Vertical Microchannel Research Articles

Wood‐Derived Monolithic Ultrathick Porous Carbon Electrodes Filled with Reduced Graphene Oxide for High‐Performance Supercapacitors with Ultrahigh Areal Capacitances

AbstractA monolithic ultrathick and dense carbon electrode with high specific surface area and high hydrophilicity is fabricated for high‐performance supercapacitors with ultrahigh areal capacitance by infiltration of graphene oxide inside the aligned pores of the wood followed by lyophilization, carbonization, thermal reduction at 800 °C, and activation with KOH. The porous and vertical microchannels in the wood‐derived carbon facilitate the transport of ions and electrons, while the presence of reduced graphene oxide provides the electrode with high specific surface area, high electrochemical stability, and compactness. Benefiting from the efficient pores resulted from the KOH activation, the monolithic electrode exhibits a high specific surface area of 1049.9 m2 g−1 with an enhanced permeability, and remains its primary structure as a freestanding electrode without the use of conductive additives or binders. Consequently, the directly sliced pellet electrode with a thickness of 2.2 mm delivers a high areal capacitance of 26.6 F cm−2 at a current density of 1 mA cm−2 in 6 M KOH electrolyte. The resultant symmetrical supercapacitor with a KOH gel electrolyte reaches an energy density of 0.91 mWh cm−2 and a maximum power density of 11.90 W cm−2.

Lattice Boltzmann mesoscopic modeling of flow boiling heat transfer processes in a microchannel

Microchannel flow boiling has received increasing attention for its high heat dissipation intensity with a small temperature difference. In the present study, a multi-relaxation-time (MRT) phase change lattice Boltzmann model is employed to investigate the flow boiling heat transfer processes in a vertical microchannel under constant heat flux boundary conditions. The bubble dynamic behavior including nucleation, growth, slide, coalescence and departure is obtained and the periodic variations of wall superheat and heat transfer coefficient are evaluated using the Jacob (Ja) number and Nusselt (Nu) number. It is demonstrated that the cavity microstructure on the heating surface can enhance the flow boiling heat transfer, further the cavity height and cavity distance are optimized with single-cavity model and multi-cavity model, respectively. Phase change heat transfer and flow boiling curves from single-phase forced convection, nucleate boiling regime, transition boiling regime to stable film boiling regime are numerically predicted for both the single-cavity structure and multi-cavity structures. The simulation results show that increasing Reynolds (Re) number accelerates the bubble departure frequency and intensifies the external forced convection, thereby achieving enhanced flow boiling heat transfer performance. These findings advance our understanding of the bubble dynamics and heat transfer mechanisms in the microchannel flow boiling processes.

Investigation of asymmetric wall temperature and concentration on free convection of conducting fluid in a microchannel

AbstractIt is of great practical interest to study the fluid flow phenomena using buoyancy‐induced flow in a microchannel. The present problem is concerned with the natural convection in a vertical microchannel within electrically nonconducting surfaces with asymmetric wall temperature and concentration distributions. The interplay of velocity slip, temperature, and concentration jump at the walls contribute to salient features of the analysis. The governing equations of the fully developed flow are solved analytically as well as numerically. The results of both methods are in good agreement, providing the consistency and reliability of the applied numerical method. The striking outcome of the present study is that temperature, as well as concentration jump conditions at the walls, lead to flow reversal and non‐laminarity distributions of the induced current intensity and magnetic field. The higher rarefaction parameter, which ascribes the microchannel flow characteristics, prevents the growth of negative current intensity, whereas it favors the growth of velocity with induced magnetization.

Fully Biomass-Based Hybrid Hydrogel for Efficient Solar Desalination with Salt Self-Cleaning Property.

Solar-driven interfacial steam generation provides an opportunity for solar harvesting and freshwater yield as a promising and eco-friendly technology. Here, we demonstrate a sustainable, nontoxic, and highly efficient fully biomass-based GG/CI hydrogel evaporator consisting of gellan gum (GG) hydrogel as the matrix and cuttlefish ink (CI) as the photothermal material. Induced by the ice-template method and freeze-drying method, vertically aligned microchannels are generated along the ice crystal growth direction. Efficient photothermal conversion is enabled by the natural black cuttlefish ink powder and enhanced by the light trapping effect within vertical microchannels. The hydrophilic property of the gellan gum hydrogel and water capillary force in those microchannels boost water pumping to the top interfacial evaporation region. Effective rapid salt self-cleaning behavior is achieved due to the rapid ion diffusion within vertical microchannels. An evaporation rate of 3.1 kg m-2 h-1 under one sun irradiance is demonstrated by this fully biomass-based GG/CI hydrogel evaporator. This work offers a promising alternative for eco-friendly and sustainable freshwater generation with abundant natural biomasses.

