NUMERICAL ANALYSIS OF GASEOUS FLOW IN MICRO-CHANNELS

The algorithm of gaseous flow in bi-dimensional micro-channels is set up andthe corresponding program based on micro-flow theory is presented. Gaseous flow in micro-channels isnumerically analyzed and the pressure drop along the duct as well .as the velocity profile in themicro-channels is obtained. The numerical results agreed well with the experimental results in thereferences. Moreover, the effects of Kn, sigma_v and Re on the velocity profiles are analyzed. It isfound that for Kn>0.001, with increasing Kn number, the slip velocity on the wall boundaryincreases; the tangential momentum coefficient sigma_v affects the slip velocity greatly. The slipvelocity increases with decreasing a, In the slip flow regime and for low Re numbers, the slipvelocity is little influenced by the Re number.

A Review on Flow Boiling of the Fluid with Lower Boiling Point in Micro-Channels

With the advancement of micro machining technology,the high-heat-flux removal from miniature electronic devices and components has become an attractive topic.Flow boiling in micro-channels is an optimal form of heat transfer and has been widely employed in high-heat-flux cooling applications.This comprehensively-reviewed article focused on the available recent literatures of experimental investigation regarding the flow boiling heat transfer and unstable behaviors of the fluid with lower boiling point in micro-channels.The thermal-fluid characteristics and potential heat transfer mechanisms of low-boiling-point fluids flow boiling in different narrow passages were summarized and discussed.The literatures regarding the pressure drop and occurrence of the unstable phenomena existing in two-phase flow boiling process were also discussed.The emphasis was given to the heat transfer enhancement methods as well as instability elimination,and various methods such as modification of surface and channel flow geometries were considered.Some future researches in the field of micro-scale flow boiling were suggested.

Fabrication of Highly Porous Interconnected Three-dimensional Scaffolds with Micro-channels

A novel highly porous 3-D poly（e-caprolactone） （PCL） scaffold with micro-channels was fabricated by injection molding and diluent acetic acids leaching technologies. In this study, the chitosan fiber was employed to form the microchannel in PCL matrix. The morphology, porosity and mechanical properties of the scaffolds were studied and calculated. It was found that the larger the content of chitosan fiber is, the higher the porosity would be, due to the volumetric expansion of chitosan fiber in PCL matrix during it being leached. In addition, the less the content of chitosan fiber is, the higher the compressive modulus would be.

Improved catalytic performance of Ni catalysts for steam methane reforming in a micro-channel reactor

Milliseconds process to produce hydrogen by steam methane reforming （SMR） reaction, based on Ni catalyst rather than noble catalyst such as Pd, Rh or Ru, in micro-channel reactors has been paid more and more attentions in recent years. This work aimed to further improve the catalytic performance of nickel-based catalyst by the introduction of additives, i.e., MgO and FeO, prepared by impregnation method on the micro-channels made of metal-ceramic complex substrate. The prepared catalysts were tested in the same micro-channel reactor by switching the catalyst plates. The results showed that among the tested catalysts Ni-Mg catalyst had the highest activity, especially under harsh conditions, i.e., at high space velocity and/or low reaction temperature. Moreover, the catalyst activity and selectivity were stable during the 12 h on stream test even when the ratio of steam to carbon （SIC） was as low as 1.0. The addition of MgO promoted the active Ni species to have a good dispersion on the substrate, leading to a better catalytic performance for SMR reaction.

RESEARCH ON DIFFUSION IN MICRO-CHANNEL FLOW DRIVEN BY ELECTROOSMOSIS

Numerical simulation using the finite differential method was carried out to analyze the diffusion of an impulse sample in the micro-channel driven by electroosmosis. The results show that the electrical field strength applied externally and the concentration of buffer solution play a significant role in the diffusion of sample, however, hydraulic diameter and aspect ratio of height to width of channel play a small role in it. Weakening the electrical field strength applied externally and the concentration of buffer solution properly can prevent the sample band from broadening effectively, and promote the efficiency of testing and separation as well as keep a faster speed of transport. The conclusions are helpful to the optimal design for micro-channel.

