Development of high-aspect-ratio microchannel heat exchanger based on multi-tool milling process

A high-aspect-ratio microchannel heat exchanger based on multi-tool milling process was developed. Several slotting cutters were stacked together for simultaneously machining several high-aspect-ratio microchannels with manifold structures. On the basis of multi-tool milling process, the structural design of the manifold side height, microchannel length, width, number, and interval were analyzed. The heat transfer performances of high-aspect-ratio microchannel heat exchangers with two different manifolds were investigated by experiments, and the influencing factors were analyzed. The results indicate that the magnitude of heat transfer area per unit volume dominates the heat transfer performances of plate-type micro heat exchanger, while the velocity distribution between microchannels has little effects on the heat transfer performances.

Molecular dynamics simulation of the flow mechanism of shear-thinning fluids in a microchannel

Shear-thinning fluids have been widely used in microfluidic systems,but their internal flow mechanism is still unclear.Therefore,in this paper,molecular dynamics simulations are used to study the laminar flow of shear-thinning fluid in a microchannel.We validated the feasibility of our simulation method by evaluating the mean square displacement and Reynolds number of the solution layers.The results show that the change rule of the fluid system's velocity profile and interaction energy can reflect the shear-thinning characteristics of the fluids.The velocity profile resembles a top-hat shape,intensifying as the fluid's power law index decreases.The interaction energy between the wall and the fluid decreases gradually with increasing velocity,and a high concentration of non-Newtonian fluid reaches a plateau sooner.Moreover,the velocity profile of the fluid is related to the molecule number density distribution and their values are inversely proportional.By analyzing the radial distribution function,we found that the hydrogen bonds between solute and water molecules weaken with the increase in velocity.This observation offers an explanation for the shear-thinning phenomenon of the non-Newtonian flow from a micro perspective.

Overall Assessment of Heat Transfer for a Rarefied Flow in a Microchannel with Obstacles Using Lattice Boltzmann Method

The objective of this investigation is to assess the effect of obstacles on numerical heat transfer and fluid flow momentum in a rectangular microchannel(MC).Two distinct configurations were studied:one without obstacles and the other with alternating obstacles placed on the upper and lower walls.The research utilized the thermal lattice Boltzmann method(LBM),which solves the energy and momentum equations of fluids with the BGK approximation,implemented in a Python coding environment.Temperature jump and slip velocity conditions were utilized in the simulation for the MC and extended to all obstacle boundaries.The study aims to analyze the rarefaction effect,with Knudsen numbers(Kn)of 0.012,0.02,and 0.05.The outcomes indicate that rarefaction has a significant impact on the velocity and temperature distribution.The presence of nine obstacles led to slower fluid movement inside the microchannel MC,resulting in faster cooling at the outlet.In MCs with obstacles,the rarefaction effect plays a crucial role in decreasing the Nusselt number(Nu)and skin friction coefficient(Cf).Furthermore,the study demonstrated that the obstacles played a crucial role in boosting fluid flow and heat transfer in the MC.The findings suggest that the examined configurations could have potential applications as cooling technologies in micro-electro-mechanical systems and microdevice applications.

Numerical Simulation of Droplet Generation in Coaxial Microchannels

In this study,numerical simulations of the pinching-off phenomena displayed by the dispersed phase in a continuous phase have been conducted using COMSOL Multiphysics(level-set method).Four flow patterns,namely“drop flow”,“jet flow”,“squeeze flow”,and“co-flow”,have been obtained for different flow velocity ratios,channel diameter ratios,density ratios,viscosity ratios,and surface tension.The flow pattern map of two-phase flow in coaxial microchannels has been obtained accordingly,and the associated droplet generation process has been critically discussed considering the related frequency,diameter,and pinch-off length.In particular,it is shown that the larger the flow velocity ratio,the smaller the diameter of generated droplets and the shorter the pinch-off length.The pinch-off length of a droplet is influenced by the channel diameter ratio and density ratio.The changes in viscosity ratio have a negligible influence on the droplet generation pinching frequency.With an increase in surface tension,the frequency of generation and pinch-off length of droplets decrease,but for small surface tension the generation diameter of droplet increases.

