Showing papers in "International Journal of Thermal Sciences in 2012"
TL;DR: In this paper, the effects of slip boundary condition in the presence of dynamic effects of nano particles have not been investigated yet, and the results of the present paper show the flow velocity and the surface shear stress on the stretching sheet and also reduced Nusselt number and reduced Sherwood number are strongly influenced by the slip parameter.
Abstract: The objective of the present study is to analyze the development of the slip effects on the boundary layer flow and heat transfer over a stretching surface in the presence of nanoparticle fractions. In the modeling of nanofluid the dynamic effects including the Brownian motion and thermophoresis are taken into account. In the case of constant wall temperature a similarity solution is presented. The solution depends on a Prandtl number, slip factor, Brownian motion number, Lewis number, and thermophoresis number. The dependency of the local Nusselt and local Sherwood numbers on these five parameters is numerically investigated. To the best of author's knowledge, the effects of slip boundary condition in the presence of dynamic effects of nano particles have not been investigated yet. The results of the present paper show the flow velocity and the surface shear stress on the stretching sheet and also reduced Nusselt number and reduced Sherwood number are strongly influenced by the slip parameter.
251 citations
TL;DR: In this paper, a two component non-homogenous equilibrium model is used for the nanofluid that incorporates the effects of Brownian motion and thermophoresis, and variable thermal conductivity and variable viscosity are taken into account in this work.
Abstract: Natural convection heat transfer and fluid flow of CuO–Water nanofluids is studied using the Rayleigh–Benard problem. A two component non-homogenous equilibrium model is used for the nanofluid that incorporates the effects of Brownian motion and thermophoresis. Variable thermal conductivity and variable viscosity are taken into account in this work. Finite volume method is used to solve governing equations. Results are presented by streamlines, isotherms, nanoparticle distribution, local and mean Nusselt numbers and nanoparticle profiles at top and bottom side. Comparison of two cases as absence of Brownian and thermophoresis effects and presence of Brownian and thermophoresis effects showed that higher heat transfer is formed with the presence of Brownian and thermophoresis effect. In general, by considering the role of thermophoresis and Brownian motion, an enhancement in heat transfer is observed at any volume fraction of nanoparticles. However, the enhancement is more pronounced at low volume fraction of nanoparticles and the heat transfer decreases by increasing nanoparticle volume fraction. On the other hand, by neglecting the role of thermophoresis and Brownian motion, deterioration in heat transfer is observed and this deterioration elevates by increasing the volume fraction of nanoparticles.
226 citations
TL;DR: The steady boundary layer free convection flow past a horizontal flat plate embedded in a porous medium filled by a water-based nanofluid containing gyrotactic microorganisms is investigated in this article.
Abstract: The steady boundary layer free convection flow past a horizontal flat plate embedded in a porous medium filled by a water-based nanofluid containing gyrotactic microorganisms is investigated. The Oberbeck-Boussinesq approximation is assumed in the analysis. The effects of bioconvection parameters on the dimensionless velocity, temperature, nanoparticle concentration and density of motile microorganisms as well as on the local Nusselt, Sherwood and motile microorganism numbers are investigated and presented graphically. In the absence of bioconvection, the results are compared with the existing data in the open literature and found to be in good agreement. The bioconvection parameters strongly influence the heat, mass, and motile microorganism transport rates.
200 citations
TL;DR: In this article, a numerical study of unconstrained melting of nano-enhanced phase change materials (NEPCM) inside a spherical container using RT27 and copper particles as base material and nano-particle, respectively.
Abstract: This paper presents a numerical study of unconstrained melting of nano-enhanced phase change materials (NEPCM) inside a spherical container using RT27 and copper particles as base material and nano-particle, respectively. Numerical studies are performed for three different Stefan number and volume fraction of nano-particles with an initial sub-cooling of 6 °C. Transient numerical simulations are performed for axi-symmetric melting inside a sphere. The simulation results show that the nano-particles cause an increase in thermal conductivity of NEPCM compared to conventional PCM. The enhancement in thermal conductivity with a decrease in latent heat results in higher melting rate of NEPCM.
191 citations
TL;DR: In this article, a carbon paper gas diffusion layer (GDL) of interdigitated PEMFC is reconstructed using the stochastic method, and its macroscopic transport properties are numerically predicted.