Flow characteristics of supersonic gas passing through a circular micro-channel under different inflow conditions

Gas flow in a micro-channel usually has a high Knudsen number. The predominant predictive tool for such a microflow is the direct simulation Monte Carlo(DSMC) method, which is used in this paper to investigate primary flow properties of supersonic gas in a circular micro-channel for different inflow conditions, such as free stream at different altitudes, with different incoming Mach numbers, and with different angles of attack. Simulation results indicate that the altitude and free stream incoming Mach number have a significant effect on the whole micro-channel flow field, whereas the angle of attack mainly affects the entrance part of micro-channel flow field. The fundamental mechanism behind the simulation results is also presented. With the increase of altitude, thr free stream would be partly prevented from entering into micro-channel.Meanwhile, the gas flow in micro-channel is decelerated, and the increase in the angle of attack also decelerates the gas flow. In contrast, gas flow in micro-channel is accelerated as free stream incoming Mach number increases. A noteworthy finding is that the rarefaction effects can become very dominant when the free stream incoming Mach number is low. In other words, a free stream with a larger incoming velocity is able to reduce the influence of the rarefaction effects on gas flow in the micro-channel.

Heat Transfer Performance and Structural Optimization of a Novel Micro-channel Heat Sink

With the advent of the 5G era,the design of electronic equipment is developing towards thinness,intelligence and multi-function,which requires higher cooling performance of the equipment.Micro-channel heat sink is promising for the heat dissipation of super-thin electronic equipment.In this study,thermal resistance theoretical model of the micro-channel heat sink was first established.Then,fabrication process of the micro-channel heat sink was introduced.Subsequently,heat transfer performance of the fabricated micro-channel heat sink was tested through the developed testing platform.Results show that the developed micro-channel heat sink has more superior heat dissipation performance over conventional metal solid heat sink and it is well suited for high power LEDs application.Moreover,the micro-channel structures in the heat sink were optimized by orthogonal test.Based on the orthogonal optimization,heat dissipation performance of the micro-channel radiator was further improved.

Modelling two-layer nanofluid flow in a micro-channel with electro-osmotic effects by means of Buongiorno’s model

A fully developed steady immiscible flow of nanofluid in a two-layer microchannel is studied in the presence of electro-kinetic effects.Buongiorno’s model is employed for describing the behavior of nanofluids.Different from the previous studies on two-layer channel flow of a nanofluid,the present paper introduces the flux conservation conditions for the nanoparticle volume fraction field,which makes this work new and unique,and it is in coincidence with practical observations.The governing equations are reduced into a group of ordinary differential equations via appropriate similarity transformations.The highly accurate analytical approximations are obtained.Important physical quantities and total entropy generation are analyzed and discussed.A comparison is made to determine the significance of electrical double layer(EDL)effects in the presence of an external electric field.It is found that the Brownian diffusion,the thermophoresis diffusion,and the viscosity have significant effects on altering the flow behaviors.

Flowing simulation of injection molded parts with micro-channel

In the micro-molding of component with a micro-sized channel, the ability for polymer melt to flowing into the micro-channel in a macro-sized part is a big challenge. The multidimensional flow behaviors are included in the injection molding the macro-component with a micro-channel. In this case, a simplified model is used to analyze the flow behaviors of the macro-sized part within a micro-channel. The flow behaviors in the macro-cavity are estimated by using the finite element and finite difference methods. The influence of the injection rate, micro-channel size, heat transfer coefficient, and mold temperature on the flowing distance is investigated based on the non-isothermal analytic method. The results show that an increase in the radius of the micro-channel and mold temperature can improve effectively the flowing distance in the micro-channel.

Modeling and Simulation of a Hybrid Jet-Impingement/Micro-Channel Heat Sink

With the progressive increase in the number of transistors that can be accommodated on a single integrated circuit,new strategies are needed to extract heat from these devices in an efficient way.In this regard methods based on the combination of the so-called“jet impingement”and“micro-channel”approaches seem extremely promising for possible improvement and future applications in electronics as well as the aerospace and biomedical fields.In this paper,a hybrid heat sink based on these two technologies is analysed in the frame of an integrated model.Dedicated CFD simulation of the coupled flow/temperature fields and orthogonal tests are performed in order to optimize the overall design.The influence of different sets of structural parameters on the cooling performance is examined.It is shown that an optimal scheme exists for which favourable performance can be obtained in terms of hot spot temperature decrease and thermal uniformity improvement.

