interface area between fluids varied,because the rotating direction of Dean vortices was unchanged.The interface area and Dean vortices intensity were found to be the two main factors influenced the mixing process.Coupling the concentration gradient contour length which reflected the interface area and the Dean numbers which indicated Dean vortices intensity,the Vortices advection intensity parameter was proposed for quantitative evaluation of the chaotic advection mixing process intensity induced by Dean vortices.Finally,there were two application cases of micromixers in chemical engineering including the synthesis of Cu2O nanoparticles and the conversion of biodiesel,in order to reveal the enhancement of micromixer with excellent mixing performance in chemical engineering.The ellipse curve micromixer was employed to synthesize Cu2O nanoparticles for photocatalytic degradation of organic dyes based on liquid phase reducing.The formation process of Cu2O nanoparticles synthesized by micromixing and stirring in beaker was clarified using the crystal forming theory.The influence mechanism of mixing performance on formation of nanoparticles was revealed.It was shown that the sufficient reducing of Cu(OH)2 was achieved due to the excellent mixing performance of micromixer.Compared with the nanoparticles synthesized by stirring in beaker,the Cu2O nanoparticles synthesized by micromixer were smaller and more uniform,and had better photocatalytic performance.It was also found that the better the mixing performance,the more sufficient reducing of Cu(OH)2,the higher the degree of refinement and uniformity of particles,and the better the photocatalytic performance.Besides,the spiral micromixer was applied for the production of biodiesel from castor oil based on esterification reaction.The uniform mixing between castor oil and ethanol was realized in the spiral micromixer,which promoted the esterification reaction.A conversion rate of 90%was achieved with a residence time of only 15s,which was an efficient biodiesel conversion process.Compared with other passive micromixers structures,the residence time of spiral microchannel was shorter so that the spiral micromixer had higher biodiesel production rate. |
