Journal of Transport Phenomena in Nano and Micro Scales
Volume:2 Issue: 2, Summer - Autumn 2014

In this study heat transfer and fluid flow characteristics of Al2O3/water nanofluid in a serpentine microchannel is numerically investigated. A constant heat flux is applied on microchannel wall and a single-phase model has been adopted using temperature-dependent properties. The effects of pertinent factors such as Reynolds number (Re=10, 20, 50 and 100), particle volume fraction (𝛷=0(distilled water), 2, 4 and 8%) and heat flux (q=5, 10 and 15 W/cm2), on the velocity and temperature field, average heat transfer coefficient (havg), pressure drop (Δp), and thermal-hydraulic performance (η) are evaluated. The results show that the use of nanofluid causes increased velocity gradient near the wall which is more remarkable for φ = 8%. The results also reveal that the heat transfer rate increases as nanoparticle volume fraction and Reynold number increase and a maximum value 51% in the average heat transfer coefficient is detected among all the considered cases when compared to basefluid (i.e., water). It is found that a higher heat flux leads to heat transfer enhancement and reduction in pressure drop. Finally, thermal-hydraulic performance is calculated and it is seen that the best performance occurs for Re =10 and φ = 4%

Thin bonded films have many applications in antireflection and reflection coating, insulating and conducting films and semiconductor industries. Thermal conductivity is one of the most important parameter for power packaging since the thermal resistance of the interconnections is directly related to the heat removal capability and thermal management of the power package. The defects in materials play very important role on the effective thermal conductivity. In this paper, finite element method (FEM) was utilized to simulate the effect of pores on the effective thermal conductivity of sintered silver nanoparticles film. The simulation results indicate that the effective thermal conductivity of film is different at different directions and would be enhanced when the pore angle is 90. The simulation results will help us to further understand the heat transfer process across highly porous structures and will provide us a powerful guide to design coating with high thermal insulation or conductor property. Because of there is no similar experimental data for this simulation results, this paper is a comparative work among three different models.

The falling and settling of solid particles in gases and liquids is a natural phenomenon happens in many industrial processes. This phenomenon has altered pure forced convection to a combination of heat conduction and heat convection in a flow over a plate. In this paper, the coupling of conduction (inside the plate) and forced convection of a non-homogeneous nanofluid flow (over a flat plate) is investigated, which is classified in conjugate heat transfer problems. Two-component four-equation non-homogeneous equilibrium model for convective transport in nanofluids has been applied that incorporates the effects of nanoparticle migration due to the thermophoresis Nt, Brownian motion Nb, and Lewis number Le simultaneously. Employing similarity variables, we have transformed the basic non-dimensional partial differential equations to ordinary differential ones and then solved numerically. Moreover, variation of the heat transfer and concentration rates with thermal resistance of the plate is studied in detail. Setting the lowest dependency of heat transfer rate to the thermal resistance of the plate as a goal, we have shown that for two nanofluids with similar heat transfer characteristics, the one with higher Brownian motion is desired.

Flow field and heat transfer of a nanofluid, whose non-Newtonian behavior has been demonstrated in the laboratory, in a square enclosure have been numerically modeled and investigated. To estimate the viscosity of nanofluid, experimental data of Hong and Kim, 2012 have been used, and a new model has been proposed. Finally, the obtained results have been compared to those of Newtonian behavior. The results obtained by the numerical simulation indicate that the average Nusselt number with non-Newtonian behavior has a value less than the Newtonian behavior. Also for the case in which the nanofluid is non-Newtonian, the buoyancy force is often insignificant, and forced convection dominates. By adding the nanoparticles, the average Nusselt number for the non-Newtonian nanofluid increases, but for the Newtonian nanofluid, depending on the dominant of natural or forced convection in the flow, it decreases or increases, respectively. On the other hand, with increasing the Reynolds number, the heat transfer rate increases for both Newtonian and non-Newtonian fluid at any constant Grashof number, while with increasing of Grashof number at a given temperature difference and a constant Reynolds number, the heat transfer rate increases and decreases in Newtonian and non-Newtonian nanofluids, respectively.

Gas mixing in a T-shape micro mixer has been simulated using the Direct Simulation Monte Carlo (DSMC) method. It is considered that the adequate mixing occurs when the mass composition of the species, CO or N2, deviates below 1 % from their equilibrium composition. The mixing coefficient is defined as the ratio of the mixing length to the main channel’s height. As the inlet Kn increases, while the diffusion of the molecules behaves more active, the mixing coefficient decreases. Furthermore, increasing the inlet pressure will cause the mixing length to increase, since the convection effect of the gas stream is more pronounced compared with the diffusion effect. Increasing the gas flow temperature or the wall temperature can enhance the mixing performance, while the effect of increasing the wall temperature is more significant. Walls with diffuse reflectors show more enhancement in mixing coefficient compared with the specular reflectors.

In this work, the characteristics of flow and heat transfer of a fluid containing nano particles of aluminum oxide with the water volume fraction (0.1-0.2-0.3)(V/V) percent of the reports. The overall heat transfer coefficient, heat transfer and the average heat transfer fluid containing nano water - aluminum oxide in a horizontal double pipe counter flow heat exchanger under turbulent flow conditions is studied. In the present study, aluminum oxide nanoparticles with a diameter of about 20 nm are used. The results show that the overall heat transfer coefficient and the overall heat transfer fluid based on nano-fluid heat transfer coefficient is slightly higher (up to about 5-12 percent).Nano-fluid heat transfer coefficient and average heat transfer increased with nano-fluid mass flow rate increases with increasing temperature and water nano-fluid, fluid temperature increases and Heated (Author) nano-fluid heat transfer coefficient is greatly influenced. The use of nano-fluid pressure may cause slight errors in the calculation.

In this paper, the measurement of the viscosity of ZnO in ethylene glycol, propylene glycol, mixture of ethylene glycol and water (60:40 by weight), and a mixture of propylene glycol and water (60:40 by weight) and the thermal conductivity in ethylene glycol and propylene glycol as base fluids in the range of temperature from 25 ºC to 60 ºC are investigated. The results indicate that as the temperature increase the viscosity of nanofluid decrease and the thermal conductivity of both base fluid and nanofluid increase. Several existing models for thermal conductivity and viscosity are compared with the experimental data, and they do not demonstrate good comparison agreement. Finally, some new models for predicting the effective viscosity and thermal conductivity are proposed. Furthermore, the viscosity of the base fluid affects the thermal conductivity variation of the nanofluids. The results indicate that the largest enhancements in thermal conductivity are 15% and 9% for EG and PG as base fluids, respectively.