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.

Microchannel reactive distillation for the conversion of aqueous ethanol to ethylene

Here we demonstrate the proof-of-concept for microchannel reactive distillation for alcohol-to-jet application:combining ethanol/water separation and ethanol dehydration in one unit operation.Ethanol is first distilled into the vapor phase,converted to ethylene and water,and then the water co-product is condensed to shift the reaction equilibrium.Process intensification is achieved through rapid mass transfer-ethanol stripping from thin wicks using novel microchannel architectures-leading to lower residence time and improved separation efficiency.Energy savings are realized with integration of unit operations.For example,heat of condensing water can offset vaporizing ethanol.Furthermore,the dehydration reaction equilibrium shifts towards completion by immediate removal of the water byproduct upon formation while maintaining aqueous feedstock in the condensed phase.For aqueous ethanol feedstock(40%_w),71% ethanol conversion with 91% selectivity to ethylene was demonstrated at 220℃,600psig,and 0.28 h^(-1) wt hour space velocity.2.7 stages of separation were also demonstrated,under these conditions,using a device length of 8.3 cm.This provides a height equivalent of a theoretical plate(HETP),a measure of separation efficiency,of ^(3).3 cm.By comparison,conventional distillation packing provides an HETP of ^(3)0 cm.Thus,9,1 × reduction in HETP was demonstrated over conventional technology,providing a means for significant energy savings and an example of process intensification.Finally,preliminary process economic analysis indicates that by using microchannel reactive distillation technology,the operating and capital costs for the ethanol separation and dehydration portion of an envisioned alcoholto-jet process could be reduced by at least 35% and 55%,respectively,relative to the incumbent technology,provided future improvements to microchannel reactive distillation design and operability are made.

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.

Effect of boundary slip on electroosmotic flow in a curved rectangular microchannel

The aim of this study is to numerically investigate the impact of boundary slip on electroosmotic flow(EOF) in curved rectangular microchannels. Navier slip boundary conditions were employed at the curved microchannel walls. The electric potential distribution was governed by the Poisson–Boltzmann equation, whereas the velocity distribution was determined by the Navier–Stokes equation. The finite-difference method was employed to solve these two equations. The detailed discussion focuses on the impact of the curvature ratio, electrokinetic width, aspect ratio and slip length on the velocity. The results indicate that the present problem is strongly dependent on these parameters. The results demonstrate that by varying the dimensionless slip length from 0.001 to 0.01 while maintaining a curvature ratio of 0.5 there is a twofold increase in the maximum velocity. Moreover, this increase becomes more pronounced at higher curvature ratios. In addition, the velocity difference between the inner and outer radial regions increases with increasing slip length. Therefore, the incorporation of the slip boundary condition results in an augmented velocity and a more non-uniform velocity distribution. The findings presented here offer valuable insights into the design and optimization of EOF performance in curved hydrophobic microchannels featuring rectangular cross-sections.

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.

Gas microchannel plate-pixel detector for X-ray polarimetry

POLAR-2 is a gamma-ray burst(GRB)polarimeter that is designed to study the polarization in GRB radiation emissions,aiming to improve our knowledge of related mechanisms.POLAR-2 is expected to utilize an on-board polarimeter that is sensitive to soft X-rays(2-10 keV),called low-energy polarization detector.We have developed a new soft X-ray polari-zation detector prototype based on gas microchannel plates(GMCPs)and pixel chips(Topmetal).The GMCPs have bulk resistance,which prevents charging-up effects and ensures gain stability during operation.The detector is composed of low outgassing materials and is gas-sealed using a laser welding technique,ensuring long-term stability.A modulation factor of 41.28%±0.64% is obtained for a 4.5 keV polarized X-ray beam.A residual modulation of 1.96%±0.58% at 5.9 keV is observed for the entire sensitive area.

Applying the Shearlet-Based Complexity Measure for Analyzing Mass Transfer in Continuous-Flow Microchannels

