Mohammad Reza Safaei

INVESTIGATION OF HEAT TRANSFER ENHANCEMENT IN A FORWARD-FACING CONTRACTING CHANNEL USING FMWCNT NANOFLUIDS

The turbulent forced convection heat transfer of water/functionalized multi-walled carbon nanotube (FMWCNT) nanofluids over a forward-facing step was studied in this work. Turbulence was modeled using the shear stress transport K-ω model. Simulations were performed for Reynolds numbers ranging from 10,000 to 40,000, heat fluxes from 1,000 to 10,000 W/m2, and nanoparticle volume fractions of 0.00% to 0.25%. The two-dimensional governing equations were discretized with the finite volume method. The effects of nanoparticle concentration, shear force, heat flux, contraction, and turbulence on the hydraulics and thermal behavior of nanofluid flow were studied. The model predictions were found to be in good agreement with previous experimental and numerical studies. The results indicate that the Reynolds number and FMWCNT volume fraction considerably affect the heat transfer coefficient; a rise in local heat transfer coefficient was noted when both Reynolds number and FMWCNT volume fraction were increased for all cases. Moreover, the contraction of the channel passage leads to the formation of two recirculation regions with augmented local heat transfer coefficient value.

Investigation of rib's height effect on heat transfer and flow parameters of laminar water-Al2O3 nanofluid in a rib-microchannel

The laminar forced convection heat transfer of water-Al2O3 nanofluids through a horizontal rib-microchannel was studied. The middle section of the down wall of microchannel was at a lower temperature compared to the entrance fluid. Simulations were performed for Reynolds numbers 10 and 100 and nanoparticle volume fractions of 0.00 to 0.04, inside a two-dimensional rectangular microchannel with 2.5mm length and 25µm width. The two-dimensional governing equations were discretized using a finite volume method. The effects of ribs hight and position, nanoparticle concentration and Reynolds number on the thermal and hydraulics behavior of nanofluid flow were investigated. The results were portrayed in terms of velocity, temperature and Nusselt number profiles as well as streamlines and isotherm contours. The model predictions were found to be in good agreements with those from previous studies. The results indicate that the normal internal ribs or turbulators, can significantly enhance the convective heat transfer within a microchannel. However, the added high ribs can cause a larger friction factor, compared to that in the corresponding microchannel with a constant height of the ribs. The results also illustrate that by increasing the ribs heights and volume fraction of nanoparticles, friction coefficient, heat transfer rate and average Nusselt number of the ribbed-microchannels tend to augment. In addition, the simulation results confirm that changing the solid volume fraction and the ribs height, cause significant changes in temperature and dimensionless velocity along the centerline of the flow, through the ribbed areas.

Numerical simulation of laminar to turbulent nanofluid flow and heat transfer over a backward-facing step

This paper presents a numerical study of heat transfer to turbulent and laminar Cu/water flow over a backward-facing step. Mathematical model based on finite volume method with a FORTRAN code is used to solve the continuity, momentum, energy and turbulence equations. Turbulence was modeled by the shear stress transport (SST) K-ω Model. In this simulation, three volume fractions of nanofluid (0%, 2% and 4%), a varying Reynolds number from 50 to 200 for the laminar range and 5000 to 20,000 for the turbulent range, an expansion ratio of 2 and constant heat flux of 4000 W/m2 were considered. The results show the effect of nanofluid volume fraction on enhancing the Nusselt number in the laminar and turbulent ranges. The effect of expansion ratio was clearly observed at the downstream inlet region where the peak of the Nusselt number profile was referred to as enhanced heat transfer due to the generated recirculation flow. An increase of pressure drop was evident with an increasing Reynolds number and decreasing nanofluid volume fraction, while the maximum pressure drop was detected in the downstream inlet region. A rising Reynolds number caused an increasing Nusselt number, and the highest heat transfer augmentation in the present investigation was about 26% and 36% for turbulent and laminar range, respectively compared with pure water.

Effect of Magnetic Field on Free Convection in Inclined Cylindrical Annulus Containing Molten Potassium

Three-dimensional (3D) numerical simulation of natural convection of an electrically conducting fluid under the influence of a magnetic field in an inclined cylindrical annulus has been performed. The inner and outer cylinders are maintained at uniform temperatures and other walls are thermally insulated. The governing equations of this fluid system are solved by a finite volume (FV) code based on SIMPLER solution scheme. Detailed numerical results of heat transfer rate, Lorentz force, temperature and electric fields have been presented for a wide range of Hartmann number (0 ≤ Ha ≤ 60) and inclination angle (0 ≤ γ ≤ 90). The results indicate that a magnetic field can control the magnetic convection of an electrically conducting fluid. Depending on the direction and strength of the magnetic field, the suppression of convective motion was observed. For vertical cylindrical annulus, increasing the strength of the magnetic field causes the loss symmetry, and as the consequence, isotherms lose their circular shape. With increasing the Hartmann number the average Nusselt number approaches a constant value. For vertical annulus, the effect of Hartmann number on the average Nusselt number is not prominent compared to the case of horizontal annulus.

Basic effects of pulp refining on fiber properties—A review

The requirement for high quality pulps which are widely used in paper industries has increased the demand for pulp refining (beating) process. Pulp refining is a promising approach to improve the pulp quality by changing the fiber characteristics. The diversity of research on the effect of refining on fiber properties which is due to the different pulp sources, pulp consistency and refining equipment has interested us to provide a review on the studies over the last decade. In this article, the influence of pulp refining on structural properties i.e., fibrillations, fine formation, fiber length, fiber curl, crystallinity and distribution of surface chemical compositions is reviewed. The effect of pulp refining on electrokinetic properties of fiber e.g., surface and total charges of pulps is discussed. In addition, an overview of different refining theories, refiners as well as some tests for assessing the pulp refining is presented.

