Vertical Microchannel Research Articles

Entropic and heat-transfer analysis of EMHD flows with temperature-dependent properties

This paper focusses on a theoretical analysis of the entropic generation and heat-transfer characteristics of electromagnetohydrodynamic (EMHD) flow in vertical hydrophobic microchannels. The flow viscosity, electrical conductivity, and thermal conductivity are assumed to be temperature variant. The fluid velocity and energy transfer equations associated with a system of coupled non-linear equations dealing with hydrophobic slip conditions are solved using a finite volume method associated with lubrication theory. The Debye–Hückel approximation is employed in an electrical double layer combined with the Poisson–Boltzmann equation to acquire an analytical solution for the electrical potential function. Slip velocities along with constant temperatures are provided to obtain numerical solutions for the case of a fully developed EMHD flow, in order to reveal the influence of fluid rheology. The results are presented for electromagnetic transport with variable viscosity over hydrophobic interfaces. Numerical and analytical validations are performed using the existing experimental results. In this study, we vizualize the significance of variable viscosity, electrical conductivity, and thermal conductivity on temperature distributions in the presence of a magnetic field. In this work, entropy generation is represented in terms of the Bejan number, which greatly impacts the normalized electroosmotic flow as well as the thermophysical parameters, leading to a minimization of the entropy-generation rate.

Microchannel–engraved and Cu–dispersed carbon nanocomposite film as a chemiresistive sensor for aqueous metal ions

A Cu-carbon nanocomposite film (2 mm thickness) is fabricated as an efficient chemiresistive sensor for aqueous Cd. The novelty of the study is in fabrication of the laser-ablated vertical microchannels (200 µm wide × 100 µm deep × 200 µm inter-spacing) on both sides of the carbon film. The microchannels speed up sensing by inducing a reproducible capillary action in the film, with a rapid rise of the liquid and wetting of the material, and augmenting exposure of the electroconductive Cu nanoparticles to the analyte through the porous textured walls. Capillary action and wetting are facilitated by hydrophilic functional groups in the material. Notably, the sensor response (electrical resistance of the film) is approximately linear (R2 = 0.977) over the concentration range 10 – 150 ppb. The microfluidics-based approach used in this study can be easily extended to develop fast chemiresistive sensors for biofluids, and the other metals in wastewater.

Magnetohydrodynamics Eyring‐Powell fluid in a vertical porous microchannel with convective boundary condition subjected to entropy generation

AbstractThis investigation is concentrated on the second law analysis of a magnetohydrodynamics Eyring‐Powell fluid in a vertical microporous channel with the convective boundary conditions under the impacts of heat generation, viscous dissipation, exponential space, temperature dependence, and Joule heating. The reduced form of the governing equations is obtained with the aid of applying nondimensional variables and is resolved using Runge‐Kutta‐Fehlberg's fourth fifth‐order method. The various relevant parameters that affect the heat transfer and entropy have been discussed in detail through graphs. It is found that the impacts of suction/injection, viscous dissipation, and convective conditions are important and the thermal performance can be improved with these factors. The generation of entropy boosts up with the impacts of radiation, space/temperature‐dependent, and Biot number and slows down with Eyring‐Powel parameters. Furthermore, the heat transfer rate amplifies with the magnetic number and the drag force intensifies with the Brinkman parameters. The comparison of results has been performed and it provides an excellent agreement.

Conditions of convective instability in a vertical circular microchannel with slippage effects

The paper outlines a novel mathematical model of convective instability in a vertical cylindrical microchannel. The model yields a transcendental equation, whose solution represents a dependence of the critical Rayleigh number on the harmonic number of the perturbation function, the ratio of thermal conductivities of the fluid and the solid wall, as well as the Prandtl and Knudsen numbers. The most dangerous are harmonics with n = 1, which corresponds to diametrically antisymmetric motions. An increase in the Knudsen number causes weakening of the system stability and a decrease in the critical Rayleigh number. This tendency is observed in the entire range of the ratio of thermal conductivities (0 … ∞) and Prandtl numbers (0.01 … 100) studied in the paper. A decrease in the thermal conductivity of the solid wall and the Prandtl numbers causes loss of the system stability, which can be explained by an increase in the amount of heat spent to arouse instability.

