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In this paper the effects of the inlet fluid temperature on the electro-osmotic flow pattern in a two-dimensional microchannel with constant walls temperature is investigated with solving the governing equations by the Lattice Boltzmann method. The main objective of this research is to study the effects of temperature variations on the distribution of ions and consequently internal electric potential and velocity field. For make possible to use the Boltzmann ion distribution equation, cup mean temperature for every cross section of the microchannel is used. At the used Lattice Boltzmann method, LBGK model for modeling the Boltzmann collision function and the Zou-He boundary conditions method for velocity field has been used. Wang model for solving the Poisson-Boltzmann and He-Chen model for solving the energy equation has been used. The results show that, with increase the temperature difference between the inlet flow and the walls, the electro-osmotic flow rate increases. Also, observed that with decrease the external electric potential and the electric double layer thickness and increase the temperature difference at the inlet zone of the microchannel, a region with return flow is formed which can be used for controlling the internal flow pattern.

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Flow analysis in the microchannels has recently accelerated dramatically. In this paper, numerical investigation of Joule heating effects on the electroosmotic flow through a microchannel with the trapezoidal cross-section and constant wall temperature have been presented. The energy equation for the temperature distribution, Navier–Stokes equation for the velocity distribution and a Poisson equation for the electric potential distribution have been solved by using the finite-volume method in a system curvilinear coordinates. Thermophysical properties such as the dynamic viscosity and electric conductivity vary with temperature. Results show that by increasing the Joule number, the temperature, velocity and mass flow rate increase with constant EDL number. Without considering the Joule heating effects, the increments of EDL number causes in the mass flow rate to increase, but with considering the joule heating effects, the increasing of mass flow rate continues until EDL number 15 and after that the flow rate decreases. On the other hand, when the cross-section is reduced by the increasing aspect ratio, the joule number remains constant while the mean temperature decreases.

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Abstract- Hydrodynamics and Heat transfer of a gaseous flow in microchannels is performed numerically. Velocity and temperature at the channel inlet is uniform and the rarefaction effect is imposed to the problem via velocity slip and temperature jump boundary conditions, according to the slip flow regime. The channel is sufficiently long to reach fully developed flow at the outlet. The numerical methodology is based on the control volume finite difference scheme and discrete equations are solved using SIMPLE algorithm. Effects of various parameters such as viscous dissipation, rarefaction, axial conduction and thermal creep on heat transfer have been considered. The results indicate that the Nusselt number in microchannels has a different value than in conventional channels. Local Nu number is found to experience a jump by the presence of viscous dissipation. The magnitude of the jump is independ of the Brinkman number values. Heat transfer is affected in two opposite directions by rarefaction increasing. Also, as Peclet number increases, there is a weak increase in fully developed Nu number values but there is significant effect of Kn number on it.

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In this study, the forced convective heat transfer of pure water and alumina-water nanofluid with volume concentration of 0.5% and 1%, as a cooling fluid through a microchannel heat exchanger was experimentally investigated. This microchannel consists of 17 parallel channels with a rectangular cross section with 400 µm width, 560 µm height and 50 mm length. The experiments were performed in the range 600 to 1800 of Reynolds and constant heat flux conditions (19 W/cm2). Stability studies showed that alumina-water nanofluid at pH = 3 for 3 hours in a bath of the ultrasonic vibrating demonstrate the maximum stability. The variations of microchannel surface temperature, fluid temperature at the entrance region of the microchannel, average heat transfer coefficient of nanofluid and pure water, and their friction factor measured experimentally. Also comparison between average Nusselt number with existing heat transfer relationships was performed. The results show that heat transfer using nanofluid shows considerably increase in comparison to water. So that the maximum amount of average heat transfer coefficient for alumina-water nanofluid with 0.5% concentrations is about 32.8% and for alumina-water nanofluid with 1% concentrations about 49.7% in comparison to pure water. It was also found that the heat transfer coefficient increases with increasing Reynolds number and nanoparticle volume fraction.

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Emulsion consists of drops of one liquid dispersed into another immiscible liquid, is a novel technique for producing monodisperse droplets. The aim of this research is using the Lattice Boltzmann Method (LBM) to simulate two-phase flows in micro-channels to access the emulsification process. To this approach, The Index-Function Model proposed by He, is used to simulate drop formation in emulsification process in a co-flowing micro-channel with a complex geometry and three inlets. The simulation is performed to investigate the mechanism of drop generation due to dripping and jetting modes and the mode between them. Index function model, which is a new reliable model to evaluate two-phase flows, is applied to track the motion and deformation of the interface between the two immiscible fluids. Accuracy of our results is examined by two well-known basic analytical models including Relaxation of a rectangular drop and coalescence of two static droplets. Our results indicate good agreements with analytical data. The dimensionless numbers such as Capillary and Velocity ratio were used. The Capillary number is one of the most important dimensionless numbers in determination of fluid flow characteristics in micro-channels. The simulations reproduce dripping, widening jetting and narrowing jetting simultaneously in a coflowing microchannel in agreement with the experimental ones. This indicates that index function LBM model has a good accuracy and high stability to simulate this kind of flow.

