فهرست مطالب

Transport Phenomena in Nano and Micro Scales - Volume:5 Issue:1, 2017
  • Volume:5 Issue:1, 2017
  • تاریخ انتشار: 1395/10/27
  • تعداد عناوین: 7
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  • A. R. Rahmati, R. Ehsani Pages 1-8
    Because of its kinetic nature and computational advantages, the Lattice Boltzmann method (LBM) has been well accepted as a useful tool to simulate micro-scale flows. The slip boundary model plays a crucial role in the accuracy of solutions for
    micro-channel flow simulations. The most used slip boundary condition is the Maxwell slip model. The results of Maxwell slip model are affected by the accommodation coefficient significantly, but there is not an explicitly relationship between properties at wall and accommodation coefficient. In the present wok, Langmuir slip model is used beside LBM to simulate micro-channel and micro-orifice flows. Slip velocity and nonlinear pressure drop profiles are presented as two major effects in such flows. The results are in good agreement with existing results in the literature.
    Keywords: Lattice Boltzmann method, Langmuir slip model, Micro, channel, Micro, orifice
  • P. Elyasi, A. R. Shateri Pages 9-22
    Laminar boundary layer flow and heat transfer of Newtonian nanofluid over a stretching sheet with the sheet velocity distribution of the form (Uw=CXβ) and the wall temperature distribution of the form (Tw= T∞ axr) for the steady magnetohydrodynamic(MHD) is studied numerically. The governing momentum and energy equations are transformed to the local non-similarity equations using the appropriate transformations. The set of ODEs are solved using Keller–Box implicit finite-difference method. The effects of several parameters, such as magnetic parameter, volume fraction of different nanoparticles (Ag, Cu, CuO, Al2O3 and TiO2), velocity parameter, Prandtl number and temperature parameter on the velocity and temperature distributions, local Nusselt number and skin friction coefficient are examined. The analysis reveals that the temperature profile increases with increasing magnetic parameter and volume fraction of nanofluid. Furthermore, it is found that the thermal boundary layer increases and momentum boundary layer decreases with the use of water based nanofluids as compared to pure water. At constant volume fraction of nanoparticles, it is also illustrated that the role of magnetic parameter on dimensionless temperature becomes more effective in lower value.
    Keywords: Boundary Layer Flow, MHD, Nanofluid, Stretching Sheet
  • A.R. Noghrehabadi, E. Hajidavaloo, M. Moravej Pages 23-29
    One of the effective methods to improve the thermal efficiency of solar collectors is using nanofluids as the coolant. The present study experimentally investigated the effect of SiO2/water nanofluid with 1% mass fraction on the performance of a symmetric collector, i.e. conical solar collector. The conical solar collector with 1 m2 area and normal to the earth was tested in Ahvaz, a city in the southwest of Iran. The experiments performed under ASHRAE standard without any surfactants based on the solar radiation, mass flow rate, and temperatures variation. Results demonstrated that the thermal efficiency and the temperature performance can be enhanced through SiO2/water nanofluid in comparison with pure water. The maximum efficiency and outlet-inlet difference temperature of conical collector using nanofluid was about 62% and 6.8 0C respectively. Moreover, the collector behaviors are more efficient with the nanofluid than with pure water in the higher values of the flow rate and sun radiation.
    Keywords: Conical solar collector, SiO2, water nanofluid, Solar radiation, Flow rate, Efficiency
  • A. A. Abbasian Arani, M. Abbaszadeh, A. Ardeshiri Pages 30-43
    Mixed convection fluid flow and heat transfer of water-Al2O3 nanofluid inside a lid-driven square cavity has been examined numerically in order to find the optimal distribution of discrete heat sources on the wall of a cavity. The effects of different heat source length, Richardson number and Grashof number on optimal heat source location has been investigated. Moreover, the average Nusselt number on the heat source for two models of nanofluid, constant properties and variable properties, are compared. The obtained results showed that by decreasing the Richardson number and increasing the Grashof number, heat transfer rate decreases.