MHD free convection flow in a vertical porous super-hydrophobic micro-channel

A free convective flow of an incompressible and electrically conducting fluid through a vertical micro-channel of rectangular geometry was considered. Both plates were porous and heated alternately. A transverse magnetic field was applied across the channel. One channel wall surface was no slip and the other was super-hydrophobic. The purpose of the study is to examine the effects of super-hydrophobicity, magnetism and wall porosity on the main characteristics of the flow. The exact solutions of the formulated differential equations were provided. A few highlights of the results obtained include: (1) the magnetic parameter lowered the skin friction at both surfaces when either of them were heated, (2) the suction/injection parameter raised the fluid temperature when the super-hydrophobic surface (SHS) was heated and brought it down when the no slip surface (NSS) was heated, (3) a critical temperature jump coefficient was observed at which the flow rates in both cases (only SHS heated, and only NSS heated) were equal. A few application areas of the research include micro-fluidics and micro-electronics.

Heat transfer analysis of mixed convection flow in a vertical microchannel with electrokinetic effect

In this article, the transient/steady mixed convective flow formation inside a vertical microchannel subjected to uneven wall zeta potentials in the presence of an electric body force is carried out theoretically. Employing the Navier–Stokes, Poisson–Boltzmann and energy equations, the velocity profile, electric potential and temperature distributions respectively governing flow formation and heat transfer in dimensional form are presented. Using appropriate dimensionless parameters, the corresponding dimensionless form of the governing equations are obtained. At the transient state, the Laplace transform technique is utilized while at the steady-state, direct integration as well as undetermined coefficients are employed to obtain solutions to governing equations. To justify solutions obtained, the steady state solution is found and compared with the implicit finite difference method at large time; this comparison gives an excellent agreement. Graphical simulations show that the skin-friction could have incurred an error of up to 19% if the buoyancy term in the momentum equation had been neglected. In addition, it is established that for a large electric double layer, the interval of no reverse flow at the microchannel walls is significantly extended.

Free Convection in a Vertical Slit Micro-channel with Super-hydrophobic Slip and Temperature Jump Conditions

The main objective of the convective incompressible fluid flow placed in a magnetic field, with vertical motion through an isothermally heated porous channel. The Super-hydrophobic the slip was applied on one wall along with temperature jump, while there is no slip on another wall. The analytical results are discussed qualitatively for the parameters which were on the mathematical formulation on the basis of the model equations designed for the linear momentum and energy balance. Decreasing the value of the slip parameter is noticed due to decreasing in temperature. The effects of heat source and porous medium on the micro channel flow is analysized in detail. Coparesion of current results is made with the earlier work. The effect of Dray parameter is to increase the fluid velocity in the channel.

Irreversibility analysis of nanofluid flow in a vertical microchannel with the influence of particle shape

Augmentation of thermal performance in heat transfer system has become research hotspot nowadays. Numerous techniques are carried out to pick up the effective heat transport mechanism for designing high efficient thermal frameworks which has extensive practical uses in industrial process. In the current study, mixture model has been implemented for better describing the characteristics of nanoparticles in a vertical microchannel. The nondimensional equations are computed by using Runge Kutta Fehlberg method. Effect of heat source, buoyancy force and convective boundary on the thermal system has been demonstrated. The role of spheroidal nanoparticles on thermal conductivity of the conventional fluid has been examined. The causes of irreversibilities in a microchannel due to nanoliquid flow has been reported in the current research work. It is obtained that Aluminum foam has higher thermal field compared to Al2 O3. Entropy generation is reduced by lowering Eckert number and Grashof number. It is explored that nanofluid containing oblate shaped nanoparticels has higher thermal conductivity ratio.