Abstract: Lattice Boltzmann method (LBM) is employed to investigate pore-scale flow and mass transport in a carbon paper gas diffusion layer (GDL) of interdigitated PEMFC. The carbon paper GDL is reconstructed using the stochastic method, and its macroscopic transport properties are numerically predicted. The predicted anisotropic permeabilities and effective diffusivity of the reconstructed GDL agree well with existing measurements. Then, effects of the porous structures of the carbon paper GDL are explored in terms of fluid flow, species transport and electrochemical reaction. The GDL porous structures greatly affect flow and mass transport, creating distinct specie concentration distribution and local current density distribution. Besides, simulations are performed to explore liquid water behaviors in the reconstructed GDL. The simulation results present a detailed description of the pore-scale liquid water behaviors. Further, simulations are performed to investigate the effects of land width and GDL contact angle on liquid water removal time and residual saturation. Narrower land reduces liquid water removal time and residual saturation. Higher contact angle increases the removal time and reduces the residual saturation.
190 citations
TL;DR: In this paper, the authors presented a simulation of multi-longitudinal vortices in a tube induced by triple and quadruple twisted tapes insertion, and the results verified the theory of the core flow heat transfer enhancement.
Abstract: The principle of heat transfer enhancement in the core flow of tube has been proposed to improve the temperature uniformity and reduce flow resistance, which is different from that of heat transfer enhancement in the boundary flow of tube. This article presents a simulation of multi-longitudinal vortices in a tube induced by triple and quadruple twisted tapes insertion. The simulation is conducted in order to gain an understanding of physical behavior of the thermal and fluid flow in the tube fitted with triple and quadruple twisted tapes for the Reynolds number from 300 to 1800. The obtained results show that, a maximum increase of 171% and 182% are observed in the Nusselt number by using triple and quadruple twisted tapes. And the friction factors of the tube fitted with triple and quadruple twisted tapes are around 4.06–7.02 times as that of the plain tube. The PEC of the tubes varies from 1.64 to 2.46. And the results verify the theory of the core flow heat transfer enhancement. Physical quantity synergy analysis is performed to investigate the mechanism of heat transfer enhancement. The synergy angles β and θ, are calculated, and the numerical results verify the synergy regulation among physical quantities of fluid particle in the flow field of convective heat transfer, which can guide us to get the optimum design.
171 citations
TL;DR: In this paper, the authors describe recent efforts in development and characterization of nanostructured thermal interface materials (TIMs) and identify possible future research directions, as well as some possible future directions for using these materials as TIMs.
Abstract: Due to high heat dissipation rates in current and projected future semiconductor devices, much attention has been given to improving paths of heat transport within the device package. A key focus is on improved thermal interface materials (TIMs), used in joining surfaces in a microelectronic package to reduce interface thermal resistance. Recently, due to the reported enhanced thermal performance of nanostructured materials, research has focused on using these materials as TIMs, or incorporating them within existing TIMs to improve thermal transport. We describe recent efforts in development and characterization of nanostructured TIMs and identify possible future research directions.
168 citations
TL;DR: In this article, a numerical analysis of natural convection has been performed for a two-dimensional triangular enclosure with partially heated from below and cold inclined wall filled with nanofluid in presence of magnetic field.
Abstract: A numerical analysis of natural convection has been performed for a two-dimensional triangular enclosure with partially heated from below and cold inclined wall filled with nanofluid in presence of magnetic field. Governing equations are solved by finite volume method. Flow pattern, isotherms and average Nusselt number are presented for 0 < Ha<100, 104 < Ra<107,0<ϕ < 0.05 and six cases that are made by location of heat sources. The results show in presence of magnetic field flow field is suppressed and heat transfer decreases. Furthermore it is observed that maximum reduction of average Nusselt number in high value of Ha occurs at Ra = 106. It is found the nanoparticles are more effective at Ra = 104 where conduction is more pronounced.
162 citations
TL;DR: In this paper, the steady MHD laminar flow of an electrically conducting fluid on a radially stretchable rotating disk in the presence of a uniform vertical magnetic field is investigated.