A Numerical Investigation into the Influence of Electrode-Related Parameters on Electroosmotic Mixing and Related Mechanisms

Electroosmosis is an effective method for liquid mixing.It is associated with the motion of a liquid in a microchannel induced by an applied electric field.In this manuscript,a numerical model is elaborated and implemented for the case of a straight channel with a single electrode pair.In particular,the Navier-Stokes equation combined with the Convection-diffusion and Helmholtz-Smoluchowski equation are used to simulate the resulting flow field.The influence of various electrode parameters on the mixing efficiency and the related mechanisms are investigated.The numerical results show that a pair of eddies are produced alternately by the changing electric field.The two liquids are mixed by the interaction of this pair of eddies.The length of the electrode affects the distance between these eddies,while the amplitude and frequency of electrode voltage determine the intensity and frequency of the eddy current,respectively.It is shown that by tuning properly the electrode parameters,the mixing efficiency can reach 97.5%.The optimization process implemented in the present work may lead in the future to a new approach to obtain controllable electroosmotic flow in microfluidic platforms.

Scaling of the bubble/slug length of Taylor flow in a meandering microchannel

In order to reduce or avoid the fluctuations from interface breakup, a meandering microchannel with curved multi-bends(44 turns) is fabricated, and investigations of scaling bubble/slug length in Taylor flow in a rectangular meandering microchannel are systematically conducted. Based on considerable experimental data,quantitative analyses for the influences of two important characteristic times, liquid phase physical properties and aspect ratio are made on the prediction criteria for the bubble/slug length of Taylor flow in a meandering microchannel. A simple principle is suggested to predict the bubble formation period by using the information of Rayleigh time and capillary time for six gas–liquid systems with average deviation of 10.96%. Considering physical properties of the liquid phase and cross-section configuration of the rectangular mcirochannel,revised scaling laws for bubble length are established by introducing Ca, We, Re and W/h whether for the squeezing-driven or shearing-driven of bubble break. In addition, a simple principle in terms of Garstecki-type model and bubble formation period is set-up to predict slug lengths. A total of 107 sets of experimental data are correlated with the meandering microchannel and operating range: 0.001 b CaTPb 0.05, 0.06 b WeTPb 9.0,18 b ReTPb 460 using the bubble/slug length prediction equation from current work. The average deviation between the correlated data and the experimental data for bubble length and slug length is about 9.42% and9.95%, respectively.

Charge resolution in the isochronous mass spectrometry and the mass of ^(51)Co

Isochronous mass spectrometry(IMS)of heavyion storage rings is a powerful tool for the mass measurements of short-lived nuclei.In IMS experiments,masses are determined through precision measurements of the revolution times of the ions stored in the ring.However,the revolution times cannot be resolved for particles with nearly the same mass-to-charge(m/q)ratios.To overcome this limitation and to extract the accurate revolution times for such pairs of ion species with very close m/q ratios,in our early work on particle identification,we analyzed the amplitudes of the timing signals from the detector based on the emission of secondary electrons.Here,the previous data analysis method is further improved by considering the signal amplitudes,detection efficiencies,and number of stored ions in the ring.A sensitive Z-dependent parameter is introduced in the data analysis,leading to a better resolution of ^(34)Ar^(18+) and ^(51)Co^(27+) with A/Z=17/9.The mean revolution times of ^(34)Ar^(18+) and ^(51)Co^(27+) are deduced,although their time difference is merely 1.8 ps.The uncorrected,overlapped peak of these ions has a full width at half maximum of 7.7 ps.The mass excess of ^(51)Co was determined to be-27;332e41T keV,which is in agreement with the previous value of-27;342e48T keV.

Development of a new correlation to calculate permeability for flows with high Knudsen number

Flows with high Knudsen number play a prominent role in many engineering applications. The present study is an effort toward the simulation of flow with high Knudsen number using modified lattice Boltzmann method （LBM） through a porous medium in a channel. The effect of collision between molecules and solid walls, which is required to accurately simulate transition flow regime, is taken into account using a modified relaxation time. Slip velocity on the wall, which is another significant difficulty in simulating transition flow regime, is captured using the slip reflection boundary condition （SRBC）. The geometry of porous medium is considered as in-line and staggered. The results are in good agreement with previous works. A new correlation is obtained between permeability, Knudsen number and porosity for flows in transition flow regimes.