A survey on experimental and numerical studies of convection heat transfer of nanofluids inside closed conduits

Application of nanofluids in heat transfer enhancement is prospective. They are solid/liquid suspensions of higher thermal conductivity and viscosity compared to common working fluids. A number of studies have been performed on the effect of nanofluids in heat transfer to determine the enhancement of properties in addition to rearrangement of flow passage configurations. The principal objective of this study is to elaborate this research based on natural, forced, and the mixed heat transfer characteristics of nanofluids exclusively via convection for single- and two-phase mixture models. In this study, the convection heat transfer to nanofluids has been reviewed in various closed conduits both numerically and experimentally.

Comparison of the Finite Volume and Lattice Boltzmann Methods for Solving Natural Convection Heat Transfer Problems inside Cavities and Enclosures

Different numerical methods have been implemented to simulate internal natural convection heat transfer and also to identify the most accurate and efficient one. A laterally heated square enclosure, filled with air, was studied. A FORTRAN code based on the lattice Boltzmann method (LBM) was developed for this purpose. The finite difference method was applied to discretize the LBM equations. Furthermore, for comparison purpose, the commercially available CFD package FLUENT, which uses finite volume Method (FVM), was also used to simulate the same problem. Different discretization schemes, being the first order upwind, second order upwind, power law, and QUICK, were used with the finite volume solver where the SIMPLE and SIMPLEC algorithms linked the velocity-pressure terms. The results were also compared with existing experimental and numerical data. It was observed that the finite volume method requires less CPU usage time and yields more accurate results compared to the LBM. It has been noted that the 1st order upwind/SIMPLEC combination converges comparatively quickly with a very high accuracy especially at the boundaries. Interestingly, all variants of FVM discretization/pressure-velocity linking methods lead to almost the same number of iterations to converge but higher-order schemes ask for longer iterations.

Performance Evaluation of Nanofluids in an Inclined Ribbed Microchannel for Electronic Cooling Applications

Nanofluids are liquid/solid suspensions with higher thermal conductivity, compared to common working fluids. In recent years, the application of these fluids in electronic cooling systems seems prospective. In the present study, the laminar mixed convection heat transfer of different water–copper nanofluids through an inclined ribbed microchannel–– as a common electronic cooling system in industry––was investigated numerically, using a finite volume method. The middle section of microchannel’s right wall was ribbed, and at a higher temperature compared to entrance fluid. The modeling was carried out for Reynolds number of 50, Richardson numbers from 0.1 to 10, inclination angles ranging from 0° to 90°, and nanoparticles’ volume fractions of 0.0–0.04. The influences of nanoparticle volume concentration, inclination angle, buoyancy and shear forces, and rib’s shape on the hydraulics and thermal behavior of nanofluid flow were studied. The results were portrayed in terms of pressure, temperature, coefficient of friction, and Nusselt number profiles as well as streamlines and isotherm contours. The model validation was found to be in excellent accords with experimental and numerical results from other previous studies. The results indicated that at low Reynolds’ flows, the gravity has effects on the heat transfer and fluid phenomena considerably; similarly, with inclination angle and nanoparticle volume fraction, the heat transfer is enhanced by increasing the Richardson number, but resulting in a less value of friction coefficient. The results also represented that for specific Reynolds (Re) and Richardson (Ri) numbers, heat transfer and pressure drop augment‐ ed by increasing the inclination angle or volume fraction of nanoparticles. With regard to the coefficient of friction, its value decreased by adding less nanoparticles to the fluid or by increasing the inclination angle of the microchannel.

Numerical Study of Entropy Generation in a Flowing Nanofluid Used in Micro- and Minichannels

This article mainly concerns theoretical research on entropy generation influences due to heat transfer and flow in nanofluid suspensions. A conventional nanofluid of alumina-water (Al2O3-H2O) was considered as the fluid model. Due to the sensitivity of entropy to duct diameter, mini- and microchannels with diameters of 3 mm and 0.05 mm were considered, and a laminar flow regime was assumed. The conductivity and viscosity of two different nanofluid models were examined with the help of theoretical and experimentally determined parameter values. It was shown that order of the magnitude analysis can be used for estimating entropy generation characteristics of nanofluids in mini- and microchannels. It was found that using highly viscous alumina-water nanofluid under laminar flow regime in microchannels was not desirable. Thus, there is a need for the development of low viscosity alumina-water (Al2O3-H2O) nanofluids for use in microchannels under laminar flow condition. On the other hand, Al2O3-H2O nanofluid was a superior coolant under laminar flow regime in minichannels. The presented results also indicate that flow friction and thermal irreversibility are, respectively, more significant at lower and higher tube diameters.

Entropy Generation during Turbulent Flow of Zirconia-water and Other Nanofluids in a Square Cross Section Tube with a Constant Heat Flux

The entropy generation based on the second law of thermodynamics is investigated for turbulent forced convection flow of ZrO2-water nanofluid through a square pipe with constant wall heat flux. Effects of different particle concentrations, inlet conditions and particle sizes on entropy generation of ZrO2-water nanofluid are studied. Contributions from frictional and thermal entropy generations are investigated, and the optimal working condition is analyzed. The results show that the optimal volume concentration of nanoparticles to minimize the entropy generation increases when the Reynolds number decreases. It was also found that the thermal entropy generation increases with the increase of nanoparticle