Abstract: The steady MHD laminar flow of an electrically conducting fluid on a radially stretchable rotating disk in the presence of a uniform vertical magnetic field is the subject of the present paper. The problem is an extension of the well-known von Karman viscous pump problem to the configuration with a stretchable disk with or without rotation first imposed in [1] . The governing equations of motion are reduced to a set of nonlinear differential equations by means of conventional similarity transformations. Energy equation accounts for the viscous dissipation and Joule heating terms. Employing a highly accurate spectral numerical integration scheme, the effects of a rotation parameter based on the wall stretching and angular velocity are examined. The quantities of particular physical interest, such as the torque, the wall shear stresses, the vertical suction velocity and the rate of heat transfer are calculated and discussed.
146 citations
TL;DR: In this paper, a numerical investigation on the application of nanofluids in micro-pin-fin heat sinks (MPFHSs) has been presented, where the three-dimensional steady Navier-Stokes and energy equations were discretized using a finite volume approach and have been solved iteratively, using the SIMPLE algorithm.
Abstract: Numerical investigation on the application of nanofluids in Micro-Pin-Fin Heat Sinks (MPFHSs) has been presented in this paper. To investigate flow and heat transfer behavior in MPFHS the three-dimensional steady Navier–Stokes and energy equations were discretized using a finite volume approach and have been solved iteratively, using the SIMPLE algorithm. DI-water is used as a base coolant fluid while the nanoparticles used in the present study are CuO nanoparticles with mean diameters of 28.6 and 29 nm and Al 2 O 3 nanoparticles with mean diameters of 38.4 and 47 nm. The results show that (i) a significant enhancement of heat transfer in the MPFHS due to suspension of CuO orAl 2 O 3 nanoparticles in the base fluid in comparison with pure water, (ii) enhancement of heat transfer is intensified with increasing volume fraction of nanoparticles and Reynolds number, (iii) increasing volume fraction of nanoparticles which is responsible for higher heat transfer performance leads to higher pressure drop or Euler number in MPFHS but the enhancements are small, especially for lower particle volume fractions, (iv) with decreasing particle diameters the Nusselt number increases for Al 2 O 3 –water nanofluid while the trend is reverse for CuO–water nanofluid.
124 citations
TL;DR: In this paper, the authors investigated the effects of different nozzle structures on the performance of a steam ejector with five different nozzles, namely, conical, elliptical, square, rectangular and cross-shaped.
Abstract: The effects of different nozzle structures on the performance of a steam ejector have been investigated numerically with the computational fluid dynamics (CFD) technique. The performance of the steam ejectors with five different nozzle structures, namely, conical, elliptical, square, rectangular and cross-shaped nozzles, have been compared under the same conditions. It is found that, compared with the CFD results of the ejector equipped with the conical nozzle, the entrainment ratio (ER) and critical back pressure (CBP) of the rectangular nozzle is 7.1% and 21.3% lower respectively; the ER and CBP of the elliptical nozzle is 7.9% and 21.3% lower respectively; the square nozzle has improved the ER by 2.0% and decreased the CBP by 2.1%; the ER and CBP of the ejector utilizing cross-shaped nozzle is 9.1% higher and 6.4% respectively lower. Based on the simulation results of the streamwise vortex and spanwise vortex distributions in the mixing chamber and the internal energy variations along the streamwise distance, the characteristics of the mixing process and the main factors accounting for the ejector performance change are clarified. The ER increase can be achieved by efficient mixing due to the interactions between the streamwise vortex and the spanwise vortex. The streamwise vortex helps to deform and rupture the spanwise vortex which has greater strength. Collides of the vortices to the mixing chamber wall at early stage would increase mechanical energy loss and reduce “effective area” for secondary flow to pass through, resulting in great decrease of the ER and CBP. This scenario should be avoided in the design of nozzles.
TL;DR: In this article, the effect of Brownian motion and thermophoresis on convective flow of a nanofluid over a convectively heated vertical plate is investigated using a similarity analysis of the transport equations.
Abstract: Natural convective flow of a nanofluid over a convectively heated vertical plate is investigated using a similarity analysis of the transport equations followed by their numerical computations The transport model employed includes the effect of Brownian motion and thermophoresis The analysis shows that velocity, temperature and solid volume fraction of the nanofluid profiles in the respective boundary layers depend, besides the Prandtl and Lewis numbers, on four additional dimensionless parameters, namely a Brownian motion parameter Nb , a thermophoresis parameter Nt , a buoyancy-ratio parameter Nr and convective parameter Nc In addition to the study of these parameters on the boundary layer flow characteristics (velocity, temperature, solid volume fraction of the nanofluid, skin friction, and heat transfer), correlations for the Nusselt and Sherwood numbers have been developed based on a regression analysis of the data These linear regression models provide a highly accurate (with a maximum standard error of 0004) representation of the numerical data and can be conveniently used in engineering practice
TL;DR: In this article, the effects of Grashof number and volume fraction of Cu-water nanofluid on natural convection heat transfer and fluid flow inside a two-dimensional wavy enclosure has been investigated numerically.
