فهرست مطالب

Transport Phenomena in Nano and Micro Scales - Volume:6 Issue:2, 2018
  • Volume:6 Issue:2, 2018
  • تاریخ انتشار: 1397/05/10
  • تعداد عناوین: 7
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  • Abdollah Goli, Iman Zahmatkesh * Pages 79-87
    In the present work, forced convection heat transfer of slip flow in porous micro-tubes with local thermal non-equilibrium between the gas and the solid matrix is investigated numerically. For this purpose, the flow is considered hydrodynamically developed but thermally developing. The Darcy-Brinkman-Forchheimer model in conjunction with separate energy equations for the gas and the solid matrix is used to describe the flow and heat transfer in the porous media. Moreover, both velocity slip and temperature jump are applied to the fluid at the wall. Simulation results are presented in terms of velocity field, distributions of the fluid and solid temperatures, the local Nusselt number, and the thermal entry length. The results indicate heat transfer improvement as a result of increase in the Peclet/Biot number. Increase in the Knudsen number or the modified conductivity ratio, however, diminishes the heat transfer rate. Although the choice of the Darcy/Forchheimer number may change the computational results, trends are not similar in the developing and the developed regions of the flow.
    Keywords: Forced convection, fully developed flow, micro-tube, Porous media, slip flow
  • Arjun Kozhikkatil Sunil *, Rakesh Kumar Pages 88-95
    The effect of thermal and solutal buoyancy induced by a discrete source of heat and mass transfer in a square duct under the influence of magnetic field, especially at the turbulent regime for the first time is reported. Al2O3/water nanofluid is used with constant heat flux from three discrete heat sources. In the present study, the effects of Reynolds number (100 to 3000), particle volume fraction (0 to 2%) and magnetic field (Ha = 10 to 90) on the Nusselt number, pressure drop and wall temperature (at the axial and height directions) are represented graphically and discussed quantitatively. Two percent Al2O3-water nanofluid under Ha = 10 can produce 81% increase in Nusselt number with only 60% increase in pressure drop when compared to base fluid at Re = 100. Four percent enhancement in nanofluids cooling effect in the vicinity of a centre of the final heat source can be utilized in hot spots cooling.
    Keywords: discrete heat source, heat transfer, Magneto Hydro Dynamics, Nusselt number, square duct
  • Sadegh Motahar *, Rahmat Khodabandeh Pages 96-103
    In the present paper, an experimental assessment was performed on the transient thermal performance of a heat sink filled by a phase change material (PCM) and PCM embedded with carbon nanofibers (CNFs) and titania (TiO2) nanoparticles as nanostructured materials. In order to enhance the thermal conductivity of PCM, CNFs and TiO2 nanoparticles at different loadings (0.5wt. % and 2 wt.% of CNFs and 2 wt.% and 4 wt.% TiO2) were dispersed by two-step method in to the molten PCM. The thermal conductivity and viscosity measurements showed an enhancement in composite thermal conductivity as well as an increment in viscosity. The heat sink was filled with PCM, PCM/CNF and PCM/TiO2 and experiments were accomplished by inputting power ranging from 3 W to 8 W. Results showed that filling the heat sink with PCM delayed the time to reach a typical temperature of 35°C by up to 110% for the power level of 8 W, while adding 2 wt.% of CNFs reduced this time by 15% and 4 wt.% of TiO2 nanoparticles improved by 2%. Generally, dispersion of TiO2 led to lower heat sink transient temperatures. However, adding 2 wt.% of CNFs and 4 wt.% TiO2 nanoparticles in to PCM at power level of 8 W raised the steady operation temperature by 11°C and 0.3°C, respectively.
    Keywords: heat sink, PCM, nanostructured material, thermal conductivity, transient operation
  • Tayebeh Namdar, Mohammad Mehdi Zerafat *, Samad Sabbaghi, Rahmatallah Saboori, Marziyeh Yousefifar Pages 104-109
    In order to improve the efficiency and rate of heat transfer in heat exchangers, various techniques such as increasing the heat transfer coefficient are proposed. This study is devoted to the investigation of heat transfer coefficient variations due to the addition of CuO, TiO2 and CuO/TiO2 nanoparticles into a shell and tube heat exchanger. CuO/TiO2 nanocomposite was synthesized using mechanical technique. Particle size analyzer was used to determine the size distribution of CuO/TiO2 nanocomposite. The results indicated an increase in the nanofluid heat transfer coefficient and overall heat transfer coefficient by increasing the nanoparticle composition and temperature. The heat transfer coefficients of CuO nanofluid are higher than that of TiO2 and the heat transfer coefficient of the nanocomposite is the highest. The heat transfer coefficient and overall heat transfer coefficient were enhanced 4.58 to 12.42 times and 4.01 to 12.33, respectively by increasing nanocomposite mass fraction. The highest heat transfer coefficients occur at the 75/25% CuO/TiO2 nanocomposite mass ratio.