Experimental investigation on cooling heat transfer and buoyancy effect of supercritical carbon dioxide in horizontal and vertical micro-channels

This study experimentally investigated the cooling heat transfer characteristics of supercritical carbon dioxide (CO2) in circular horizontal and vertical (downward and upward) micro-channels (Dh = 1 mm). Measurements were performed for operating pressure ranging from 7.5 to 8.2 MPa, mass flow rate ranging from 1.323 to 2.567 kg/h, wall heat flux ranging from 14.05-39.34 kW/m2. The effects of mass flow rate, wall heat flux, operating pressure and flow direction on heat transfer coefficient and buoyancy effect were explored. The experimental results revealed that heat transfer intensification benefited from increased mass flow rate and decreased operating pressure. Wall heat flux exhibited relatively mild influence on heat transfer coefficient. A significant heat transfer enhancement in downward flow was discerned compared with horizontal flow. The buoyancy effect significantly influences the heat transfer characteristics during supercritical CO2 cooling process as thermal properties dramatically changes with the bulk temperature. The forced convection aligned region where the buoyancy effect was less dominated witnessed distinctly higher heat transfer coefficient in contrast with mixed convection flow. Furthermore, a comparative study was performed by comparing several existing correlations with experimental results of Nusselt number, and large deviations were reasonably observed due to limited applicability.

Convective instability of nanofluids in vertical circular porous microchannels

• Convective instability of nanofluids in a vertical circular microchannel was considered • Dependence of the critical Rayleigh number of different parameters was obtained • An increase in the Knudsen number causes a decrease in the critical Rayleigh number • A decrease in the Prandtl number causes a decrease in the critical Rayleigh number The article presents an analysis of the convective instability of a nanofluid in a vertical porous cylindrical microchannel with slip boundary conditions on the wall. The microchannel is surrounded by a continuous medium. For the calculation, the Galerkin method with a different order of approximation was used. The influence of the Darcy, Knudsen and Prandtl numbers, the ratio of the thermal conductivities of the fluid and the wall, the intensity of sliding along the wall, as well as the properties of nanoparticles on the Rayleigh number at which instability occurs, is analyzed. It was shown that an increase in the porosity of the medium promotes stabilization. It has been determined that the conditions for the onset of convection are decisively influenced by the thermal conductivity of nanoparticles (Lewis number), whereas the slippage effects (Knudsen number) are of lesser importance.

Numerical Modeling of Flow through the Vertical Rectangular Microchannel for Drug Screening Applications

A numerical study of flow in the rectangular vertical microchannel for drug screening applications is described. The flow characteristics through the microchannel are presented by incompressible Naiver-Stroke equations. Flow rate is evaluated from numerical modeling to simulate the drug flow through the microchannel. Here in this study, computational fluid dynamics based finite element method commercial software COMSOL is used for simulation of drug flow through the artery for cancer drug screening applications. The simulation work also presents the diffusion velocity at the intersection area between the drug and cancer cells..

Convective Instability in Slip Flow in a Vertical Circular Porous Microchannel

The paper represents an analysis of convective instability in a vertical cylindrical porous microchannel performed using the Galerkin method. The dependence of the critical Rayleigh number on the Darcy, Knudsen, and Prandtl numbers, as well as on the ratio of the thermal conductivities of the fluid and the wall, was obtained. It was shown that a decrease in permeability of the porous medium (in other words, increase in its porosity) causes an increase in flow stability. This effect is substantially nonlinear. Under the condition Da > 0.1, the effect of the porosity on the critical Rayleigh number practically vanishes. Strengthening of the slippage effects leads to an increase in the instability of the entire system. The slippage effect on the critical Rayleigh number is nonlinear. The level of nonlinearity depends on the Prandtl number. With an increase in the Prandtl number, the effect of slippage on the onset of convection weakens. With an increase in the ratio of the thermal conductivities of the fluid and the wall, the influence of the Prandtl number decreases. At high values of the Prandtl numbers (Pr > 10), its influence practically vanishes.

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 non-conducting 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.

Electro‐osmotic flow of fractional second‐grade fluid with fractional Cattaneo heat flux through a vertical microchannel

AbstractThis study investigates the unsteady electro‐osmotic flow (EOF) of a fractional second‐grade fluid through a vertical microchannel with convection heat transfer. The fractional Cattaneo heat flux model will be used to modify the heat equation. The solutions for the velocity and the temperature have been derived by employing the Laplace and finite Fourier sine transforms and their numerical inverses. The results show that at the beginning of the time period, the fractional parameter postpones the movement of the fluid. Furthermore, the results show that at the high values of retardation time (non‐Newtonian case), the required time for the velocity and the flow rate to reach the steady state increases. Moreover, the heat relaxation time reduces the heat transfer until a critical time, and then the effect reverses.