Abstract: Effects of Grashof number and volume fraction of Cu–water nanofluid on natural convection heat transfer and fluid flow inside a two-dimensional wavy enclosure has been investigated numerically. Also, in the presence of nanofluid, the second law of thermodynamics is applied to predict the nature of irreversibility in terms of entropy generation. Finite-Volume numerical procedure with non orthogonal body fitted collocated grid arrangement is used to solve the governing differential equations. Calculation were performed for the Grashof numbers from 104 to 106, nanoparticles volume fraction from 0% to 10% and surface waviness ranging from 0.0 to 0.4 for different patterns of wavy enclosure. Streamlines, isothermal lines, counters of local entropy generation and the variation of local and average Nusselt number are presented and compared with considering the effects of different parameters. The results show that the average heat transfer rate decreases as nanoparticles volume fraction and Grashof number increase. Also, besides decreasing heat transfer rate, the nanoparticles can be used for decreasing the entropy generation.
TL;DR: In this article, experimental results on effective thermal conductivity and specific heat capacity of ionanofluids as a function of temperature and concentration of multi-wall carbon nanotubes in several ionic liquids are presented.
Abstract: Ionanofluids represent a new and innovative class of heat transfer fluids which possess fascinating thermophysical properties. They are designable and fine-tunable through their base ionic liquids. Experimental results on effective thermal conductivity and specific heat capacity of ionanofluids as a function of temperature and concentration of multi-wall carbon nanotubes in several ionic liquids are presented. Results showed that ionanofluids exhibit superior thermal conductivity and specific heat capacity compared to those of their base ionic liquids and they further increase with the concentration of carbon nanotubes. Although the enhancement of the effective thermal conductivity of ionanofluids was found to be independent of temperature, the specific heat capacity increases significantly with increasing temperature in the range of 60–90 °C. Interaction between ionic liquid ions and dispersed nanotubes is believed to be one of the key factors for the enhanced thermal conductivity of ionanofluids.
TL;DR: In this paper, lattice Boltzmann method is applied to investigate the natural convection flows utilizing nanofluids in a square cavity, where the fluid in the cavity is a water-based Nanofluid containing Al2O3 or Cu nanoparticles.
Abstract: In this study, lattice Boltzmann method is applied to investigate the natural convection flows utilizing nanofluids in a square cavity. The fluid in the cavity is a water-based nanofluid containing Al2O3 or Cu nanoparticles. The study has been carried out for the Rayleigh number 103–106 and the solid volume fraction 0–0.05. The effective thermal conductivity and viscosity of nanofluid are calculated by Chon and Brinkman models, respectively. The effects of solid volume fraction of nanofluids on hydrodynamic and thermal characteristics are investigated and discussed. The average and local Nusselt numbers, streamlines, temperature contours and vertical component of velocity for different values of solid volume fraction and Rayleigh number are illustrated. The results indicate that by increasing solid volume fraction, the average Nusselt number increases for both nanofluids. It is found that the effects of solid volume fraction for Cu are stronger than Al2O3.
TL;DR: In this article, a new method is proposed for estimating temperature-dependent thermal properties using solutions to transient inverse heat conduction problems, where the unknown thermal property is treated as the optimization variable, and the errors to be minimized are the differences between the calculated temperatures and the measured ones.
Abstract: A new method is proposed for estimating temperature-dependent thermal properties using solutions to transient inverse heat conduction problems. The prior information on the functional form of the thermal properties is not necessary for the proposed approach. The unknown thermal property is treated as the optimization variable, and the errors to be minimized are the differences between the calculated temperatures and the measured ones. The least-squares method is employed for the solution of the ill-posed inverse problem. The main contribution of this work is to introduce the complex-variable-differentiation method into transient inverse heat conduction problems, for the calculation of sensitivity coefficient. The inverse estimations of the heat capacity, the thermal conductivity and the thermal diffusivity are reported. Examples are given to demonstrate the effectiveness, efficiency and accuracy of the inverse approach as well as the potential to engineering applications. The effects of measurement errors on the inverse results are also investigated.