    Keywords: Nanocomposite, Nanofluid, Heat transfer coefficient, Shell, tube heat exchanger
  • Naiyer Razmara *, Naiyer Alizadeh, Hossein Namarvari Pages 110-121
    Understanding the microscopic dispersion and aggregation of nanoparticles at nanoscale media has become an important challenge during the last decades. Nanoscale modeling techniques are the important tools to tackle many of the complex problems faced by engineers and scientists. Making progress in the investigations at nanoscale whether experimentally or computationally has helped understand the physical phenomena at the molecular scale. In addition, important developments have been made in predicting behavior and transport characteristics of nanofluids. In this review, we will discuss on the progresses made on the demonstration of aggregation phenomena in nanofluids. Our main focus will be on the application of molecular modeling in the illustration of the effect of aggregation on the nano-rheology of nanofluids. By the review in the literature the new fields of studies are recommended for more consideration and investigation.
    The most important field to be considered is to study the aggregation as an important factor affecting the boiling performance of nanofluids.
    Keywords: Aggregation, Rheology, Molecular, Nanoscale
  • Gbeminiyi Musibau Sobamowo*, Lawrence Olumide Jayesimi, Akindoye M. Waheed Pages 122-132
    The various industrial, biological and engineering applications of flow of squeezing flow of fluid between parallel plates have been the impetus for the continued interest and generation renewed interests on the subject. As a part of the renewed interests, this paper presents the study of axisymmetric magnetohydrodynamic squeezing flow of nanofluid in porous media under the influence of slip boundary condition using differential transformation method. Good agreements are established when the results of the differential transformation method are compared with the results of numerical method Runge-Kutta coupled with shooting method. Also, the analytical solution is used to investigate the effects of porous medium, magnetic field and slip boundary on the steady two-dimensional axisymmetric flow of the nanofluid. It is shown from the results that the velocity of the fluid increases as the magnetic field and porous parameters increase under slip condition while the velocity of the fluid decreases with increase in the magnetic field and porous parameter under no slip condition. By increasing the slip parameter, the velocity of the fluid increases while the velocity of the fluid decreases as the Reynolds number increases. Studies on nanofluidics such as energy conservation, friction reduction and micro mixing biological samples can be enhanced and better understood by the insights given in this present study.
    Keywords: Nanofluid, Porous media, Squeezing flow, Magnetic field, Slip boundary condition
  • Hossien Khorasanizadeh *, Mojtaba Seperhnia Pages 133-151
    Nowadays, microchannels as closed circuits channels for fluid flow and heat removal are an integral part of the silicon-based electronic microsystems. Most of previous numerical studies on microchannel heat sinks (MCHS) have been performed for a two-dimensional domain using constant properties of the working fluid. In this study, laminar fluid flow and heat transfer of variable properties Al2O3-water nanofluid in a trapezoidal MCHS, consisted of five trapezoidal microchannels have been studied. The three dimensional solution domains include the whole flow field and the complete MCHS solid parts. Four inlet/outlet arrangements, three pressure drops of 5, 10 and 15 kPa and nanoparticles volume fractions between 0 and 4% are assumed and the effects of these arrangements, properties variations and the Brownian motion on the heat sink performance quantified. The results indicate that the A-type heat sink, for which the inlet and outlet are placed horizontally at the center of the north and the south walls, has a better heat transfer performance, smaller thermal resistance and provides more uniform substrate temperature distribution. Temperature-dependent properties increases the heat transfer between 2.7% and 3.39%, decreases the thermal resistance between 3.49% and 6.29 % and decreases the ratio of difference between the maximum and minimum substrate temperatures to the heat flux between 3.3% and 7.19%. Presence of the Brownian motion showed a similar trend but with a slighter importance.
    Keywords: Trapezoidal microchannel, Trapezoidal heat sink, Variable properties nanofluid, Inlet, outlet arrangements, KKL Brownian motion model