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In this paper, magnetogasdynamics with outlet Knudsen of 0.2 is studied in a pressure-driven microchannel. By using a developed code, the effects of changing magnetic field parameters including power and length with implementation of slip velocity at the walls has been simulated numerically. The geometry is a two dimensional planar channel having a constant width through all. The flow is assumed to be laminar and steady in time. In order to analyze the variation of velocity, pressure, Lorentz force and induction magnetic field, the governing equations for flow and magnetic fields have been solved simultaneously using the lattice Boltzmann. No assumption of being constant for parameters like Knudsen and volumetric forces are made. Another feature of this research is to improve the quantity of results which is a major problem in this method and many studies have been done in this area. This study represents the results tending to that of analytical relations by using a second order accuracy for calculation of slip velocity and correction of pressure deviation curve in compare with the past studies if a proper relaxation time is determined. The simulation results show a change in Fx profile to M if the length of external magnetic field length reduces to 40% of the whole. Removing applied magnetic field from both ends of the channel will increase pressure gradient at the intermediate part and displaces the section at which the maximum pressure deviation occurs. Slip velocity and centerline velocity behave different for the reduced magnetic field length.

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Effects of different inlet/outlet arrangements on thermal performance of porous microchannel heat sink MCHS of any geometry has not been studied yet. In this investigation, the effects of utilization of four different inlet/outlet arrangements on electronic chip cooling utilizing trapezoidal MCHS with porous microchannels with porosity of 0.88 have been studied numerically. For this purpose, three dimensional simulations of laminar forced convection flow in microchannels and conduction in solid parts of MCHS by applying constant heat flux of 150 kWm-2 at its base plate have been performed utilizing the finite volume method and the commercial Ansys-CFX code. The results show that the A- and B-type arrangements, for wich the inlet and outlet are in direction of flow in the microchannels, have a better heat transfer performance, smaller thermal resistance and provide more uniform temperature distribution in the MCHS base plate. The results indicate that using porous media is effective in reducing the MCHS base plate temperature and in this regard the D-type arrangement has the best performance among the heat sinks studied. Considering both the positive effect of using porous media on increasing the heat transfer coefficient and its negative effect on increasing the required pumping power, the A-type arrangement has the best performance.

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In this paper, motion of a flexible membrane and hydrodynamic interaction of multiple membranes in a microchannel are simulated by developing a computer code written in C. The membranes are considered as flexible boundaries immersed in the fluid. First a single biconcave shaped membrane with high rigidity is considered. Due to the rigidity of the membrane, it experiences tumbling motion and its vertical displacement becomes oscillatory. Then, the effects of initial position of a circular membrane on its deformation, vertical velocity and displacement are investigated. It was observed that as the initial location of the membrane approaches the channel’s central axis, its vertical displacement and velocity decreased, but its horizontal velocity component increased. Finally, the simultaneous motion of multiple membranes in a microchannel and their interaction with each other and with flow are evaluated. The membranes do not collide and hence the collision mechanism is not modeled. It was found that the upstream membrane experienced greatest deformation and the greatest force was exerted on it by the fluid on it. In addition, simultaneous presence of multiple membranes would result in a reduction in the flow velocity. The current numerical results have good agreement with the available valid numerical ones.

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In this present study, the mixing of two incompressible miscible fluids with different density and viscosity has been investigated in a two-dimensional microchannel equipped with an oscillating stirrer in different excitation frequency. Although most studies in the field of fluid mixing, have been studied the mixer performance when the two fluids were absolutely identical, but the mixing make sense when two fluids has been non-uniformity such as different temperature, concentration or properties. The aim of this study is to evaluating the effect of various properties of the fluids in mixer performance and mixing value. Simulation has been performed in Re=100 and Sc=10, between 0.1 to 1 strouhal number by using element based finite volume method by means of commercial code CFX. Mixer performance has been evaluated in three different modes: mixing of two identical fluids, mixing of two fluids with different density and mixing of two fluids with different viscosity. The results show that, mixing of the fluids with different properties leads to change in mixer performance, and has unique performance in each case. In comparison with similar properties fluids, mixing of fluids with different viscosity and density show lesser inclined in mixing. It has been shown that variation of strouhal number has lesser effect on mixing index changes. The ratio of maximum mixing index changes to base mixing index in the case of different density and viscosity is 54.01 and 51.15 percent, respectively, while the value is 577.94 percent for the mixing of similar fluids.