TL;DR: In this paper, the effect of internal heat source on the onset of Darcy-Brinkman convection in a porous layer saturated by nanofluid is studied, and the resulting eigenvalue problem is solved numerically using the Galerkin technique with the Rayleigh number as the eigen value.
Abstract: The effect of internal heat source on the onset of Darcy–Brinkman convection in a porous layer saturated by nanofluid is studied. The boundaries are considered to be free–free, rigid–rigid and lower-rigid and upper-free boundaries. The Brinkman–Darcy equation with fluid viscosity different from effective viscosity is used to characteristic the nanofluid motion. The model used for nanofluid includes the effects of Brownian motion and thermophoresis. The linear stability theory is employed and the resulting eigenvalue problem is solved numerically using the Galerkin technique with the Rayleigh number as the eigenvalue. The influence of internal heat source strength, nanoparticle Rayleigh number, modified particle-density increment, modified diffusivity ratio, Lewis number, Darcy number and the porosity on the stability of the system is investigated graphically. It is found that the internal heat source, nanoparticle Rayleigh number, modified diffusivity ratio and Lewis number have a destabilizing effect while Darcy number and the porosity show stabilizing effects on the system.
TL;DR: In this paper, the second order slip flow and heat transfer over a stretching sheet were analyzed and the governing partial differential equations of the flow were reduced into non-linear ordinary differential equations.
Abstract: In this paper, we analyze the second order slip flow and heat transfer over a stretching sheet. The governing partial differential equations of the flow and heat transfer are reduced into non-linear ordinary differential equations. An exact solution for the momentum equation is obtained and the governing energy equation is solved numerically by a fourth order Runge–Kutta method with shooting technique. The effects of various physical parameters such as the mass transfer parameter s, the first order slip parameter γ and the second order slip parameter δ on the fluid flow are analyzed (through graphs). Also the effects of the above said parameters (s, γ, δ) and the Prandtl number Pr on heat transfer are investigated and discussed for two general heating conditions (i) prescribed surface temperature (PST case) and (ii) prescribed heat flux (PHF case). Furthermore, the numerical results for the wall temperature gradient (the Nusselt number) in PST case and wall temperature in PHF case are presented in a table and the salient features are discussed.
TL;DR: In this paper, the effect of Rayleigh number, solid volume fraction and both Hartmann number and heat generation or absorption coefficient on the iso-contours of streamline and temperature were investigated in a square cavity filled with different nanofluids.
Abstract: Natural convection in a square cavity filled with different nanofluids is studied numerically. Both upper and lower surfaces are being insulated, whilst a uniform magnetic field is applied in a horizontal direction. Constant different temperatures are imposed along the vertical walls of the enclosure, steady state laminar regime is considered. The transport equations for continuity, momentum, energy are solved. The numerical results are reported for the effect of Rayleigh number, solid volume fraction and both Hartmann number and heat generation or absorption coefficient on the iso-contours of streamline and temperature. In addition, the predicted results for average Nusselt are presented for various parametric conditions. This study was done for 103 ≤ Ra ≤ 107, 0 ≤ Ha ≤ 60, 0 ≤ ϕ ≤ 0.06 and −10 ≤ q ≤ 10 while the Prandtl number represent water is kept constant at 6.2. The results show that for weak magnetic field; the addition of nanoparticles is necessary to enhance the heat transfer but for strong magnetic field there is no need for nanoparticles because the heat transfer will decrease. On the other hand to augment the heat transfer; nanoparticles volume fraction must be increased but with a small value of heat absorption coefficient (q < 0) at constant Hartmann and Rayleigh numbers.
TL;DR: In this article, the conservation equations of mass, momentum and energy are solved using the finite volume method for varying inclination angles between 0° and 90° and two cavity height based Rayleigh numbers, Ra = 104 and 105, a Prandtl number of Pr = 102 and three cavity aspect ratios of 1, 4 and 8.