Effects of channel wall wettability on gas-liquid dynamics mass transfer under Taylor flow in a serpentine microchannel

The wall wettability of microchannels plays an important role in the gas-liquid mass transfer dynamics under Taylor flow.In this study,we regulated the contact angle of the wall surface through surface chemical grafting polymerization under controlled experimental conditions.The dynamic changes of CO_(2)bubbles flowing along the microchannel were captured by a high-speed video camera mounted on a stereo microscope,whilst a unit cell model was employed to theoretically investigate the gas-liquid mass transfer dynamics.We quantitatively characterized the effects of wall wettability,specifically the contact angle,on the formation mechanism of gas bubbles and mass transfer process experimentally.The results revealed that the gas bubble velocity,the overall volumetric liquid phase mass transfer coefficients(kLa),and the specific interfacial area(a)all increased with the increase of the contact angle.Conversely,gas bubble length and leakage flow decreased.Furthermore,we proposed a new modified model to predict the gas-liquid two-phase mass transfer performance,based on van Baten’s and Yao’s models.Our proposed model was observed to agree reasonably well with experimental observations.

Liquid-liquid two-phase flow in a wire-embedded concentric microchannel: Flow pattern and mass transfer performance

In this work, flow pattern and mass transfer of liquid-liquid two-phase flow in a wire-embedded concentric microchannel are studied using toluene-water system. Droplet flow, slug flow, oval flow and annular flow are observed in the wire-embedded concentric microchannel. The effects of embedded wires and physical properties on flow patterns are investigated. The embedded wire insert is conducive to the formation of annular flow. The flow pattern distribution regions are distinguished by the Caaq(capillary number)±We_(org)(Weber number) flow pattern map. When Weorg<0.001, slug flow is the main flow pattern, and when Weorg>0.1, annular flow is the main flow pattern. Oval flow and droplet flow are between We_(org)= 0.001-0.1, and oval flow is transformed into droplet flow with the increase of Caaq. The effect of flow rate, phase ratio, initial acetic acid concentration, insert shape and flow patterns on mass transfers are studied. Mass transfer process is enhanced under annular flow conditions, the volumetric mass transfer coefficient is up to 0.36 s^(-1) because of the high interfacial area and interface renewal rate of annular flow.

Microchannel reactors for Fischer-Tropsch synthesis:Experimental investigation and mathematical modeling

The Fischer-Tropsch synthesis is a significant technology for converting coal,natural gas,and biomass into synthetic fuels.In recent years,the use of microchannel reactors for the Fischer-Tropsch synthesis has attracted significant attention.Fischer-Tropsch synthesis experiments were carried out in a microchannel reactor and the influences of reaction conditions on the experimental results were investigated in this study.Based on the experimental data,a dynamic multi-component pseudo-homogeneous variable-volume flow model of microchannel reactors for the Fischer-Tropsch synthesis was built to determine the pressure-,velocity-,conversion-and(component-wise)concentration-distributions in reaction channels.The model takes into account the combined effects of gas volume expansion caused by the frictional pressure drop and gas volume contraction caused by reaction consumption.A novel effective method for calculating the pressure and superficial gas velocity values in microchannel reactors was proposed in the model.Besides that,two sets of experimental data were selected from references to evaluate the validity and accuracy of the model.The reaction performances in the microchannels were analyzed carefully based on the calculated results.

