Numerical study of flow and heat transfer characteristics of alumina-water nanofluids in a microchannel using the lattice Boltzmann method

In the present study, mathematical modeling is performed to simulate force d convection flow of Al₂O₃/water nanofluids in a microchannel using the lattice Boltzmann method (LBM). Simulations are conducted at low Reynolds numbers (Re ≦ 16). Results indicate that the average Nusselt number increases with the increase of Reynolds number and particle volume concentration. The fluid temperature distribution is more uniform with the use of nanofluid than that of pure water. Furthermore, great deviations of computed Nusselt numbers using different models associated with the physical properties of a nanofluid are revealed. The results of LBM agree well with the classical CFD method for predictions of flow and heat transfer in a single channel and a microchannel heat sink concerning the conjugate heat transfer problem, and consequently LBM is robust and promising for practical applications.     

Viscous dissipation effects on thermal transport characteristics of combined pressure and electroosmotically driven flow in microchannels

This study investigates the influence of viscous dissipation on thermal transport characteristics of the fully developed combined pressure and electroosmotically driven flow in parallel plate microchannels subject to uniform wall heat flux. Closed form expressions are obtained for the transverse distributions of electrical potential, velocity and temperature and also for Nusselt number. From the results it is realized that the Brinkman number has a significant effect on Nusselt number. Generally speaking, to increase Brinkman number is to decrease Nusselt number. Although the magnitude of Joule heating can affect Brinkman number dependency of Nusselt number, however the general trend remains unchanged. Depending on the value of flow parameters, a singularity may occur in Nusselt number values even in the absence of viscous heating, especially at great values of dimensionless Joule heating term. For a given value of Brinkman number, as dimensionless Debye–Huckel parameter increases, the effect of viscous heating increases. In this condition, as dimensionless Debye–Huckel parameter goes to infinity, the Nusselt number approaches zero, regardless of the magnitude of Joule heating. Furthermore, it is realized that the effect of Brinkman number on Nusselt number for pressure opposed flow is more notable than purely electroosmotic flow, while the opposite is true for pressure assisted flow.     

Electrokinetically modulated flow of couple stress magneto‐nanofluids in a microfluidic channel

In the present article, the theoretical investigation is presented for the mixed electrokinetic and pressure‐driven transport of couple stress nanoliquids in a microchannel with the effect of magnetic field and porous medium. This topic has gained a remarkable scope in nanoscale electro‐osmotic devices. The formulation of the present mathematical problem is simplified using the Debye‐Hückel linearization assumption. The merging model has important features such as the thermal Grashof number, solutal Grashof number, Joule heating, Helmholtz‐Smoluchowski velocity. The analytical solutions are presented for the axial velocity, temperature, and solute concentration. The expressions for the heat transfer rate, solute mass transfer rate, and surface shear stress function at the walls are also presented. The results display that, the velocity of the couple stress nanofluid is less in the case of pure electro‐osmotic flow as compared to that of combined electro‐osmotic and pressure‐driven flow. When the Joule heating parameter vanishes, the temperature and solute concentration profiles are linear, otherwise nonlinear. The shear stress function is larger in the case of pure electro‐osmotic flow and it is smaller for the combined effects of electro‐osmotic and pressure gradient. The present analysis places a significant observation that the various zeta potential plays an influential role in heartening fluid velocity. The analysis is relevant to electrokinetic hemodynamics and microfluidics.     

Joule heating impacts on MHD pulsating flow of Au/CuO-blood Oldroyd-B nanofluid in a porous channel

This article deals, the pulsating flow of blood carrying Au/CuO Oldroyd-B nanofluid through a porous channel with the effects of viscous dissipation, thermal radiation, and Joule (Ohmic) heating, and applied magnetic field. The perturbation technique is employed to get analytic solutions for flow variables. A comparison between analytical and numerical results shows a good agreement. The effect of various parameters is addressed extensively aided by pictorial results. The obtained results present that the velocity is reduced with the higher values of Hartmann number and volume fraction of nanoparticles. The temperature of nanofluid is enhanced with an enhancement of Eckert number and radiation parameter while it reduces with a rise in Hartmann number. Furthermore, the rise of the volume fraction of nanoparticles boosts up the rate of heat transfer.     

Fully-developed thermal transport in combined electroosmotic and pressure driven flow of power-law fluids in microchannels

Thermally fully-developed heat transfer has been analyzed for combined electroosmotic and pressure driven flow of power-law fluid in a microchannel. Analytical expressions for transport parameters are presented in terms of the flow behavior index, the length scale ratio (ratio of Debye length to half channel height), dimensionless pressure gradient, and Joule heating parameter (ratio of Joule heating to surface heat flux). Closed form solutions are obtained for some specific values of the flow behavior index, while numerical solutions are presented for general cases. The results show that the temperature variation across the channel increases with increasing the pressure gradient. To reduce the length scale ratio is found to decrease the temperature variation, particularly for shear-thinning fluids. To increase the Joule heating parameter is to enlarge the temperature variation in the channel, especially for shear-thickening fluids. The Nusselt number can be increased by decreasing the length scale ratio due to the electroosmotic effect. Also, the Nusselt number increases with decreasing the values of flow behavior index and dimensionless pressure gradient.     