Significance of increasing Lorentz force and buoyancy force on the dynamics of water conveying SWCNT and MWCNT nanoparticles through a vertical microchannel

Carbon nanotubes are used to achieve high heat transfer rates in a variety of engineering applications include thermal storage systems, electronic component cooling, high-performance building insulation, heat exchangers and drying technologies. Hence the aim of this article is to examine the addition of single walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT) to water in a vertical microchannel to improve heat transfer. The effects of MHD, slip, convective boundary condition and heat source/sink are incorporated. The Brinkman-Forchheimer flow model and type II hybrid nanofluid model is adopted. Converted dimensionless differential equations are solved numerically via Dsolve command with the aid of Maple. The simulation assessment is worked out by graphs. One of the main tasks of the analysis is to compare MWCNT/water and SWCNT-MWCNT/water. It is shown that the improvement of the heat source/sink parameter improves the temperature and the rate of heat transfer in MWCNT/water is higher than SWCNT-MWCNT/water. Also larger values of Lorentz force and buoyancy force decreases the drag coefficient.

A Self-Priming Microfluidic Chip with Cushion Chambers for Easy Digital PCR.

A polydimethylsiloxane (PDMS)-based self-priming microfluidic chip with cushion chambers is presented in this study for robust and easy-operation digital polymerase chain reaction (dPCR). The chip has only one inlet and can partition samples autonomously through negative pressure, provided by a de-gassed PDMS layer with a multi-level vertical branching microchannel design. Meanwhile, cushion chambers make the chip capable of very robust use for sample partitioning. Finally, the proposed microfluidic chip showed excellent performance in the absolute quantification of a target gene by performing quantitative detection of a 10-fold serial dilution DNA template. Owing to its characteristics of easy operation, low cost, and high robustness, the proposed dPCR chip is expected to further promote the extensive application of digital PCR, especially in resource-limited settings.

Heat transfer and entropy generation in a MHD Couette–Poiseuille flow through a microchannel with slip, suction–injection and radiation

Flow, heat transfer and entropy generation in a vertical microchannel made of two parallel porous plates (the injection plate kept at rest while the suction plate is moving in upward/downward direction) under the combined action of buoyancy force and transverse magnetic field are studied. Variable pressure gradient (favorable/adverse) due to Couette–Poiseuille (C–P) flow is considered here. This is one of the fundamental problems of applied science and engineering. Slip conditions for the velocity at plates are implemented for both moving-plate directions and pressure gradients with the help of classical C–P flow. The basic equations are solved numerically by using Runge–Kutta–Fehlberg method along with shooting technique. Numerical results are in concordance with previously published results for specific cases. The present results are frequently compared with those of the classical C–P flow. Influence of operational parameters (plate movement, pressure gradient, injection–suction rate, slip length, Grashof Number, temperature ratio (between hotter to colder side) and viscous dissipation effect) on the flow and heat transfer characteristics (velocity, temperature, Nusselt number (Nu) distribution, entropy generation and Bejan Number) are investigated here. An effort is made to search singularity in the variation of Nu with some parameters, and find the parameters’ values at which the global entropy is minimally generated in the channel. Finally, a critical analysis is conducted on the individual contribution of irreversibilities due to heat flow, fluid friction and Joule heating to the total entropy generation.

Thermal Radiation Effect on Fully Developed Natural Convection Flow in a Vertical Micro-Channel

The effect of thermal radiation on steady fully developed natural convection flow in a vertical micro-channel is presented in this article. Effects of velocity slip and temperature jump conditions are taken into account due to their counter effects on both the volume flow rate and the rate of heat transfer. Due to the presence of thermal radiation, the momentum and energy equations are coupled system of ordinary differential equations. Governing coupled nonlinear equations are solved analytically by employing the perturbation analysis method to obtain an expression for fluid temperature, fluid velocity, rate of heat transfer and skin friction on the microchannel walls. The effect of various parameters controlling the physical situation such as thermal radiation, temperature difference, Knudsen number, and fluid wall interaction are discussed with the aid of line graphs and Tables. Results indicate that both velocity and temperature enhanced with the increase of the thermal radiation parameter.
 Keywords: Thermal radiation, Natural convection, Micro-channel, Velocity slip, Temperature jump

Thermal analysis of MHD Powell–Eyring fluid flow through a vertical microchannel

This investigation addresses the impact of entropy generation analysis of Eyring–Powell fluid flow through a microchannel in the presence of thermal/exponential heat source coefficients. The non-dimensional forms of the corresponding governing equations of the physical phenomenon along with the boundary conditions are solved by using Finite Element Method. Also, the physical features of viscous, joule heating as well as thermal and exponential heat source coefficient are accounted in the energy expression. The obtainable expressions of velocities and temperature distribution are used for the determination of the entropy generation and Bejan number distribution. The influence of numerous appropriate parameters is computed and deliberated through sketches. The thermal performance can be improved through the effects of magnetic field, viscous heating, Joule heating and convective condition. Furthermore, it is figured out that the entropy generation is enhanced for Grashof number.

