Nanofluid In Microchannel Research Articles

Experimental and numerical investigation into the hydrodynamics of nanofluids in microchannels

The hydrodynamic study of nanofluids in microchannel is carried out. For this, three microchannels of hydraulic diameters of 130, 211 and 300μm are fabricated by photolithographic and wet etching processes on silicon wafers. Alumina nanofluids with concentrations 0.25vol.%, 0.5vol.% and 1.0vol.% with particle sizes 45nm and 150nm are prepared, stabilized and characterized by standard methods like sonication, pH variation, Transmission Electron Microscope (TEM) and Dynamic Light Scattering (DLS) measurements. The base fluids used are water and Ethylene Glycol. The effect of volume fraction, channel size, particle size and base fluids are presented and analyzed. It is shown that there is an early transition to turbulence for 211 and 301μm channels. In addition, numerical modeling is carried out by using mixture rule and discrete phase modeling approach. This approach is found to be more suitable at higher Reynolds numbers whereas at lower Reynolds numbers the conventional mixture model can be used. Shear induced particle migration is identified to be an important phenomenon at higher Reynolds number and hence the reason for difference between these two models.

Entropy generation of viscous dissipative nanofluid flow in microchannels

An analytical study on the viscous dissipation effect on entropy generation in laminar fully developed forced convection of water–alumina nanofluid in circular microchannels is reported. In the first-law analysis, closed form solutions of the temperature distributions in the radial direction for the models with and without viscous dissipation term in the energy equation are obtained. The results show that the heat transfer coefficient decreases with nanoparticle volume fraction largely in the laminar regime of nanofluid flow in microchannel when the viscous dissipation effect is taken into account. In the second-law analysis, the two models are compared by analyzing their relative deviations in entropy generation for different Reynolds number and nanoparticle volume fraction. When the viscous dissipation is taken into account, the temperature distribution is prominently affected and consequently the entropy generation ascribable to the heat transfer irreversibility is significantly increased. The increase of entropy generation induced by the increase of nanoparticle volume fraction is attributed to the increase of both the thermal conductivity and viscosity of nanofluid which causes augmentation in the heat transfer and fluid friction irreversibilities, respectively. By incorporating the viscous dissipation effect, both thermal performance and exergetic effectiveness for forced convection of nanofluid in microchannels dwindle with nanoparticle volume fraction, contrary to the widespread conjecture that nanofluids possess advantage over pure fluid associated with higher overall effectiveness from the aspects of first-law and second-law of thermodynamics.

Experimental thermal–hydraulic evaluation of CuO nanofluids in microchannels at various concentrations with and without suspension enhancers

This experimental study focuses on the effect of two important factors on the heat transfer and flow properties of copper oxide (CuO)/water nanofluids in a parallel microchannel flow configuration. The first factor considered is the solid media (CuO) concentration. In this investigation, concentration values of 0.005%, 0.01%, and 0.1% by volume were tested. The second factor is the use of a surfactant, cetyltrimethylammonium bromide (CTAB), as a suspension enhancer. All together, these two factors led to a total of six types of nanofluids, which were tested in addition to pure water, the reference fluid. The experimental setup allowed for the determination of the hydrodynamic and thermal performance of each nanofluid. In addition, a selection of the nanofluids were characterized by the use of scanning transmission electron microscopy (STEM) and Dynamic Light Scattering (DLS) techniques. The DLS transient settling measurements showed that for a nanofluid with a concentration of 0.1% by volume, the nanoparticle dispersion and suspension is negatively affected unless a surfactant is used. Hydrodynamic losses, which were evaluated by comparing the effect of the imposed pressure drop on the mass flow rate, were not meaningfully affected by the composition of the nanofluids tested. The measurements also showed that nanofluids containing a surfactant generally provided a modest increase in heat transfer rate when compared with tests performed using pure water. The largest increase was about 17% for a fluid with a concentration of 0.01% by volume. Consequently, the gains in heat transfer do not appear to be accompanied by a significant pumping power penalty. The results of this study suggest that the use of a surfactant is essential in maintaining a proper suspension of nanoparticles in the fluid, especially at higher concentrations.

Experimental and Numerical Investigation Into the Heat Transfer Study of Nanofluids in Microchannel

There are very few detailed experimental investigations about the heat transfer behavior of nanofluids in microchannel. The heat transfer behavior of nanofluids in microchannel is investigated. Two microchannels with hydraulic diameters 218 and 303 μm are fabricated by wet etching process on silicon wafer. An experimental set-up having provision of flow in the channel and temperature measurement along with bottom wall temperature is built-up. Alumina nanofluids with concentrations of 0.25 vol. %, 0.5 vol. %, and 1 vol. % with 45 nm are prepared, stabilized, and characterized by standard methods. The thermal conductivity and viscosity used in the study were measured and analyzed. The base fluids used are water and ethylene glycol. The effect of volume fraction, channel size, particle size, and base fluids are presented and analyzed. An important phenomenon of dispersion is observed. In addition, numerical modeling is carried out by using discrete phase approach. Shear induced particle migration is identified to be the reason of difference for dispersion of particles. The Brownian and thermophoretic forces are responsible for major changes in particle concentration and their movement. Also, it was found that better heat transfer characteristics can be obtained by higher concentration of nanofluids and by low viscous base fluids.