Continuous-flow microchannels are widely employed for synthesizing various materials,including nanoparticles,polymers,and metal-organic frameworks(MOFs),to name a few.Microsystem technology allows precise control over reaction parameters,resulting in purer,more uniform,and structurally stable products due to more effective mass transfer manipulation.However,continuous-flow synthesis processes may be accompanied by the emergence of spatial convective structures initiating convective flows.On the one hand,convection can accelerate reactions by intensifying mass transfer.On the other hand,it may lead to non-uniformity in the final product or defects,especially in MOF microcrystal synthesis.The ability to distinguish regions of convective and diffusive mass transfer may be the key to performing higher-quality reactions and obtaining purer products.In this study,we investigate,for the first time,the possibility of using the information complexity measure as a criterion for assessing the intensity of mass transfer in microchannels,considering both spatial and temporal non-uniformities of liquid’s distributions resulting from convection formation.We calculate the complexity using shearlet transform based on a local approach.In contrast to existing methods for calculating complexity,the shearlet transform based approach provides a more detailed representation of local heterogeneities.Our analysis involves experimental images illustrating the mixing process of two non-reactive liquids in a Y-type continuous-flow microchannel under conditions of double-diffusive convection formation.The obtained complexity fields characterize the mixing process and structure formation,revealing variations in mass transfer intensity along the microchannel.We compare the results with cases of liquid mixing via a pure diffusive mechanism.Upon analysis,it was revealed that the complexity measure exhibits sensitivity to variations in the type of mass transfer,establishing its feasibility as an indirect criterion for assessing mass transfer intensity.The method presented can extend beyond flow analysis,finding application in the controlling of microstructures of various materials(porosity,for instance)or surface defects in metals,optical systems and other materials that hold significant relevance in materials science and engineering.

A comprehensive review on microchannel heat sinks for electronics cooling

The heat generation of electronic devices is increasing dramatically,which causes a serious bottleneck in the thermal management of electronics,and overheating will result in performance deterioration and even device damage.With the development of micro-machining technologies,the microchannel heat sink(MCHS)has become one of the best ways to remove the considerable amount of heat generated by high-power electronics.It has the advantages of large specific surface area,small size,coolant saving and high heat transfer coefficient.This paper comprehensively takes an overview of the research progress in MCHSs and generalizes the hotspots and bottlenecks of this area.The heat transfer mechanisms and performances of different channel structures,coolants,channel materials and some other influencing factors are reviewed.Additionally,this paper classifies the heat transfer enhancement technology and reviews the related studies on both the single-phase and phase-change flow and heat transfer.The comprehensive review is expected to provide a theoretical reference and technical guidance for further research and application of MCHSs in the future.

Air-Side Heat Transfer Performance Prediction for Microchannel Heat Exchangers Using Data-Driven Models with Dimensionless Numbers

This study explores the effectiveness of machine learning models in predicting the air-side performance of microchannel heat exchangers.The data were generated by experimentally validated Computational Fluid Dynam-ics(CFD)simulations of air-to-water microchannel heat exchangers.A distinctive aspect of this research is the comparative analysis of four diverse machine learning algorithms:Artificial Neural Networks(ANN),Support Vector Machines(SVM),Random Forest(RF),and Gaussian Process Regression(GPR).These models are adeptly applied to predict air-side heat transfer performance with high precision,with ANN and GPR exhibiting notably superior accuracy.Additionally,this research further delves into the influence of both geometric and operational parameters—including louvered angle,fin height,fin spacing,air inlet temperature,velocity,and tube temperature—on model performance.Moreover,it innovatively incorporates dimensionless numbers such as aspect ratio,fin height-to-spacing ratio,Reynolds number,Nusselt number,normalized air inlet temperature,temperature difference,and louvered angle into the input variables.This strategic inclusion significantly refines the predictive capabilities of the models by establishing a robust analytical framework supported by the CFD-generated database.The results show the enhanced prediction accuracy achieved by integrating dimensionless numbers,highlighting the effectiveness of data-driven approaches in precisely forecasting heat exchanger performance.This advancement is pivotal for the geometric optimization of heat exchangers,illustrating the considerable potential of integrating sophisticated modeling techniques with traditional engineering metrics.

Effect of the Geometrical Parameter of OpenMicrochannel on Pool Boiling Enhancement

High heat dissipation is required for miniaturization and increasing the power of electronic systems.Pool boiling is a promising option for achieving efficient heat dissipation at low wall superheat without the need for moving parts.Many studies have focused on improving heat transfer efficiency during boiling by modifying the surface of the heating element.This paper presents an experimental investigation on improving pool boiling heat transfer using an open microchannel.The primary goal of this work is to investigate the impact of the channel geometry characteristics on boiling heat transfer.Initially,rectangular microchannels were prepared on a circular copper test piece with a diameter of 20 mm.Then,the boiling characteristics of these microchannels were compared with those of a smooth surface under saturated conditions using deionized water.In this investigation,a wire-cutting electrical discharge machine(EDM)machine was used to produce parallel microchannels with channel widths of 0.2,0.4,and 0.8 mm.The fin thicknesses were 0.2,0.4,and 0.6 mm,while the channel depth remained constant at 0.4 mm.The results manifested that the surface featuring narrower fins and broader channels achieved superior performance.The heat transfer coefficient(HTC)was enhanced by a maximum of 248%,and the critical heat flux(CHF)was enhanced by a maximum of 101%compared to a plain surface.Eventually,the obtained results were compared with previous research and elucidated a good agreement.