    Also by reducing the Richardson number, optimal heat source location move to the top of the wall and with augmentation of Richardson number, heat source optimal location move to the middle of the wall. Furthermore, the overall heat transfer increases by increasing nanoparticles volume fraction. Moreover, it was found that for two different models of nanofluids and in Ri=1, the values of the average Nusselt number are close together.
    Keywords: Mixed convection, Nanofluids, Heat sources, Optimization
  • H. Hassanzadeh Afrouzi, A. Moshfegh, M. Farhadi, K. Sedighi Pages 44-53
    The microphase separation of hydrated perfluorinated sulfonic acid membrane Nafion was investigated using Dissipative Particle Dynamics (DPD). The nafion as a polymer was modelled by connecting coarse grained beads which corresponds to the hydrophobic backbone of polytetrafluoroethylene and perfluorinated side chains terminated by hydrophilic end particles of sulfonic acid groups [1, 2]. Each four water molecule coarse grained in a bead to obtain the same bead size as built in Nafion model. The morphology of hydrated Nafion is studied for branching density of 1144, an example of Nafion EW1200, water content of 10%, 20% and 30% and polymer molecular weight of 5720, 11440 and 17160. The results show water particles and hydrophilic particles of Nafion side chains spontaneously form aggregates and are embedded in the hydrophobic phase of Nafion backbone. The averaged water pore diameter and the averaged water clusters distance were found to rises with water volume fraction.
    Keywords: Fuel cell, Membrane, Nafion, Microphase separation, water network, DPD
  • H. Nowruzi, H. Ghassemi Pages 54-63
    In the current study, five different atomistic water models (AWMs) are implemented, In order to investigate the impact
    of AWMs treatment on the water velocity profile and density number. For this purpose, Molecular dynamics simulation (MDS) of Poiseuille flow in a nano-channel is conducted. Considered AWMs are SPC/E, TIP3P, TIP4P, TIP4PFQ and TIP5P. To assessment of the ability of each model in prediction of velocity profile, it is compared with analytic velocity profile. Furthermore, MDS results of density number are evaluated by real non-dimensional value for density number of water (Rho*). Based on computational results,predicted velocity profile from MDS is in appropriate accordance to analytic solution based on the Navier–Stokes equations. In addition, SPC/E and TIP4P models prepare the best prediction of the velocity profile, and are recommended where the averaged magnitude of velocity across the nano-channel is essential. Furthermore, a jump in velocity of TIP5P and TIP4P models is revealed in the vicinity of the nano-channel walls. However, approximately similar quantity is detected in the flow velocity of all different AWMs near the nano-channel walls. Finally, numerical results related to density number show, the TIP5P water model has higher compliance with the intended Rho*, and thus this model is suggested, where density number plays an important role in our MDS.
    Keywords: Molecular dynamics simulation, Atomistic water models, Analytic solution, Velocity profile, Density number, Lennard, Jones
  • B. Raei, F. Shahraki, M. Jamialahmadi, S.M. Peyghambarzadeh Pages 64-77
    In this paper, overall heat transfer coefficient and friction factor of water based γ-Al2O3 nanofluid in a double tube counter flow heat exchanger have been measured experimentally under turbulent flow condition. For better dispersion of γ-Al2O3 nanoparticles in distilled water, magnetic stirrer and ultrasonic vibrator (with a power of 240 kW and frequency of 35 kHz) were implemented. The stabilized γ-Al2O3 /water nanofluid have been examined at the concentrations of 0.05 and 0.15 vol. % with variation of flow rates in the range of 7–9 l/min. Nanofluid enters the inner tube of the heat exchanger at different temperatures including 45, 55,and 65 °C. Results demonstrated that increasing the nanofluid flow rate, concentration and inlet temperature can improve the overall heat transfer coefficient and heat transfer rate. Also, the ratio of the overall heat transfer coefficient of nanofluid to that of pure water decreased with increasing the nanofluid flow rate. Meanwhile, the maximum enhancements of the overall heat transfer coefficient and heat transfer rate and friction factor compared with those of base fluid (distilled water) are respectively equal to 19.3%, 10% and 25% which is occurred at the concentration of 0.15 vol. %.
    Keywords: Double tube heat exchanger, Nanofluid, Overall heat transfer coefficient