Abstract: Steady two-dimensional natural convection in rectangular two-dimensional cavities filled with non-Newtonian power law-Boussinesq fluids is numerically investigated. The conservation equations of mass, momentum and energy are solved using the finite volume method for varying inclination angles between 0° and 90° and two cavity height based Rayleigh numbers, Ra = 104 and 105, a Prandtl number of Pr = 102 and three cavity aspect ratios of 1, 4 and 8. For the vertical inclination of 90°, computations were performed for two Rayleigh numbers Ra = 104 and 105 and three Prandtl numbers of Pr = 102, 103 and 104. In all of the numerical experiments, the channel is heated from below and cooled from the top with insulated side walls and the inclination angle is varied. A comprehensive comparison between the Newtonian and the non-Newtonian cases is presented based on the dependence of the average Nusselt number N u ¯ on the angle of inclination together with the Rayleigh number, Prandtl number, power law index n and aspect ratio dependent flow configurations which undergo several exchange of stability as the angle of inclination ɸ is gradually increased from the horizontal resulting in a rather sudden drop in the heat transfer rate triggered by the last loss of stability and transition to a single cell configuration. A correlation relating N u ¯ to the power law index n for vertically heated cavities for the range 104 ≤ Ra ≤ 105 and 102 ≤ Pr ≤ 104 and valid for aspect ratios 4 ≤ AR ≤ 8 is given.
TL;DR: In this article, a bio-heat transfer equation was developed based on the dual phase-lag model for considering the effect of micro-structural interaction, which was regarded as a tri-layer composite.
Abstract: A bio-heat transfer equation was developed based on the dual-phase-lag model for considering the effect of micro-structural interaction. This work employs the corresponding equation to analyze the bio-heat transfer problem in skin, which was regarded as a tri-layer composite. Comparison of the present results with the results from the Pennes model and thermal wave model of bio-heat transfer is made. As stated in the literature, the DPL model would reduce to Fourier's law as τ q = τ T . However, another literature does not completely agree with it. This controversy would be reviewed in the present work. Also, the conclusion that the DPL bio-heat conduction equation with the effect of blood perfusion can be reduced to the Fourier bio-heat conduction equation only if both τ q and τ T are zero was made in the literature. The authors are very interested in such a conclusion, and it becomes one of studying points of the present work.
TL;DR: In this article, a parametric study was conducted and effects of pertinent parameters such as Rayleigh number, the aspect ratio of the C-shaped enclosure, and the volume fraction of the Cu nanoparticles on the flow and temperature fields and the rate of heat transfer inside the enclosure were investigated.
Abstract: Natural convection fluid flow and heat transfer inside C-shaped enclosures filled with Cu–Water nanofluid has been investigated numerically using finite volume method and SIMPLER algorithm. A parametric study was conducted and effects of pertinent parameters such as Rayleigh number, the aspect ratio of the C-shaped enclosure, and the volume fraction of the Cu nanoparticles on the flow and temperature fields and the rate of heat transfer inside the enclosure were investigated. It was found from the obtained results that the mean Nusselt number increased with increase in Rayleigh number and volume fraction of Cu nanoparticles regardless aspect ratio of the enclosure. Moreover the obtained results showed that the rate of heat transfer increased with decreasing the aspect ratio of the cavity. Also it was found that the rate of heat transfer increased with increase in nanoparticles volume fraction. Also at low Rayleigh numbers, the effect of Cu nanoparticles on enhancement of heat transfer for narrow enclosures was more than that for wide enclosures.
TL;DR: In this paper, the gravity effects on the mixed convection heat transfer in a microchannel using lattice Boltzmann method were investigated. But the results showed that buoyancy forces generated a rotational cell in the microchannel flow, leading to the negative slip velocity at Kn ǫ = 0.005.
Abstract: This paper aims to study the gravity effects on the mixed convection heat transfer in a microchannel using lattice Boltzmann method. To include these effects, hydrodynamic boundary condition equations are modified. In this problem, cold fluid enters the microchannel and leaves it after cooling the hot walls. For a wide range of inlet Knudsen number (Kn), computations are performed, and for validation, appropriate comparisons between present and previous available results are made. As the results, stream lines, longitudinal variations of friction coefficient, Nusselt number, slip velocity and temperature jump, and velocity and temperature profiles in different cross sections are presented. The results show that lattice Boltzmann method can be used to simulate mixed convection in a microchannel, and the effects of buoyancy forces are important for Kn 0.05, these effects can be ignored. In addition, it is observed that buoyancy forces generate a rotational cell in the microchannel flow, leading to the negative slip velocity at Kn = 0.005.