竖插翅片微通道换热器空气侧积灰特性研究

竖插翅片微通道换热器表面积尘导致其长效性能衰减。文章建立了颗粒物沉积仿真模型,研究了竖插翅片表面积灰分布特性及其对换热器性能的影响。结果表明,积灰分布特性受颗粒物粒径和空气流速的显著影响:小粒径颗粒物主要沉积在翅片迎风面的百叶窗区域,而大粒径颗粒物则沉积在背风面。沉积率随粒径增加先增加后减小,峰值为粒径10μm。小粒径颗粒物沉积率随空气流速增大而增加,大粒径颗粒物沉积率则先增大后减小。积灰厚度的增加导致Nu下降,f因子增加以及性能评价因子(PEC)减小。当Re=1100时,0.15mm积灰厚度使Nu降低了4.0%,f增加了61.6%,PEC减少了18.2%。研究结果为竖插翅片换热器的长效性能提升提供了理论依据。

Growth behavior of CVD diamond in microchannels of Cu template

Deposition of diamond inside the trenches or microchannels by chemical vapor deposition （CVD） is limited by the diffusion efficiency of important radical species for diamond growth （H, CH3） and the pore depth of the substrate template. By ultrasonic seeding with nanodiamond suspension, three-dimensional （3D） penetration structure diamond was successfully deposited in cylindrical microchannels of Cu template by hot-filament chemical vapor deposition. Micro-Raman spectroscopy and scanning electron microscopy （SEM） were used to characterize diamond film and the effects of microchannel depth on the morphology, grain size and growth rate of diamond film were comprehensively investigated. The results show that diamond quality and growth rate sharply decrease with the increase of the depth of cylindrical microchannel. Individual diamond grain develops gradually from faceted crystals into micrometer cluster, and finally to ballas-type nanocrystalline one. In order to modify the rapid decrease of diamond quality and growth rate, a new hot filament apparatus with a forced gas flow through Cu microchannels was designed. Furthermore, the growth of diamond film by new apparatus was compared with that without a forced gas flow, and the enhancement mechanism was discussed.

微通道换热器中制冷剂分布的量化:红外热成像法与电容法

微通道换热器具有高比表面积,高换热系数,高热流密度等优势,应用于多个领域。然而,两相流流量分布不均问题导致了其换热量有所损失。目前,可视化、光学法、超声波等方法应用于两相流研究,但在工程应用上仍有一定问题。因此,该研究提出了以红外热成像以及电容法作为研究微通道换热器流量分布的手段。结合红外热成像图片以及换热器模拟仿真,可以获得微通道管的入口状态的分布情况。通过在集流管内放入电容传感器,可以估算集流管内的干度分布。通过这两种方法,可以有效地量化微通道换热器的制冷剂两相流分布情况,从而深入探究分液不均的机理与改善分液的方法。

液体脂肪酶催化制备甘油二酯油连续化工艺研究

旨在强化酶催化大豆油的甘油解反应为甘油二酯油的工业化生产提供技术支持,以大豆油和甘油为原料,液体脂肪酶为催化剂,利用微通道反应器制备甘油二酯油。通过单因素实验考察了液体脂肪酶用量、大豆油与甘油质量比、反应温度、反应时间对反应粗产物中甘油二酯质量分数的影响,设计中试放大实验考察工艺的稳定性,并采用分子蒸馏对中试粗产物进行纯化。结果表明:甘油二酯油制备的最佳工艺条件为液体脂肪酶(10 000 U/mL)用量3%(以大豆油质量计)、大豆油与甘油质量比10∶1、反应温度50℃、反应时间9.0 s,在此条件下粗产物中甘油二酯的质量分数达44%;经中试实验放大后,粗产物中各组分(游离脂肪酸、单甘酯、甘油二酯和甘油三酯)的质量分数无明显变化,甘油二酯的质量分数为42%,反应可在微通道反应器中连续进行;经分子蒸馏纯化后,产物中甘油二酯的质量分数达到48%。综上,通过微通道反应器可以实现液体脂肪酶催化大豆油甘油解制备甘油二酯油的连续化生产,且该方法稳定性较好。