Flow of mixed convection for radiative and magnetic hybrid nanofluid in a porous material with Joule heating

This paper analyzes the mixed convection flow and transport of heat in a hybrid nanofluid via an exponentially extending/contracting surface. Joule heating, magnetic field, permeability of a porous medium, thermal radiation, and slip condition are taken into consideration. Magnetite (Fe₃O₄) and copper (Cu) are used as a mixture of nanoparticles while ethylene glycol as a regular liquid. The paradigm is dissolved by utilizing the method of Runge–Kutta–Fehlberg with the shooting technique in MATLAB software. The effect of controlling parameters on the coefficient of drag force, heat transfer coefficient, and the distributions of temperature and velocity for physical parameters are discussed numerically, physically, and graphically. The outcomes ended up illustrating that the transport of heat is diminished by upsurging the Joule heating and magnetic field parameters for both contracting and extending states. For larger values of permeability parameter and parameter of mixed convection, the coefficient of local skin friction upsurges in extending situations.     

Extended Graetz problem including streamwise conduction and viscous dissipation in microchannel

Extended Graetz problem in microchannel is analyzed by using eigenfunction expansion to solve the energy equation. The hydrodynamically developed flow is assumed to enter the microchannel with uniform temperature or uniform heat flux boundary condition. The effects of velocity and temperature jump boundary condition on the microchannel wall, streamwise conduction and viscous dissipation are all included. From the temperature field obtained, the local Nusselt number distributions are shown as the dimensionless parameters (Peclet number, Knudsen number, Brinkman number) vary. The fully developed Nusselt number for each boundary condition is obtained also in terms of these parameters.     

Joule heating induced heat transfer for electroosmotic flow of power-law fluids in a microcapillary

Capillary electrophoresis systems mainly used for chemical analyses and biomedical diagnoses usually involve biofluids in electrolyte buffers which cannot be treated as Newtonian fluids. In addition, the presence of Joule heating can limit the performance of capillary electrophoresis systems. This study presents a detailed analysis of Joule heating induced heat transfer for electroosmotic flow (EOF) of power-law fluids in a microcapillary. The steady, fully developed EOF field of power-law fluids governed by the Cauchy momentum equation is solved analytically by using two approximate schemes for modified Bessel functions, I₀(x) and I₁(x). Subsequently, under the widely accepted assumption of thin electric double layer (EDL) in microfluidics, an exact solution for temperature field induced by Joule heating is analytically solved from the energy equation subject to a mixed thermal boundary condition outside the capillary. Closed form expressions are obtained for the two-dimensional temperature field, the average fluid temperature and the local Nusselt number in both thermally developing and thermally developed regions. It is found that the rheological properties of power-law fluids affect the heat transfer characteristics mainly through the thermal Peclet number.     

Field-Synergy and Figure-of-Merit Analysis of Two Oxide–Water-Based Nanofluids' Flow in Heated Tubes

Field-synergy analysis is performed on the water–oxide nanofluid flow in circular heat sinks to examine the synergetic relation between the flow and temperature fields for heating processes. By varying the Reynolds number and the nanoparticle volume fraction, the convective heat transfer of nanofluid is investigated based on the field synergy number. For heating, the degree of synergy between the velocity and temperature fields of nanofluid flow deteriorates with the Reynolds number increase, leading to a decreased heat transfer performance of the nanofluid. By increasing the particle volume fraction, the degree of synergy between the velocity and temperature fields of the nanofluid flow can be intensified, thus going to convection heat transfer enhancement. After generating results, one can notice that the heat transfer enhancement is strongly dependent on nanoparticle type, Reynolds number, and volume fraction. The results are similar, even if the thermal conductivity of the two considered oxide nanoparticles are quite different. Additionally, a convenient figure of merit that is known as the Mouromtseff number was used as base of comparison, and the results indicated that the considered nanofluids can successfully replace water in specific applications for single-phase forced convection flow in a tube.     

Joule heating effects on magnetohydrodynamic free convection flow of a micropolar fluid

An analysis is presented to study the effect of viscous and Joule heating on MHD-free convection flow with a variable plate temperature in a micropolar fluid in the presence of uniform transverse magnetic field. The presence of dissipation increases both the skin friction and the rate of heat transfer at the surface. The friction factor and heat transfer rate decrease with an increase in the magnetic field parameter M and micropolar parameter Δ.     