TL;DR: In this article, the Lattice Boltzmann simulation of natural convection in an open enclosure which subjugated to water/copper nanofluid has been investigated, and the results show that the average Nusselt number increases with augmentation of Rayleigh number and the volume fraction of nanoparticles for whole ranges of aspect ratios.
Abstract: In this paper Lattice Boltzmann simulation of natural convection in an open enclosure which subjugated to water/copper nanofluid has been investigated. Calculations were performed for Rayleigh numbers (Ra = 10 4 –10 6 ), volume fractions of nanoparticles ( 0 ≤ φ ≤ 0.05 ) and open enclosure aspect ratios ( 0.5 ≤ A ≤ 2 ). The comparisons show that the average Nusselt number increases with augmentation of Rayleigh number and the volume fraction of nanoparticles for whole ranges of aspect ratios. The average Nusselt number decreases as the aspect ratio increases at various Rayleigh numbers and different the nanoparticle volume fractions. But generally the most effect of nanoparticles on heat transfer enhancement is observed at the aspect ratio of A = 2. Also nanoparticles influence the heat transfer less at Ra = 10 5 among studied Rayleigh numbers.
TL;DR: In this paper, thermal conductivity and viscosity of surfactant free and stable ethylene glycol (EG) based ZnO-nanofluids, prepared using long duration sonication, have been investigated.
Abstract: Thermal conductivity and viscosity of surfactant free and stable ethylene glycol (EG) based ZnO-nanofluids, prepared using long duration sonication, have been investigated both as a function of ZnO nanoparticle volume fraction and temperature (10–70 °C). Thermal conductivity enhancement of > 40% have been observed (ZnO volume fraction: 0.0375), which is substantially higher than that reported earlier on EG based oxide nanofluids. The observed enhancement of the studied ZnO-EG nanofluids is comparable to that of graphene oxide (0.04 volume fraction)–EG nanofluids. Viscosity of ZnO-EG nanofluids displays transition from Newtonian behavior at lower ZnO concentration to non-Newtonian characteristics at higher ZnO content and lower temperatures. Most importantly, viscosity of ZnO-EG nanofluid is found to be nearly independent of ZnO nanoparticle loading. The large thermal conductivity enhancement and marginal viscosity penalty of ZnO-EG nanofluids is believed to be the consequence of prolonged ultrasonication (∼60 h), which results in superior fragmentation and uniform distribution of ZnO nanoparticle clusters in the base fluid (EG). Results on the pool boiling characteristics of ZnO-EG nanofluids are reported first time and an enhancement of ∼22% in the boiling heat transfer coefficient is achieved with 0.016 volume fraction of ZnO nanoparticle loading. Finally, surfactant free ZnO-EG based nanofluids, prepared using long duration sonication, possesses all the features (viz, high thermal conductivity enhancement, nearly no viscosity penalty and enhanced heat transfer coefficient) required for an energy efficient coolant.
TL;DR: In this article, the problem of mixed convection fluid flow and heat transfer of Al 2 O 3 -water nanofluid with temperature and nanoparticles concentration dependent thermal conductivity and effective viscosity inside a square cavity was investigated numerically.
Abstract: The problem of mixed convection fluid flow and heat transfer of Al 2 O 3 –water nanofluid with temperature and nanoparticles concentration dependent thermal conductivity and effective viscosity inside a square cavity has been investigated numerically. The geometry of the present work was a square cavity with a heat source on the bottom wall, insulated top wall and moving downward cold side walls. The effects of increase in shear force while the buoyancy force was constant and effects of increase in buoyancy force when the shear force was kept constant were investigated. When the heat source was located in the middle of bottom wall, when the Rayleigh number was kept constant, the effect of addition of nanoparticles on enhancement of heat transfer increased with increase in Reynolds number. For a constant Reynolds number and for high Rayleigh numbers, the rate of heat transfer decreased with increase in nanoparticle volume fraction. Moreover it was found that the rate of this decrease increased with increase in Rayleigh number. Also the obtained results showed that when the heat source moved toward the side wall, the rate of heat transfer increased. The results obtained using variable thermal conductivity and variable viscosity models were compared to the results obtained by the Maxwell-Garnett model and the Brinkman model. The results showed that significant differences existed between the calculated overall heat transfers for the two different combinations of formulas. Moreover the difference increased with increase in nanoparticles volume fraction.