Adomian decomposition method for combined effect of Hall and ion-slip on mixed convection flow of chemically reacting Newtonian fluid in a microchannel with heat absorption/generation

In this work, theoretical analysis is carried out on fully developed hydromagnetic flow of heat generating/absorbing fluid in a vertical microchannel in the presence of Hall and ion-slip effects. The governing coupled flow equations are obtained in dimensional form and thereafter simplified by employing some similarity transformation variables and quantities. The transformed coupled equations are solved by adopting the Adomian Decomposition Method (ADM) and their solution represented in closed form. Interesting influences of some active parameter on different flow features are shown using line graphs and Table. The numerical values for the results obtained in this work using ADM were validated with results obtained from existing benchmark. Results from the analysis reveal that in the existence of heat generation/absorption, the primary velocity and the temperature distribution could be enhanced by growing the ion-slip and Hall current parameter, whereas its influence on velocity is contrary along the secondary direction.

Electroosmotic natural convection flow in a vertical microchannel with asymmetric heat fluxes

This article inspects the role of wall asymmetric heat fluxes and buoyant force on steady natural convection flow in a vertical microchannel with electrokinetic effect. The governing Poisson–Boltzmann, momentum and energy equations representing the electric potential, velocity profile and temperature distribution in the microchannel are obtained for the current investigation. Using the Debye–Huckel linearization, exact solutions for electric potential, velocity and temperature are found by method of undetermined coefficients and direct integration. In the course of graphical simulations, result shows that the skin-friction and mass flow rate in the microchannel would have been overestimated by approximately 37% and 30% respectively if the slip-condition at the microchannel walls for current investigation had not been taken into consideration regardless of the magnitude of buoyant force. In addition, the inclusion of buoyancy parameter acts as an enhancer on velocity profile and mass flow rate.

In-situ construction of lithiophilic interphase in vertical micro-channels of 3D copper current collector for high performance lithium-metal batteries

Although the great demand for high-energy-density batteries has inspired the renewed interest of rechargeable lithium metal batteries that use metallic lithium as the anode, short lifespan and serious safety risks caused by the substantial volume change of the Li electrode during plating/stripping and the uncontrollable growth of Li dendrites are two main issues of Li metal anode. In this study, a new 3D Cu current collector with a pore structure of vertical micro-channels coated with a lithiophilic Li-Sb interphase is prepared via a facile synthesis procedure. The introduced lithiophilic interphase can dramatically reduce the Li nucleation barrier to inhibit the formation of Li dendrites. The vertical micro-channels pore structure can effectively lower the local current density to regulate a smooth Li deposition. Consequently, the symmetrical cells with the hybrid architecture substrate demonstrates an ultra-long lifespan over 2000 h at 12 mA cm−2 for 12 mAh cm−2. Full cells made of such a 3D copper supported Li anode coupled with a LiFePO4 cathode with a similar architeture can achieve a remarkable retention of 91.23% at a high areal capacity (~ 6 mAh cm−2) over 150 cycles under a harsh condition (low N/P capacity ratio of 1.5).

초발수 특성을 이용한 기체투과성 방수 필름 제작 및 특성 평가

In this paper, a gas permeable waterproof film with superhydrophobic characteristics of irregular microstructures and vertical microchannels, is presented. A wet etching technique of aluminum alloys was employed for the preparation of a superhydrophobic surface, and a typical punching technique was used for the fabrication of vertical microchannels. A waterproof film was created via the casting process of polydimethylsiloxane (PDMS). The contact angle of the prepared film was measured as 156°, and the diameter of the vertical microchannel was approximately 100 μm. In the experiment involving maximum liquid entry pressure (LEP) measurement, the LEP of the microtextured PDMS film with 50 vertical microchannels was approximately 16 kPa, whereas the smooth PDMS film could not withstand any pressure. In the long-term reliability test, by using a pressure relief valve of 14 kPa, the fabricated film maintained the water pressure without any leakage.