微通道换热器专利技术综述

目前微通道换热器技术已经广泛应用在汽车、家用空调、除湿机以及热泵热水器等行业中。文章针对微通道换热器的专利申请情况进行检索、统计和分析,梳理微通道换热器技术的主要技术分支,分析中外申请人专利申请的不同侧重点,介绍每个行业中微通道换热器的专利申请情况、主要申请人、所面临的具体技术需求以及重点专利,指出了国内申请人应当保持挖掘技术需求和解决技术问题的积极性,借鉴成熟的专利技术,同时继续拓展微通道换热器的应用领域,以期望进一步推动微通道换热器技术的发展。

AIN Monolithic Microchannel Cooled Heatsink for High Power Laser Diode Array

A novel AIN monolithic microchannel cooled heatsink for high power laser diode array is introduced.The high power stack laser diode array with an AIN monolithic microchannel heatsink is fabricated and tested.The thermal impedance of a 10 stack laser diode array is 0 121℃/W.The pitch between two adjacent bars is 1 17mm.The power level of 611W is achieved under the 20% duty factor condition at an emission wavelength around 808nm.

微通道气液两相环状流传热特性及场协同角分析

微通道内流体传热特性及机制的研究是微反应器调控、传递强化及设计的基础。对于微通道内多相流体系,各流动相的流动、分散等特征直接影响多相体系的传热特性,决定了微通道传热效率。利用数值方法研究十字型微通道气液两相环状流传热性能。在环状流传热过程中,气相引入一方面让气液间的剪切力带动液相提高了液相流速;另一方面增加扰动从而减小了场协同角,两方面共同作用强化环状流总传热性能。从环状流液膜波动出发,引入场协同角研究液膜波动与环状流局部传热性能的关系。在液膜波动处局部场协同角变小,场协同角越小,高速气相与高温液相之间的热量交换越强,因此高速气相带走更多的热量,从而强化了环状流的总传热性能。最后根据研究初步绘制传热流程图,为微反应器内部温度控制和传热强化技术提供理论支撑。

挤压扁管和成型扁管微通道换热器长效特性研究

对喷锌挤压扁管和成型扁管微通道换热器样品进行了SWAAT腐蚀前后热力性能对比分析研究。挤压扁管微通道换热器样品SWAAT测试400h出现翅片腐蚀现象,800h后出现大面积翅片脱落。换热性能在400h后出现下降,800h后剧烈下降,最大性能衰减20%;成型扁管微通道换热器样品在腐蚀测试时长内未发现翅片与扁管脱离现象,性能衰减缓慢,最大性能下降10%左右。腐蚀对两种扁管样品均造成风阻增加,最大为40%。在测试时长内挤压扁管样品风阻震荡上升,成型扁管样品风阻单调增加。

Liquid–liquid two-phase mass transfer characteristics in a rotating helical microchannel

In this work,the mass transfer characteristics of two immiscible fluids were investigated in a rotating helical microchannel with hydraulic diameter of 932μm.Aqueous phosphoric acid solution and 80%tri-n-butyl phosphate(TBP)in kerosene were selected for the investigation of mass transfer performance in quartz glass/high density polyethylene(HDPE)microchannel.High dispersion between the two immiscible fluids can be obtained in the microchannel due to the intensifying action of centrifugal force,and the majority of the droplets with average diameter of 20–100μm were produced in the microchannel.The flow rate and rotation speed were found to have great effects on the extraction efficiency and average residence time.The empirical correlation of average residence time based on experimental data was developed by theoretical analysis and data fitting method,and a mathematical model of the mass transfer coefficient in dispersed phase was proposed.

Laminar flow of micropolar fluid in rectangular microchannels

Compared with the classic flow on macroscale, flows in microchannels have some new phenomena such as the friction increase and the flow rate reduction. Papautsky and co-workers explained these phenomena by using a micropolar fluid model where the effects of micro-rotation of fluid molecules were taken into account. But both the curl of velocity vector and the curl of micro-rotation gyration vector were given incorrectly in the Cartesian coordinates and then the micro-rotation gyration vector had only one component in the z-direction. Besides, the gradient term of the divergence of micro-rotation gyration vector was missed improperly in the angular moment equation. In this paper, the governing equations for laminar flows of micropolar fluid in rectangular microchannels are reconstructed. The numerical results of velocity profiles and micro-rotation gyrations are obtained by a procedure based on the Chebyshev collocation method. The micropolar effects on velocity and micro-rotation gyration are discussed in detail.