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In this article, an immiscible two-phase flow in two dimensional ordinary and modified T-junction microchannels is numerically studied. To this approach, the Lattice Boltzmann method with Pseudo-Potential model is used. The accuracy of the present model is examined by the Laplace test, drop contact angle, and drop formation in an ordinary T-junction microchannel. The comparison shows that the present results have good agreement with previous numerical and experimental data. The effects of various parameters including Capillary number, flow rate ratio, width ratio, and drop contact angle on the width of the drop and on the distance between drops for ordinary and modified T-junction microchannels are investigated in details. The results reveal that by simple modifications to the ordinary T-junction, smaller drops and lower distances between them are generated in the comparison of ordinary T-junction geometry under the same conditions. On the other hand, this study demonstrates that the multiphase flows in micro-devices are very sensitive to even small changes in the channel geometry. It also indicates that lattice Boltzmann method with Pseudo-Potential model is an effective numerical technique to simulate the generation of drops in microchannels.

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In this numerical study, three dimensional laminar flow, heat transfer and other thermal characteristics of a microchannel heat sink, consisting of seven isosceles triangular microchannels, have been investigated. For this purpose, conduction in the solid parts has been considered and two different horizontal inlet/outlet (I-type) and vertical inlet/outlet (U-type) arrangements have been considered. Simulations have been performed for a constant heat flux of 125 kWm-2 entering from the substrate. In previous studies flow of water in rectangular microchannles has been considered, but in this study CuO-water nanofluid has been utilized. The effects of the Brownian motion of nanoparticles and variation of thermophysical properties of the nanaofluid with termperature have been considered and their importances studied. The results show that with increasing pressure drop, the heat sink performance in terms of heat transfer, thermal resistance and uniform temperature distribution at subtrate improves for the two nominated arrangements. Also increasing the volume fraction to 2% improves the heat sink performance, but as it increases further the thermal resistance and the non-uniformity of temperature at the bottom plate enlarge with no heat transfer improvement. Making comparison with the results of the previous studies on the effect of inlet/outlet arrangement proves that the thermal performance is affected by both of the inlet/outlet arrangement as well as the shape and geometry of the microchannels. For the heat sink of this study with triangular microchannels the performance of the I-type arrangement is better than the U-type arrangement.

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In this study, effects of zeta potential distribution and geometrical specifications are numerically investigated on mixing efficiency in electroosmotic flows. Considered geometries include straight, converging, diverging, and converging-diverging microchannels. Electroosmotic flow simulations are conducted based on the N-S and Nernst-Planck equations for momentum and ionic charges distributions, respectively, by lattice Boltzmann method. Numerical simulations are validated against available analytic electroosmotic flow solutions in homogeneous straight channels, and then flow patterns and mixing performances in the presence of non-uniform zeta potential distributions are investigated in search for enhanced mixing performances. Numerical results indicate that converging channel leads to a sizable increase in mixing efficiencies, while the flow rate decreases at the same time. In contrast, diverging channels increase the flow rate, while decrease the mixing efficiency. Therefore, it is expected to achieve a balance between the mixing efficiency and mass flow rate using converging-diverging geometries. Numerical results indicate that mixing efficiency of about 90% can be reached with a converging-diverging microchannel with a reasonable decrease in mass flow rate as compared to its geometrical diverging-converging counterpart channel.

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In the present study, thermal performance of a microchannel heat sink with superhydrophobic walls is studied for different ratios of the wall convergence. To this end, three-dimensional Navier-Stokes equations and energy equation subject to the slip boundary conditions, viz. velocity slip and temperature jump, are numerically solved using the finite volume method. Then, the variations of thermal resistance of the heat sink with the number of channels, width- and height-tapered ratios, are studied for a fixed pumping power. The results show that by utilizing the superhydrophobic walls, the optimum width-tapered ratio of the channel is higher than that of the hydrophilic walls. The accentuated effect of the number of channels on thermal performance in the presence of liquid-solid interfacial slip weakens the effect of converging the width of the channel. It is also revealed that the optimum number of channels also increases to give prominence to the effect of interfacial slip by diminishing the smallest dimension of the channel. Finally, it is shown that for a pumping power of 0.05 W, using a heat sink with converging microchannels and superhydrophobic walls, reduces the overall thermal resistance by 28 percent, compared to that with conventional microchannels. In fact, the increase in fluid flow rate resulting from the use of converging microchannels with superhydrophobic walls outweighs the undesirable effect of temperature jump on heat transfer, in a sense that the heat sink performance augments considerably.

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These days, investigation on using acoustofluidic microchannels in separation of microparticles and cells is under consideration. Working under optimum efficiency, these microchannels should be designed and manufactured truly. In this work, a new methodology for designing and manufacturing of acoustofluidic microchannels are explained. Then, a metallic microchannel with 2-nodes of pressure wave based on this method was developed. For mass production purpose, a low cost and reliable method which is CNC micromachining is used. Also, to conduct the heat generated by the wave, this microchannel was made out of aluminum and then polishing technique is applied. Then, the performance of this microchannel in agglomerating of human blood cells and BT-20 breast cancer cells to nodal lines was experimentally studied. The results showed that the applied design and manufacturing technique are suitable. Although some tests were performed to find temperature rise of microchannel due to damping effect, it was found that true design method and also using metals with high thermal conductivity can prevent the temperature increase to the point beyond which living cells will be hurt.