Analytical Study on Forced Convection of Nanofluids With Viscous Dissipation in Microchannels

This article presents an analytical study on forced convection of laminar fully developed flow of incompressible, constant-property nanofluids in microchannels. Closed-form solutions for the temperature distributions in the radial direction with the incorporation of viscous dissipation are obtained under isoflux boundary condition. The effects of the governing parameters, including modified Brinkman number, thermal conductivity ratio, and nanoparticle volume fraction of the nanofluids, on the temperature distributions are investigated and analyzed for both heating and cooling processes. The heat transfer performance characterized by the Nusselt number is investigated based on the effects induced by these parameters. In the comparison between the models with and without viscous dissipation, it is found that the thermal performance of a microchannel is overrated when viscous dissipation is excluded in the analysis. It is concluded that these governing parameters are intimately interrelated in the flow and thermal analyses of nanofluids in microchannels. The interrelationship of the viscous dissipation effect and the nanoparticle volume fraction is examined in a contour deviation map of Nusselt numbers between the model with and without considering the viscous dissipation.

Critical investigation of heat transfer enhancement using nanofluids in microchannels with slip and non-slip flow regimes

The Reynolds number in nanofluid studies depends on the inlet velocity and the kinematic viscosity of nanofluid. The nanofluid kinematic viscosity increases with an increase in nanoparticles volume fraction while the inlet velocity should be increased to keep the Reynolds number constant. Therefore, it is not clear that either increasing the nanoparticles volume fraction or increasing the inlet velocity has a major role on heat transfer enhancement in nanofluids flow studies which are done at constant Reynolds numbers. In this study, forced convection Al 2 O 3 -water nanofluid flows in two-dimensional rectangular microchannels have been investigated to study heat transfer enhancement due to addition of the nanoparticles to the base fluid especially in microchannels at low Reynolds number. Three different cases are examined to evaluate proportion impact of increasing nanoparticles volume fraction ( ϕ ) and the inlet velocity ( u in ) on heat transfer enhancement. Two-dimensional Navier–Stokes and energy equations accompany with the slip velocity and the jump temperature boundary conditions expressions have been discretized using the Finite Volume Method (FVM). The Brownian motions of nanoparticles have been considered to determine the thermal conductivity of nanofluids. The calculated results show good agreement with the previous numerical and analytical data. It is found that at a given Reynolds number, the major enhancement in the Nusselt number is not due to increasing the nanoparticle concentrations but it is due to the increasing the inlet velocity to reach a constant Re . Constant Reynolds number studies of nanofluids are not sufficient approach to evaluate the heat transfer and the skin friction factor due to the nanofluids usage. ► At a specific Reynolds number, the major enhancement on the Nusselt number is due to increasing the inlet velocity to reach a constant Re. ► In the nanofluids studies, fixing the Reynolds number is not a sufficient comparison to evaluate the heat transfer and the skin friction factor. ► A logical comparison will be achieved when the pumping power is assumed constant.

Forced convective heat transfer of nanofluids in microchannels

Convective heat transfer coefficient and friction factor of nanofluids in rectangular microchannels were measured. An integrated microsystem consisting of a single microchannel on one side, and two localized heaters and five polysilicon temperature sensors along the channel on the other side were fabricated. Aluminum dioxide (Al 2O 3) with diameter of 170 nm nanofluids with various particle volume fractions were used in experiments to investigate the effect of the volume fraction of the nanoparticles to the convective heat transfer and fluid flow in microchannels. The convective heat transfer coefficient of the Al 2O 3 nanofluid in laminar flow regime was measured to be increased up to 32% compared to the distilled water at a volume fraction of 1.8 volume percent without major friction loss. The Nusselt number measured increases with increasing the Reynolds number in laminar flow regime. The measured Nusselt number which turned out to be less than 0.5 was successfully correlated with Reynolds number and Prandtl number based on the thermal conductivity of nanofluids.

Thermally developing electroosmotic transport of nanofluids in microchannels

A theoretical analysis is presented in this work to assess the influence of nanofluids on thermally developing and hydrodynamically developed electroosmotic transport in parallel plate microchannels (Graetz problem). The hydraulic diameters of the microchannels are assumed to be beyond a certain threshold limit, so that the electric double layers formed adjacent to the plates do not overlap with each other. The volumetric heating arising from the conduction currents in the flow is modeled using Ohm’s law. The viscous generation terms in the energy equation are neglected, based on the earlier findings that the consequent effects are negligible as compared to the Joule heating effects in electroosmotically driven microchannel flows. Closed form expressions for the pertinent temperature distributions and the Nusselt number variations are obtained by employing the method of separation of variables in conjunction with an eigen value formulation, in order to assess the influence of volume fraction of the dispersed nano-particles on the overall rates of convective transport. It is revealed that the effects of nano-particles in the fluid turn out to be significant in the thermal entrance region only, especially for higher Peclet number values. The implications of the incorporation of nanofluids are demonstrated to be somewhat non-trivial in nature, and are strongly determined by the effective Peclet number values obtained on the basis of the phase-integral values of the thermo-physical properties and the pertinent flow parameters.