Flow and heat transfer analysis of MHD third-grade fluid flow through a vertical microchannel subjected to entropy generation

This paper analyses the generation of entropy in an electrically conducting third-grade fluid through a vertical channel considering the variable thermal conductivity. Aspects of radiation, viscous dissipation, porous medium, Joule heating, convective boundary condition, and heat generation are studied. Nonlinear systems of ordinary differential equations are obtained via applying suitable dimensionless variables. After that, the system is solved with the aid of using the Runge–Kutta–Fehlberg method. The numerical solutions are used to characterize the irreversibility and irreversibility ratio. It is established that the entropy is enhanced with accelerating estimations of the third-grade material parameter, Brinkman number, magnetism, Biot number, porous parameter, and the impact is decelerated with elevating values of the radiation. The rate of heat transfer is higher for the Brinkman number, and a similar impact on drag force is noticed for magnetic and Grashof numbers. All the parameters on flow, temperature, fluid irreversibility and irreversibility ratio are discussed through graphical illustration.     

Heat transfer enhancement and pressure drop characteristics of TiO2–water nanofluid in a double-tube counter flow heat exchanger

This article reports an experimental study on the forced convective heat transfer and flow characteristics of a nanofluid consisting of water and 0.2 vol.% TiO₂ nanoparticles. The heat transfer coefficient and friction factor of the TiO₂–water nanofluid flowing in a horizontal double-tube counter flow heat exchanger under turbulent flow conditions are investigated. The Degussa P25 TiO₂ nanoparticles of about 21 nm diameter are used in the present study. The results show that the convective heat transfer coefficient of nanofluid is slightly higher than that of the base liquid by about 6–11%. The heat transfer coefficient of the nanofluid increases with an increase in the mass flow rate of the hot water and nanofluid, and increases with a decrease in the nanofluid temperature, and the temperature of the heating fluid has no significant effect on the heat transfer coefficient of the nanofluid. It is also seen that the Gnielinski equation failed to predict the heat transfer coefficient of the nanofluid. Finally, the use of the nanofluid has a little penalty in pressure drop.     

Artificial neural network modeling of nanofluid flow in a microchannel heat sink using experimental data

The present paper deals with the artificial neural network modeling (ANN) of heat transfer coefficient and Nusselt number in TiO₂/water nanofluid flow in a microchannel heat sink. The microchannel comprises of 40 channels; each channel has a length of 4 cm, a width of 500 μm, and a height of 800 μm. In the ANN modeling of heat transfer coefficient and Nusselt number 23 and 72 datasets have been used, respectively. The experimental Nusselt number has been calculated based on three different thermal conductivity models, four volume fractions of 0, 0.5, 1, and 2%, two values of Reynolds number i.e. 400 and 1200 and three different heating rates including 50.6, 60.7, and 69.1 W. Therefore, the inputs that are introduced to the neural network are volume fraction of nanoparticles, Reynolds number, heating rate, and model number while the output of network is the Nusselt number. It is elucidated that an appropriately trained network can act as a good alternative for costly and time-consuming experiments on the nanofluid flow in microchannels. The average relative errors in the prediction of Nusselt number and heat transfer coefficients were 0.3% and 0.2%, respectively.     

Effect of viscous heating on heat transfer performance in microchannel slip flow region

Based on the superposition principle, an analytical solution for steady convective heat transfer in a two-dimensional microchannel in the slip flow region is obtained, including the effects of velocity slip and temperature jump at the wall, which are the main characteristics of flow in the slip flow region, and viscous heating effects in the calculations. The cases of constant heat flux boundary conditions and one wall as adiabatic and the other wall at constant heat flux input are studied. The solution method is verified for the cases where micro-scale effects are neglected. The effects of viscous heating on the temperature profiles and on the heat transfer performance are analyzed in detail. It is concluded that the effect of viscous heating, like an internal energy source, heats the fluid along the flow direction and severely distorts the temperature profiles. The effects of key parameters, such as the Brinkman and Knudsen numbers, on the Nusselt number, which expresses the heat transfer performance are investigated.     

A Numerical Study on the Forced Convection of Laminar Nanofluid in a Microchannel with Both Slip and No-Slip Conditions

This article provides numerically study of the thermal performance of a microchannel, cooled with either pure water or a Cu-water nanofluid, while considering the effects of both slip and no-slip boundary conditions on the flow field and heat transfer. The microchannel is partially heated at a constant temperature and cooled by forced convection of a laminar flow at a relatively lower temperature. The effects of pertinent parameters such as Reynolds number, solid volume fraction, and slip velocity coefficient on the thermal performance of the microchannel are studied. The results of the numerical simulation indicate that the heat transfer rate is significantly affected by the solid volume fraction and slip velocity coefficient at high Reynolds numbers.     