TL;DR: In this article, a comparative study of three different mathematical models for the packed bed latent heat storage system, comprised of a cylindrical storage tank filled with paraffin encapsulated spherical containers, is presented.
Abstract: There are several methods adopted by the scientists in modeling the heat transfer phenomenon during solid–liquid phase change. However, the validity of the various models developed and the complexity required for a given problem is not well reported in the literature. This paper presents a comparative study of three different mathematical models for the packed bed latent heat storage system, comprised of a cylindrical storage tank filled with paraffin encapsulated spherical containers. The enthalpy formulation technique is used in the models to accommodate the phase change behavior of the paraffin over a range of temperatures. The results obtained by solving the models using the fully explicit finite difference method are initially validated with experimental results. Further numerical analysis is performed using the models for two different heat transfer fluids of air and water at different mass flow rates and ball sizes and the validity of the models are compared and reported. It is found that one model is sufficient when air is the heat transfer fluid and another model is recommended when water is the heat transfer fluid.
TL;DR: In this article, the nucleate boiling and critical heat flux (CHF) of Fe3O4/ethylene glycol-water nanofluid at atmospheric pressure on a horizontal thin Ni-Cr wire were investigated.
Abstract: Nucleate boiling and critical heat flux (CHF) of Fe3O4/ethylene glycol–water nanofluid at atmospheric pressure on a horizontal thin Ni–Cr wire were investigated. Fe3O4 nanoparticles were dispersed in 50% (by volume) ethylene glycol/de-ionized water as base liquid in different concentrations (0.01–0.1% (by volume)). Experiments showed that boiling heat transfer coefficients deteriorate by increasing nanoparticle concentration in nanofluid. Addition of nanoparticles delays nucleate boiling incipience and increases CHF. Scanning Electron Microscope (SEM) graphs showed that porous layer of deposited nanoparticles formed on the heating surface boiled in nanofluid. The maximum CHF enhancement was obtained for 0.1% (by volume) nanofluid to be about 100%. Enhanced CHF was measured when previously boiled wire in nanofluid was used to boil the base fluid. Two different electrodes were used for applying electric field in the boiling nanofluids. Electric field applied in both configurations augmented boiling heat transfer coefficients while CHF remained almost unchanged.
TL;DR: In this article, an experimental investigation of nanoparticle suspensions utilized as nano-enhanced phase change materials (NePCM) was conducted and the experimental results were compared with the numerical predictions of a one-dimensional Stefan model.
Abstract: An experimental investigation of nanoparticle suspensions utilized as nano-enhanced phase change materials (NePCM) was conducted. Cyclohexane-based NePCM samples were prepared with copper oxide nanoparticles with various mass concentrations. Thermal conductivity of the samples was measured using the transient plane source technique for both liquid and solid phases at multiple temperatures. Unidirectional freezing of the samples was investigated and the experimental results were compared with the numerical predictions of a one-dimensional Stefan model proposed earlier by the authors. It was shown that the measured thermal conductivity for samples in their liquid phase is enhanced with increasing concentration of nanoparticles, whereas the data in the solid phase exhibit a non-monotonic enhancement when the concentration is greater than 2%. Unidirectional freezing was found to be expedited by up to 5.2%.
TL;DR: In this paper, an advanced 3D finite element model for the coupled solution of heat transfer and fluid flow equations governing transformer thermal performance is presented, which is able to predict detailed thermal distribution in a specific transformer, without requiring prior knowledge of nodal temperature or temperature gradient values.
Abstract: Thermal performance in oil-immersed power transformers is governed by the flow of oil, acting both as an electrical insulator and a medium for the transfer of heat generated in the core and windings toward the tank and the surrounding air. This paper presents the development of an advanced three-dimensional (3D) finite element model for the coupled solution of heat transfer and fluid flow equations governing transformer thermal performance. The main advantages of the proposed method are: (i) no need to predefine the convection coefficients at the interfaces between the active part/tank walls and the circulating oil, (ii) detailed representation of specific transformer parts that play an important role in the accurate representation of oil flow and heat dissipation (such as winding cooling ducts and corrugated tank panels) through an automated design process, enhancing the model accuracy with the least possible computational effort and (iii) accurate definition of the transformer heat sources (core and windings loss). The proposed methodology provides an integrated tool for thermal simulation, able to predict detailed thermal distribution in a specific transformer, without requiring prior knowledge of nodal temperature or temperature gradient values.