Study on liquid–liquid two-phase mass transfer characteristics in the microchannel with deformed insert

In this work, the liquid-liquid two-phase mass transfer characteristics in the microchannel with deformed insert were studied. The experiment used di-(2-ethylhexyl) phosphoric acid/kerosene-Cu^(2+)as the mass transfer evaluation system. The effects of some key factors such as the total flow velocity,channel inner diameter, channel length, insert diameter, extractant concentration on the extraction efficiency and mass transfer coefficient were systematically investigated. Compared with a simple microreactor, the liquid-liquid mass transfer enhancement effect of the insert was quantitatively analyzed. The study found that the regular deformation of the insert could cause fluid interface deformation and promote flow state chaos, effectively increasing the mass transfer rate. And the enhancement effect of the insert was more significant at high flow velocities. The highest mass transfer coefficient in the microchannel with deformed insert was 7.886 s^(-1), the enhancement factor could reach 4.17. And only needed 0.095 s to approach the extraction equilibrium. The deformed center insert exhibited an effective liquid-liquid mass transfer enhancement effect, which can be used as a micro-chemical process enhancement method to be applied in the fields of higher throughput mass transfer and chemical synthesis,and at the same time provide ideas for development and structural optimization of microreactors.

Experimental and Numerical Analysis of the Influence ofMicrochannel Size and Structure on Boiling Heat Transfer

Computational fluid dynamics was used and a numerical simulation analysis of boiling heat transfer in microchannels with three depths and three cross-sectional profiles was conducted.The heat transfer coefficient and bubble generation process of three microchannel structures with a width of 80μm and a depth of 40,60,and 80μm were compared during the boiling process,and the factors influencing bubble generation were studied.A visual test bench was built,and test substrates of different sizes were prepared using a micro-nano laser.During the test,the behavior characteristics of the bubbles on the boiling surface and the temperature change of the heated wall were collected with a high-speed camera and a temperature sensor.It was found that the microchannel with a depth of 80μm had the largest heat transfer coefficient and shortest bubble growth period,the rectangular channel had a larger peak heat transfer coefficient and a lower frequency of bubble occurrence,while the V-shaped channel had the shortest growth period,i.e.,the highest frequency of bubble occurrence,but its heat transfer coefficient was smaller than that of the rectangular channel.

Simulation of gas–liquid two-phase flow in a flow-focusing microchannel with the lattice Boltzmann method

A lattice Boltzmann method for gas–liquid two-phase flow involving non-Newtonian fluids is developed. Bubble formation in a flow-focusing microchannel is simulated by the method. The influences of flow rate ratio, surface tension,wetting properties, and rheological characteristics of the fluid on the two-phase flow are analyzed. The results indicate that the flow pattern transfers from slug flow to dry-plug flow with a sufficiently small capillary number. Due to the presence of three-phase contact lines, the contact angle has a more significant effect on the dry-plug flow pattern than on the slug flow pattern. The deformation of the front and rear meniscus of a bubble in the shear-thinning fluid can be explained by the variation of the capillary number. The reduced viscosity and increased contact angle are beneficial for the drag reduction in a microchannel. It also demonstrates the effectiveness of the current method to simulate the gas–liquid two-phase flow in a microchannel.

Bionic microchannels for step lifting transpiration

Those various cross-sectional vessels in trees transfer water to as high as 100 meters,but the traditional fabrication methods limit the manufacturing of those vessels,resulting in the non-availability of those bionic microchannels.Herein,we fabricate those bionic microchannels with various cross-sections by employing projection micro-stereolithography(PμSL)based 3D printing technique.The circumradius of bionic microchannels(pentagonal,square,triangle,and five-pointed star)can be as small as 100μm with precisely fabricated sharp corners.What's more,those bionic microchannels demonstrate marvelous microfluidic performance with strong precursor effects enabled by their sharp corners.Most significantly,those special properties of our bionic microchannels enable them outstanding step lifting performance to transport water to tens of millimeters,though the water can only be transported to at most 20 mm for a single bionic microchannel.The mimicked transpiration based on the step lifting of water from bionic microchannels is also achieved.Those precisely fabricated,low-cost,various cross-sectional bionic microchannels promise applications as microfluidic chips,long-distance unpowered water transportation,step lifting,mimicked transpiration,and so on.

