Mostafa Shojaeian

Viscous dissipation effect on heat transfer characteristics of mixed electromagnetic/pressure driven liquid flows inside micropumps

This paper presents the effect of viscous dissipation on heat transfer characteristics of mixed electromagnetic/ pressure driven liquid slip flows inside parallel plate microchannels. Flow is governed by the Navier-Stokes equations subject to the imposition of electromagnetic field with the boundary condition appropriate to the slip flow regime. For isoflux walls, some closed form expressions for the local and bulk temperature profiles and the Nusselt number in terms of dimensionless slip length, Hartmann number and Brinkman number are given, while the viscous dissipation is also taken into account. Then the analytical solutions derived in this analysis are elaborated. It turns out that since the contribution of the viscous dissipation on the Nusselt number under the given circumstances, especially a stronger electromagnetic field, may reach to nearly 10%, therefore, the viscous heating should be taken into consideration. Otherwise, the heat transfer rate may be overestimated or underestimated depending on whether the fluid is being heated or cooled. Also, there are singularities in Nusselt number values, which move close together by including the viscous dissipation. Further, an increase in the Hartmann number increases the convection, which is especially reflected in smaller values of dimensionless slip length.

Investigation of change in surface morphology of heated surfaces upon pool boiling of magnetic fluids under magnetic actuation

Nanofluids are becoming a significant candidate for new generation coolants to be used in industrial applications. In order to reduce clustering and sedimentation of nanoparticles and improve the heat transfer performance simultaneously, magnetic fluids prepared with magnetic Fe3O4 nanoparticles dispersed in water, which were placed in a pool and were exposed to varying magnetic fields to actuate nanoparticles in the system. The effect of magnetic actuation on boiling heat transfer characteristics and on the surface morphology of the pool was examined. An average enhancement of 29% in boiling heat transfer was achieved via magnetic actuation with rather low magnetic field (magnetic flux densities up to 11 mT) densities. Furthermore, it was observed that magnetic actuation significantly prevented the deposition and sedimentation of the nanoparticles in the pool. Otherwise, significant destabilization of nanoparticles causing aggregation and heavy sedimentation was present as a result of the performed surface analysis. Even though magnetic actuation reduced the sedimentation on the macroscale, the deposition of a thick and porous film occurred onto the pool floor, increasing the surface roughness.

Numerical Heat Transfer and Entropy Analysis on Liquid Slip Flows Through Parallel-Plate Microchannels

In this study, two-dimensional (2D) numerical simulations of liquid slip flows in parallel-plate microchannels have been performed to obtain heat transfer characteristics and entropy generation rate under asymmetric heating conditions. Heat transfer analysis has been conducted along with second-law analysis through utilizing temperature-dependent thermophysical properties. The results indicate that temperature-dependent thermophysical properties have a positive effect on convective heat transfer and entropy generation. Nusselt numbers of the upper and lower plates and global entropy generation rates are significantly affected by slip parameter and heat flux ratio. It is shown that Nusselt number of the lower plate may have very large but finite values at a specific heat flux ratio. This finding resembles to analytical solutions, where singularities leading to an infinite Nusselt number exist.

Entropy Generation Analysis of Laminar Flows of Water-Based Nanofluids in Horizontal Minitubes under Constant Heat Flux Conditions

During the last decade, second law analysis via entropy generation has become important in terms of entropy generation minimization (EGM), thermal engineering system design, irreversibility, and energy saving. In this study, heat transfer and entropy generation characteristics of flows of multi-walled carbon nanotube-based nanofluids were investigated in horizontal minitubes with outer and inner diameters of ~1067 and ~889 µm, respectively. Carbon nanotubes (CNTs) with outer diameter of 10–20 nm and length of 1–2 µm were used for nanofluid preparation, and water was considered as the base fluid. The entropy generation based on the experimental data, a significant parameter in thermal design system, was examined for CNTs/water nanofluids. The change in the entropy generation was only seen at low mass fractions (0.25 wt.% and 0.5 wt.%). Moreover, to have more insight on the entropy generation of nanofluids based on the experimental data, a further analysis was performed on Al2O3 and TiO2 nanoparticles/water nanofluids from the experimental database of the previous study of the authors. The corresponding results disclosed a remarkable increase in the entropy generation rate when Al2O3 and TiO2 nanoparticles were added to the base fluid.

Numerical investigation of slip flows through 2-d u-shaped microchannels

Micro-electro-mechanical systems (MEMS) offer vast applications and those involving fluid flow/gas flows typically operate in the slip flow regime, where the normal non-slip condition assumptions for boundary conditions are not valid. In this study, two-dimensional numerical simulations of continuum and slip flows for U-shaped microchannels were performed to evaluate flow characteristics by means of the ANSYS-FLUENT software. In order to model slip condition existing at the solid-liquid interface of microchannels, a user defined subroutine was linked to the software. The effect of Reynolds number and geometric parameters such as the bend shape and the distance between legs on fluid flow and heat transfer characteristics were examined in detail. The computational results showed that increasing either the slip length or Reynolds number decreased the average friction factor, while the distance between legs does not have any effect on it. Moreover, it was found that the averaged Nusselt number would increase with Reynolds number and the slip length, but increasing the distance between legs decreased the averaged Nu for square bend compared to its negligible increase for circular bend.

Numerical investigation of fully developed fluid flow and heat transfer in double-trapezoidal microchannels

Fully developed fluid flow and heat transfer characteristics of double-trapezoidal microchannels with constant wall temperature are numerically investigated in the slip flow regime. The governing equations are solved together with the appropriate boundary conditions using finite volume method. The effect of rarefaction on Poiseuille number, Po, and Nusselt number, Nu, is studied for Knudsen numbers, Kn, varying from 0 to 0.1. The effects of base angle, B, and aspect ratio, A, on the fluid flow and the heat transfer characteristics are also examined. The results reveal that the rarefaction and the cross-section shapes have prominent effects on these characteristics of double-trapezoidal microchannels. According to the results, the Poiseuille number decreases with increasing Kn, while the values of the Nusselt number completely depend on the impacts of the rarefaction and the fluid-surface interaction. Po and Nu decrease with aspect ratio for A 1. Moreover, an increase in the base angle has a positive effect on Po and Nu, however this increasing trend is less pronounced for B > 60 ° and A < 1.67.Copyright