Analysis of second law on Eyring‐Powell nanoliquid flow in a vertical microchannel considering magnetic field and convective boundary

AbstractStudies related to enhancing heat transfer has attained much attention of researchers to avail optimized heat‐transfer devices. High viscous fluids are of great importance as they are widely used in petroleum products, organic chemistry, coating, printing, and so forth. In this study, heat transfer mechanism driven by Eyring‐Powell nanoliquid flow in a vertical microchannel is examined. Impact of considering buoyancy force, magnetic field, and convective boundary on the thermal system is demonstrated. The modeled nondimensional equations are computed by using the Runge‐Kutta‐Fehlberg method. The vital roles of thermophoresis and Brownian motion are discussed in detail. The significance of second law analysis for thermal systems is presented. The causes of irreversibilities in a microchannel due to Eyring‐Powell nanoliquid flow is also demonstrated in the current research study. The upshots of the current investigations are visualized through graphical elucidation. It is established that minimization of entropy generation can be achieved by enhancing the mechanism of thermophoresis. The convective boundary helps in transmitting heat from the thermal system to the ambience hence the lower thermal field is attained.

Hall and ion‐slip effects on MHD mixed convection flow in a vertical microchannel with asymmetric wall heating

AbstractIn this article, a theoretical analysis is carried out on hydromagnetic mixed convection flow of viscous incompressible and electrically conducting fluid in a vertical microchannel in the existence of Hall current and ion‐slip effects. The walls of the microchannels are preheated and the flow is prompted by the combination of buoyancy forces and pressure gradient. The governing dimensional equations are obtained and reduced to dimensionless form using suitable transformations. The flow equations are solved analytically, and their solutions are obtained in dimensionless form. Some interesting effects of the relevant flow parameters on the flow formation and volume flow rates, in both primary and secondary flow directions, are showed. The results show that, in the presence of Hall current and pressure gradient, the ion‐slip current supports flow formation along the primary flow direction, while destabilizing it along the secondary direction. In addition, it is also highlighted that, in the presence of pressure gradient and buoyancy forces, the ion‐slip current increases the volume flow rate in both primary and secondary flow directions, whereas the Hall current destabilizes the volume flow rate along the primary direction.

Pristine Titanium Carbide MXene Hydrogel Matrix.

The hydrogel matrix normally forms via covalent or noncovalent interactions that make the matrix resistant to hydration and disassembly. Herein this type of chemical transition is demonstrated in titanium carbide MXene (Ti3C2Tx), in which the exchange of intercalated Li+ with hydrated protons triggers significantly suppressed hydration in stacked Ti3C2Tx. Based on this intercalation chemistry behavior, pristine Ti3C2Tx hydrogel matrices with an arbitrary microstructures are fabricated by freezing-induced preassembly and a subsequent specially designed thawing process in protic acids. The absence of extrinsic components maximizes the materials performance of the resultant pristine Ti3C2Tx hydrogel, which produces a compressive modulus of 2.4 MPa and conductivity of 220.3 ± 16.8 S/m at 5 wt % solid content. The anisotropic Ti3C2Tx hydrogel also delivers a promising performance in solar steam generation by facilitating rapid water transport inside vertical microchannels.

The Effects of Gravity on the Pressure Drop and Heat Transfer Characteristics of Steam in Microchannels: An Experimental Study

Experiments were carried out to investigate the pressure drop and heat transfer behaviors of a microchannel condenser. The effects of gravity on the condensation of steam in the microchannels were investigated for both horizontal and vertical cases. For the experimental results, the pressure drop of vertical microchannels in the condenser is lower than for horizontal microchannels. In the case of the horizontal microchannel, as the mass flow rate of steam increases from 0.01 g·s−1 to 0.06 g·s−1, the pressure drop increases from 1.5 kPa to 50 kPa, respectively. While the mass flow rate of steam in the vertical microchannel case increases from 0.01 g·s−1 to 0.06 g·s−1, the pressure drop increases from 2.0 kPa to 44 kPa, respectively. This clearly indicates that the gravitational acceleration affects the pressure drop. The pressure drop of the vertical microchannel is lower than that obtained from the horizontal microchannel. In addition, the capacity of the condenser is the same in both cases. This leads to the performance index obtained from the vertical microchannel condenser being higher than that obtained from the horizontal microchannel condenser. These results are important contributions to the research on the condensation of steam in microchannels.