In-flight deformation measurement for high-aspect-ratio wing based on 3D speckle correlation

In order to get an effective solution of the in-flight wing deformation measurement for high-wing aircrafts with high-aspect-ratio,a method based on three-dimensional(3D)speckle correlation technique is proposed.Firstly,an in-flight wing deformation measurement system with two sets of conjugate cameras is designed based on structural characteristics and test requirements of high-wing aircrafts with large-aspect-ratio.Secondly,the in-flight wing deformation measurement method based on 3D speckle correlation technique is introduced including three aspects:measuring system and wing datum calibration,speckle image matching and 3D reconstruction,and wing deformation analysis.Fi-nally,ground simulation test of dynamic deformation measurement of a scaled model wing and flight test of dynamic deformation measurement of a large transport wing are carried out.The test results show that the measuring accuracy of single point coordinate in ground simulation test is better than 0.1 mm/m,in the airborne vibration environment,the static single-point positioning accuracy is bet-ter than 5 mm,and the in-flight wing deformation measurement data is well received by the flight test engineers.This method can satisfy the requirements of stability,reliability,high precision,non-con-tact and full-field measurement for dynamic deformation measurement of aircraft wing with high-as-pect-ratio.

Numerical study on the hydrodynamics behavior of a central insert microchannel

In this work, the computational fluid dynamics method is used to study the liquid hydrodynamics behavior in the microchannel without central insert(MC1) and the central insert microchannel(MC2), respectively. The maximum deviation between simulation and experiment is 24%. The formations of flow patterns are explained based on contours and force analysis where the flow pattern maps are established by two-phase flow rate. The effects of aqueous phase viscosity and two-phase flow rate on the characteristic sizes of each flow pattern are also explored. Specifically, four unconventional flow patterns are found in MC2, namely the unique droplet flow, the unique slug flow, the unique coarse annular flow and the unique film annular flow. Though the insert occupies part of the channel, the pressure difference in the channel is significantly reduced compared with MC1. Moreover, the insert significantly changes the formation velocity range of each flow pattern, greatly broadens the formation range of annular flow and also has an important influence on the characteristic size of the flow pattern. The organic-phase dimensionless axial size(Lo/W) and the dimensionless radial size(Do/W) of the droplet(slug) are negatively related to the aqueous-phase viscosity(μa) and flow rate(ua). The Do/W of the annular is negatively correlated with μaand positively correlated with organic-phase flow rate(uo). This study provides direct numerical evidence that the insert is key to the formation of bicontinuous phase flow pattern, as well as further strengthens our understanding of the flow characteristics and optimization design of insert microchannels.

Study on an improved rotating microchannel separator in the intensification for demulsification and separation process

An improved rotating microchannel(IRM) separator was further explored in the intensification for demulsification and separation process. Oil-in-water(O/W) emulsion system of 2-ethylhexyl phosphoric acid-2-ethylhexyl ester(P507)–water without emulsifier was employed to evaluate the performance of the new equipment. In this experiment, the influence on demulsification separation process was explored by changing the geometrical structure and channel height of the microchannel and combining the liquid–liquid two-phase flow pattern, and the correlation general graph between demulsification efficiency and dimensionless parameters was established. The total demulsification effect of the IRM and the separation capacity of the clear organic phase recovered from demulsification are significantly improved. In addition, the liquid–liquid two-phase flow pattern of the clear organic phase after demulsification and the remaining emulsion in the IRM are observed and recorded by high-speed photography. The separation ability of organic phase from the upper outlet can be significantly improved when the total demulsification rate of IRM is up to 90%. There are 3 types and 6 kinds of flow patterns observed. The results demonstrated that the suitable demulsification performance is obtained when the liquid–liquid two-phase inside the IRM is in a parallel pattern. Finally, the relation map between total demulsification efficiency and the universal flow is drawn, which provides a basis for the accurate control of the IRM device.