Thermal performance of mixed electroosmotic-pressure driven flows in microtubes with constant wall temperature

A numerical analysis is performed to explore the heat transfer characteristics of mixed electroosmotic and pressure-driven flows in a microtube with constant wall temperature. Thermally fully-developed flow with Joule heating is considered. The Joule heating is generated by imposed voltage gradient and can be regarded as volumetric heat source. The analysis combines energy equation with overall energy balance equation for a control element to generate a nondimensional governing equation. Of interest are the effects of the relative duct radius a (ratio of the duct radius to Debye length), the pressure gradient parameter P (ratio of pressure gradient to electroosmotic forces) and the Joule number S (ratio of heat generation due to Joule heating to heat transfer at the wall) on the temperature distribution and the local heat transfer rate. The results indicate that the Nusselt number increases with an increase in the relative duct radius and the Joule number or a decrease in the pressure gradient parameter. The resulting solution due to the conditions of energy unbalance in the flow is also discussed.     

节流型微通道换热特性研究

为研究节流型微通道换热特性,设计并加工制作了突缩突扩结构的微通道实验件。采用控制变量法控制改变加热电压、质量流量、入口温度,通过实验数据对比分析研究了影响节流型微通道对流换热的规律。研究结果表明:随着质量流量的增加,微通道蒸发器的对流传热系数不断减小;随着雷诺数的增大努谢尔数不断增大,对流换热效果比较明显。     

Natural convection of nanofluids in an enclosure between a circular and a sinusoidal cylinder in the presence of magnetic field

In this study natural convection heat transfer of Cu–water nanofluid in a cold outer circular enclosure containing a hot inner sinusoidal circular cylinder in the presence of horizontal magnetic field is investigated numerically using the Control Volume based Finite Element Method (CVFEM). Both circular enclosure and inner cylinder are maintained at constant temperature. The governing equations of fluid motion and heat transfer in their vorticity stream function form are used to simulate the fluid flow and heat transfer. The effective thermal conductivity and viscosity of nanofluid are calculated using the Maxwell–Garnetts (MG) and Brinkman models, respectively. The calculations were performed for different governing parameters such as the Hartmann number, Rayleigh number, values of the number of undulations of the inner cylinder and nanoparticle volume fraction. The results indicate that in the absence of magnetic field, enhancement ratio decreases as Rayleigh number increases while an opposite trend is observed in the presence of magnetic field. Also it is found that the average Nusselt number is an increasing function of nanoparticle volume fraction, the number of undulations and Rayleigh numbers while it is a decreasing function of Hartmann number.     

Influence of Viscous Dissipation on non‐Darcy Mixed Convection Flow of Nanofluid

In this paper, the steady fully developed non‐Darcy mixed convection flow of a nanofluid in a vertical channel filled with a porous medium with different viscous dissipation models is analyzed. The Brinkman‐Forchheimer extended Darcy model is used to describe the fluid flow pattern in the channel. The transport equations for a nanofluid are solved analytically using the seminumerical‐analytical method known as differential transformation method, and numerically with the Runge‐Kutta shooting method. Finally, the influence of pertinent parameters, such as solid volume fraction, different nanoparticles, mixed convection parameter, Brinkman number, Darcy number, and inertial parameter on the velocity and temperature fields are shown graphically. The results show that velocity and temperature are enhanced when the mixed convection parameter, Brinkman number, and Darcy number increases whereas solid volume fraction and inertial parameter decreases the velocity and temperature fields. The obtained results show that the nanofluid enhances the heat transfer process significantly.     

Flow and thermal analysis of Oldroyd 8 constant fluid in a porous channel

The present research is based on the thermal and flow properties of the viscoelastic Oldroyd 8 constant fluid in an upright microchannel. The energy and momentum equations were solved with the support of temperature Jump and velocity slip boundary conditions. To measure the irreversibility rate of the flow system, the acquired results of velocity and thermal equations were used. To crack the current mathematical model problem, the numerical Runge–Kutta–Fehlberg method was used. With the aid of graphs, the effect of physical parameters such as thermal radiation, thermal-dependent heat source, Joule heating, fluid parameters, velocity slip, and temperature Jump parameters on the fluid flow, thermal energy, and system entropy generation was discussed. Fluid parameters have different effects on the velocity profile. The Grashof and Hartmann numbers demonstrate opposite effects on the momentum field. The thermal energy of the system reduces with thermal radiation and temperature Jump factor. The thermal radiation, Hartmann number, and temperature Jump parameters reduce the system's irreversibility rate. With the Brinkman number and temperature Jump parameter, the irreversibility ratio increases.