Hall effects on MHD couple stress fluid flow through a vertical microchannel subjected to heat generation: A numerical study

AbstractThe present study analyzes the irreversibility in an electrically conducting couple stress fluid between two vertical channel plates with the aspects of exponential space and temperature‐dependent, radiative heat flux, heat generation, and Joule heating. A reduced system of the governing equations is obtained via applying suitable nondimensional variables. To determine the solutions of velocity, thermal field, irreversibility, and irreversibility ratio, the Runge‐Kutta‐Fehlberg method along with the shooting technique has been employed. The various pertinent parameters that are involved in the problem have been discussed in detail through graphs. The results show that the entropy production can improve with the viscous dissipation, Joule heating, exponential space and temperature‐dependent, and radiative heat flux. Furthermore, it is clear that the entropy enhances with wall ambient temperature difference, radiation parameter, and the effect is reversed with higher estimations of the fluid wall interaction parameter, couple stress parameter, and rarefaction. The present numerical results are compared with the existing results that are in good agreement.

Hall and ion‐slip effects on steady MHD free convective flow through a porous medium in a vertical microchannel

AbstractWe have considered the steady fully developed magnetohydrodynamic free convection flow through a porous medium in a microchannel bounded by two infinite vertical parallel plates due to asymmetric heating of plates taking Hall and ion‐slip effects into account. Effects of velocity slip and temperature jump have been considered on the microchannel surfaces, and the exact solutions have been obtained for momentum and energy equations under relevant boundary conditions. The influence of governing parameters on flow formation is discussed with the aid of graphs. The significant result from the study is that an increase in the value of rarefaction parameter leads to enhancement in volume flow rate. Furthermore, it is evident that the volume flow rate is found to be an increasing function of the Hall current parameter.

Experimental study on boiling performance of wetting fluids in three-dimensional rectangle microchannels

A unique boiling heat transfer study of wetting fluids in a vertical three-dimensional rectangle microchannel was carried out in this paper, to develop a novel passive three-dimensional chip stack cooling technology. The base liquids were deionized water and R113, and Sodium dodecagon sulfate (SDS) was used as a surfactant, which is added into the base fluids for developing wetting fluids. The length, gap, and width of rectangle channels were in the range of 30 mm to 100 mm,30 mm to 50 mm, and 0.4 mm to 4 mm, respectively. Metal wires with different numbers were laid in the rectangle channel along the flow direction for forming change in the width of the microchannel. The study results show that using wetting fluids can significantly enhance both heat transfer coefficients and maximum heat flux of thermosyphon boiling in three-dimensional microchannels compared with that found in pure liquids. The enhanced effects have significant relations with the three-dimensional geometric dimensions. The present experimental results provide significant technology support for passive three-dimensional chip cooling.

A New Correlation for Subcooled Flow Boiling Heat Transfer in a Vertical Narrow Microchannel

Abstract Experimental research of subcooled flow boiling heat transfer of de-ionized water in a high-aspect-ratio, one-sided heating, vertical rectangular microchannel having a hydraulic diameter of 0.94 mm (5.01 mm × 0.52 mm) is carried out. The tested heat flux, mass flux, and inlet fluid subcooling are in the ranges of 0–30 W/cm2, 200–500 kg/m2 s, and 5–20 °C, respectively. A series of 24 existing correlations for subcooled flow boiling heat transfer are systematically reviewed and evaluated against the experimental data containing 328 points, which give significantly different predictions. Some typical conventional correlations such as Kandlikar, Shah, Chen, and Liu and Winterton predict well, among which the Shah correlation performs the best with the mean absolute deviation (MAD) of 10.45%. However, they all neglect the effect of fluid subcooling, especially in low-subcooling regime. Based on the Shah correlation, a new correlation for subcooled flow boiling heat transfer in the vertical narrow microchannel considering the subcooling effect is developed, which shows higher precision of prediction with the MAD of 7.87%.