Computer simulation of Cu:AlOOH/water in a microchannel heat sink using a porous media technique and solved by numerical analysis AGM and FEM

Extensive improvements in small-scale thermal systems in electronic circuits,automotive industries,and microcomputers conduct the study of microsystems as essential.Flow and thermic field characteristics of the coherent nanofluid-guided microchannel heat sink are described in this perusal.The porous media approximate was used to search the heat distribution in the expanded sheet and Cu:γ-AlOOH/water.A hybrid blend of Boehme copper and aluminum nanoparticles is evaluated to have a cooling effect on the microchannel heat sink.By using Akbari Ganji and finite element methods,linear and non-linear differential equations as well as simple dimensionless equations have been analyzed.The purpose of this study is to investigate the fluid and thermal parameters of copper hybrid solution added to water,such as Nusselt number and Darcy number so that we can reach the best cooling of the fluid.Also,by installing a piece of fin on the wall of the heat sink,the coefficient of conductive heat transfer and displacement heat transfer with the surrounding air fluid increases,and the efficiency of the system increases.The overall results show that expanding values on the NP(series heat transfer fluid system maximizes performance with temperatures)volume division of copper,as well as boehmite alumina particles,lead to a decrease within the stream velocity of the Cu:AlOOH/water.Increasing the volume fraction of nanoparticles in the hybrid mixture decreases the temperature of the solid surface and the hybrid nanofluid.The Brownian movement improves as the volume percentage of nanoparticles in the hybrid mixture grows,spreading the heat across the environment.As a result,heat transmission rates rise.As the Darcy number increases,the thermal field for solid sections and Cu:AlOOH/water improves.

Different efficiency toward the biomimetic aerobic oxidation of benzyl alcohol in microchannel and bubble column reactors: Hydrodynamic characteristics and gas–liquid mass transfer

The selective aerobic oxidation of benzyl alcohol to benzaldehyde has attracted considerable attention because benzaldehyde is a high value-added product. The rate of this typical gas–liquid reaction is significantly affected by mass transfer. In this study, CoTPP-mediated(CoTPP: cobalt(II) mesotetraphenylporphyrin) selective benzyl alcohol oxidation with oxygen was conducted in a membrane microchannel(MMC) reactor and a bubble column(BC) reactor, respectively. We observed that 83% benzyl alcohol was converted within 6.5 min in the MMC reactor, but only less than 10% benzyl alcohol was converted in the BC reactor. Hydrodynamic characteristics and gas–liquid mass transfer performances were compared for the MMC and BC reactors. The MMC reactor was assumed to be a plug flow reactor,and the dimensionless variance was 0.29. Compared to the BC reactor, the gas–liquid mass transfer was intensified significantly in MMC reactor. It could be ascribed to the high gas holdup(2.9 times higher than that of BC reactor), liquid film mass transfer coefficient(8.2 times higher than that of BC reactor), and mass transfer coefficient per unit interfacial area(3.8 times higher than that of BC reactor). Moreover,the Hatta number for the MMC reactor reached up to 0.61, which was about 15 times higher than that of the BC reactor. The computational fluid dynamics calculations for mass fractions in both liquid and gas phases were consistent with the experimental data.

微通道技术在颈椎管肿瘤微创切除术中的应用

目的:探讨微通道技术在颈椎管肿瘤微创切除手术中的应用。方法:选择2017年2月至2020年3月于北京大学第三医院神经外科因颈椎管肿物行微创切除手术的51例患者的病例资料进行回顾性分析,其中5例患者肿物位于硬膜外,6例位于硬膜内外,40例位于髓外硬膜下(6例位于脊髓腹侧)。肿物最大径0.5~3.0 cm。患者临床表现包括颈肩部或上肢疼痛43例,感觉障碍(麻木)22例,肢体无力8例。所有患者均采用微通道锁孔技术进行肿瘤显露,显微镜下进行肿瘤切除手术。结果:35例切除半侧椎板,12例采用半侧椎板间开窗,2例在半椎板切除或半侧椎板间开窗基础上切除内侧1/4小关节,经解剖间隙(未切除骨质)2例。肿瘤切除程度包括50例全部切除和1例次全切除。肿瘤性质包括36例神经鞘瘤,12例脊膜瘤,2例肠源性囊肿和1例皮样囊肿。术后无感染、脑脊液漏。7例患者出现肢体麻木。平均随访时间15个月(3~36个月),未发现颈椎不稳或后突等畸形,肿瘤均无复发。结论:颈椎管较为宽大,不超过三个节段的颈椎管肿瘤可借助微通道技术进行充分显露,除髓内或恶性肿瘤外,可显微外科切除。患者术后更多颈椎正常结构和肌肉附着得到保留,有利于颈椎保持正常功能,神经电生理监测的辅助有助于避免脊髓或神经损伤。