Heat transfer, pressure drop and flow patterns of flow boiling on heterogeneous wetting surface in a vertical narrow microchannel

The micro-structured surfaces have a significant impact on the flow patterns and heat transfer mechanisms during the flow boiling process. As illustrated by previous studies, there is a trade-off between a hydrophobic and a hydrophilic surface. To examine the effect of heterogeneous wetting surfaces on the flow boiling process, an experimental investigation of flow boiling in a rectangular vertical narrow microchannel with the heterogeneous wetting surfaces and original silicon surface was conducted with deionized water as the working fluid. Hydrophobic patterns perpendicular (HC) and parallel (HP) to the flow direction are compared. The subcooled boiling curves, heat transfer coefficient, and pressure drop were discussed with the variation of heat flux and mass flux, the trends of which were analyzed along with the flow patterns. The heat transfer coefficient of flow boiling on heterogeneous wetting surface HC was enhanced for up to 39.55% compared to a silicon surface with less pressure drop. During the boiling process, the dominant heat transfer mechanism was nucleate boiling, with numerous nucleate sites between the hydrophilic/hydrophobic stripes. The easily formed bubble can be easily detached due to the supply of liquid from the hydrophilic regions nearby. With the increasing wall heat flux, the boiling heat transfer coefficient of the heterogeneous wetting surface HP is gradually lower than HC due to hydrophobic stripes parallel to the flow direction cannot restrict the bubbles from expanding in the axial direction.

Joule heating and viscous dissipation effect on electroosmotic mixed convection flow in a vertical microchannel subjected to asymmetric heat fluxes

The impact of Joule heating due to electric double layer (EDL) and viscous dissipation on electroosmotic mixed convection flow in a vertical microchannel with asymmetric heat fluxes is established in this article. The Poisson-Boltzmann, momentum and energy equations representing the electric potential, velocity profile and temperature distribution in the microchannel are obtained in dimensionless forms. Using the Debye-Hückel linearization, exact solutions are obtained for electric potential, velocity profile and temperature distributions by method of undetermined coefficients in the absence of viscous dissipation and Joule heating while an inbuilt Matlab function called pdepe is employed to solve the coupled nonlinear momentum and energy equations in the presence of Joule heating and viscous dissipation. Results show that the presence of Joule heating and viscous dissipation lead to decrease in velocity profile and temperature distributions throughout the microchannel.

Heat transfer and irreversibility rate in MHD flow of a hybrid nanofluid with Newton boundary condition, slip flow, and nonlinear thermal radiation

AbstractThe magnetohydrodynamic flow of a water‐based hybrid nanoliquid through a vertical microchannel has been investigated in the presence of collective effects, such as volume fraction of nanoparticle, suction/injection, magnetic field, temperature‐dependent heat source, hydrodynamic slip, and convective boundary conditions. The current mathematical formulations have been worked out numerically by using the fourth‐ and fifth‐order Runge–Kutta–Fehlberg scheme. The physical aspects of variation in velocity, temperature, entropy generation, and Bejan number with considered governing parameters have been discussed via corresponding graphs. The obtained numerical results demonstrated that radiation parameter and nanoparticle volume fraction reduce the thermal energy of the hybrid nanofluid. Moreover, entropy generation diminishes with the Hartmann number and permeability parameter, whereas it enhances with the Grashof number and Biot number.

Two-layered electroosmotic flow through a vertical microchannel with fractional Cattaneo heat flux

The unsteady electroosmotic flow (EOF) of two layers of immiscible fluids through a vertical microchannel with heat transfer will be investigated. The channel is subjected to an alternating current (AC) electric field. The channel has two fluid layers: layer (I) is filled with Newtonian fluid, while layer (II) is filled with fractional Jeffreys fluid. The fractional derivatives will be replaced by the well-known Caputo–Fabrizio fractional derivatives. The Laplace and finite Fourier sine transforms and their numerical inverses have been used to get the semi-analytical solutions for the current problem. It has been found that the electroosmosis velocity and the flow rate are higher in the Newtonian layer compared with the non-Newtonian layer. Moreover, the heat transfer delays by elevating the value of the heat relaxation time in the two layers.

In situ coating of a lithiophilic interphase on a biporous Cu scaffold with vertical microchannels for dendrite-free Li metal batteries

Copper oxide is coated in situ on a phase inversion-derived Cu scaffold for Li metal anodes, which exhibit a low nucleation overpotential, high coulombic efficiency and a long lifespan. The scaffold-Li//NCM full cell exhibits good cycling stability.