Mathematical modeling and numerical computation of the effective interfacial conditions for Stokes flow on an arbitrarily rough solid surface

The present work is concerned with a two-dimensional(2D)Stokes flow through a channel bounded by two parallel solid walls.The distance between the walls may be arbitrary,and the surface of one of the walls can be arbitrarily rough.The main objective of this work consists in homogenizing the heterogeneous interface between the rough wall and fluid so as to obtain an equivalent smooth slippery fluid/solid interface characterized by an effective slip length.To solve the corresponding problem,two efficient numerical approaches are elaborated on the basis of the method of fundamental solution(MFS)and the boundary element methods(BEMs).They are applied to different cases where the fluid/solid interface is periodically or randomly rough.The results obtained by the proposed two methods are compared with those given by the finite element method and some relevant ones reported in the literature.This comparison shows that the two proposed methods are particularly efficient and accurate.

Hydrodynamic Effect on the Iso-octane Steam-Reforming in a Monolithic Reactor Channel

The production process of clean hydrogen by iso-octane steam reforming in a micro-reactor under atmospheric pressure,and the high temperature was investigated.The simulation is done using momentum conservation,mass conservation,and convection-diffusion equations,represented by the Navier-Stocks equations,the continuity equation,and the Steffan-Maxwell equation respectively.The resolution was performed using a differential equations discretization into their conservative by the finite element method with an unconditionally stable scheme.An analysis of the hydrodynamics of the flow effect shows that the hydrogen produced by iso-octane reforming is proportional to the mixed flow Reynolds number.

Micro-Channel Heat Sink:A Review

The method to cool a high heat flux device is an important research direction for the heat exchanger design.Micro-channels are an eflfective heat exchange structure both for single-phase and two-phase flow.In this paper,the heat transfer correlations of single-phase,two-phase and nanofluid in a micro-channel are discussed and analyzed.The correlations of pressure drop for single-phase and two-phase fluids are also presented.Excluding the different working fluids used in the micro-channel,the diameter and aspect ratio,shape and structure,surface roughness,internal and external factor and layout of micro-channel pipe are considered to analyze their influence on the heat transfer performance and pressure drop.Micro-channel technology applications include industry,air-conditioning,solar energy systems,heat pipe technology and computer data center cooling.Compared to the conventional heat exchangers used in these fields,a micro-channel heat sink showed a much better heat transfer coefficient and low volume,indicating that it is a good choice and has huge potential for cooling application.Finally,existing problems and future scopes are described,and drawing up design standard,experimental and simulated methods for evaluating its performance are the urgent actions which need to be carried out.This review paper serves as guidance for researchers to design and predict the performance of micro-channel heat sinks.

Interplay of conducting and non-conducting walls on hydromagnetic natural convection flow in a vertical micro-channel with Hall current

Theoretical investigation on the interaction between conducting and nonconducting walls on hydromagnetic natural convection flow of viscous incompressible and electrically conducting fluid through a vertical micro-channel taking into account the effects of induced magnetic field in presence of Hall current is presented.Governing coupled equations responsible for the flow are obtained when either the micro-channel walls are electrically conducting or are electrically non-conducting.Using the method of undetermined coefficients,exact solution are obtained and presented in dimensionless form subject to relevant boundary conditions.Expressions for fluid velocity,induced magnetic field,skin friction,volume flow rate and induced current density in both primary and secondary flow directions are also obtained.Effects of some governing parameters like Hall current parameter,rarefaction parameter and Hartmann number on the different flow situations are given using the aid of line graphs and Tables.The main conclusion of the present analysis is that,in the existence of rarefaction parameter,primary fluid velocity could be enhanced with the increase in Hall parameter when the micro-channel walls are either insulated or when the left micro-channel wall is electrically conducting.Results obtained in this work are relevant in many magnetically controlled devices and could also be used as a benchmark in checking the accuracies of result obtained in some empirical experiments.

NUMERICAL SIMULATION OF TRANSVERSE DIFFUSION IN A MICROCHANNEL

The diffusion-based micro flow of a T-sensor with three inlets in whichvarious species are injected was simulated numerically. The results show that the Reynolds number isan important factor of affecting the efficiency of diffusion. The smaller the channel width is, themore strongly the species diffuse. The velocity gradient across the channel width plays a key rolein the diffusion of species. The conclusions are helpful to the design of micro-fluidic devices andthe analysis of data collected from such devices.