仿生正形尾鳍结构微通道流动与传热特性数值研究及结构优化

基于鱼类尾鳍在流动过程中的重要作用,仿生设计并建立了正形尾鳍结构微通道物理模型,研究了仿生尾鳍肋间距对微通道综合传热性能的影响,提取了尾鳍结构凹陷长轴相对长度Dt/Wch、决定尾鳍结构最大宽度的bt/Wch以及决定尾鳍结构最大高度的张角αt三个结构参数,数值分析了结构参数对仿生微通道平均摩擦因数、平均Nusselt数、综合性能因子以及热阻和泵功的影响规律;并通过均匀试验分析回归拟合和遗传算法多目标优化得到了正形尾鳍结构参数的最优解。结果表明,综合性能因子随着仿生肋间距的增大而减小,当间距为4 mm时,正形尾鳍结构微通道的综合性能因子变化范围为1.74~1.89,综合传热性能明显优于矩形光滑通道;随着Dt/Wch的增加、bt/Wch的减小或αt的增加,尾鳍结构微通道的综合传热性能提升,其中凹陷长轴相对长度Dt/Wch和高度张角αt影响较大。结构参数为Dt/Wch=0.476、bt/Wch=0.135、αt=146.96°时,微通道性能有较大提升。

液相剥离法高效制备石墨烯的研究进展

石墨烯是一种具有优良物理化学性质的二维纳米材料,广泛应用于电池、催化、传感器、印刷、生物医药等领域。然而,石墨烯及其衍生产品的应用与发展面临着巨大挑战——低成本、高品质、规模化生产。本文综述了液相剥离法高效制备石墨烯的研究进展,重点探讨了电化学插层法、溶剂插层法、高温膨胀法和微波膨胀法等液相剥离的前处理方法原理以及对石墨烯剥离效果的影响;分析了水基溶剂、有机溶剂和混合溶剂等剥离溶剂的优缺点与选取原则;对比了超声、高剪切和微通道等过程强化设备的剥离原理和优缺点;简述了离心分离的后处理方法以及分离效果;最后对液相剥离法宏量制备石墨烯的发展趋势进行了展望:通过结合人工智能等方法进行多目标优化,开发无残留的功能化插层剂并匹配温和快速的膨胀方法,寻找低毒、低沸点、高分散的溶剂体系,精确调控液相剥离设备作用机理,设计连续化梯级离心设备,实现液相剥离制备石墨烯的连续化、规模化、低成本快速制备。

家用空气源热泵热水器微通道冷凝器仿真研究

为提升家用空气源热泵热水器水箱外微通道冷凝器的性能,建立了家用空气源热泵热水器水箱外微通道冷凝器的准稳态模型,实验验证结果显示模型误差能控制在±9%以内。依据模型对微通道冷凝器的换热量、换热系数、压降等性能进行仿真研究。结果发现,微通道冷凝器为四流程时能够减小压降并保证换热量满足需要;关键参数微通道截面的最佳尺寸在1.6~2.6 mm^(2)系统性能最优,同时扁管内微通道数应控制在16~24根。研究结果为家用空气源热泵热水器微通道冷凝器的设计提供借鉴。