Showing papers on "Nusselt number published in 2015"

Melting and convection of phase change materials in different shape containers: A review

Abstract: A review of analytical, numerical and experimental investigations of melting and ensuing convection of phase change materials within enclosures with different shapes commonly used for thermal energy storage is presented. The common shapes of the containers being rectangular cavities, spherical capsules, tubes or cylinders (vertical and horizontal depending on orientation of gravity) and annular cavities are covered. Studies focusing on melting within rectangular cavities are categorized into two groups. The first one is melting due to isothermal heating on one or more boundaries, whereas the second is the constant heat flux-assisted melting. Moreover, constrained and unconstrained melting in both spherical and horizontal cylindrical containers were discussed. The effects of the concentric geometry and location of the heating source on melting in horizontal annular spaces are presented. The review concentrated on elucidating the heat transfer mechanisms (conduction and convection) during the multiple stages of the melting process and the effects of these mechanisms on the liquid–solid interface shape and its progress, melting rate, charging time of the storage system, etc. The strength of buoyancy driven-convection varies greatly with the dimensionless Rayleigh or Stefan numbers and depends somewhat on the location of heat source and imposed boundary condition. High dimensionless numbers and/or side position of the heat source ensure the dominant role of natural convection melting, otherwise conduction will be responsible for major melting within the container. Furthermore, the geometrical parameters such as the aspect ratio in rectangular containers and vertical cylindrical ones, diameter or radius in spherical capsules and horizontal cylindrical vessels, and eccentricity in annular cavities are reviewed. In addition, the parameters affecting the thermal behavior of the melting process in various enclosures, i.e. the Nusselt, Rayleigh, Stefan, Prandtl and Fourier numbers and are reviewed.

Effect of space dependent magnetic field on free convection of Fe3O4–water nanofluid

Abstract: Effect of spatially variable magnetic field on ferrofluid flow and heat transfer is investigated. The enclosure is filled with Fe3O4–water nanofluid. Control volume based finite element method (CVFEM) is applied to solve the governing equations. The combined effects of ferrohydrodynamic and magnetohydrodynamic have been taken into account. The influences of Magnetic number, Hartmann number, Rayleigh number and nanoparticle volume fraction on the flow and heat transfer characteristics have been examined. Results show that enhancement in heat transfer decrease with increase of Rayleigh number while for two other active parameters different behavior is observed. Also it can be concluded that Nusselt number is an increasing function of Magnetic number, Rayleigh number and nanoparticle volume fraction while it is a decreasing function of Hartmann number.

Simulation of copper-water nanofluid in a microchannel in slip flow regime using the lattice Boltzmann method

Abstract: Laminar forced convection heat transfer of water–Cu nanofluids in a microchannel was studied utilizing the lattice Boltzmann method (LBM). The entering flow was at a lower temperature compared to the microchannel walls. Simulations were performed for nanoparticle volume fractions of 0.00 to 0.04 and slip coefficient from 0.005 to 0.02. The model predictions were found to be in good agreement with earlier studies. The effects of wall slip velocity and temperature jump of the nanofluid were studied for the first time by using lattice Boltzmann method. Streamlines, isotherms, longitudinal variations of Nusselt number, slip velocity and temperature jump as well as velocity and temperature profiles for different cross sections were presented. The results indicate that LBM can be used to simulate forced convection for the nanofluid micro flows. Moreover, the effect of the temperature jump on the heat transfer rate is significant. Also, the results showed that decreasing the values of slip coefficient enhances the convective heat transfer coefficient and consequently the Nusselt number (Nu) but increases the wall slip velocity and temperature jump values.

Ferrofluid flow and heat transfer in a semi annulus enclosure in the presence of magnetic source considering thermal radiation

Abstract: In this paper, ferrofluid flow and heat transfer in a semi annulus enclosure is investigated considering thermal radiation. The enclosure has a wall with constant heat flux boundary condition. Combined effects of Ferrohydrodynamic (FHD) and magnetohydrodynamic (MHD) are considered. It is assumed that the magnetization of the fluid is varying linearly with temperature and magnetic field intensity. Control Volume based Finite Element Method (CVFEM) is applied to solve the governing equations. The calculations were performed for different governing parameters namely; the Radiation parameter, Rayleigh number, nanoparticle volume fraction, Magnetic number arising from FHD and Hartmann number arising from MHD. Results show that Nusselt number is an increasing function of Rayleigh number, nanoparticle volume fraction, magnetic number while it is a decreasing function of with Hartmann number and radiation parameter.

Study of Natural Convection MHD Nanofluid by Means of Single and Multi-Walled Carbon Nanotubes Suspended in a Salt-Water Solution

Abstract: The natural convection boundary layer flow along a vertical cone with variable wall temperature under the presence of magnetohydrodynamics is investigated. The mathematical model based on nanolayer single and multi-wall carbon nanotubes in salt-water solutions and interfacial layers is considered. The nonlinear partial differential equations are analytically solved by means of Mathematica package BVPh 2.0 based on the homotopy analysis method. Flow behavior under altering involving physical parameters such as: Eckert number, Prandtl number, power law index, and nanoparticle volume fraction on the velocity and the temperature profiles is discussed and explained through graphs and tables. The values of Skin friction and Nusselt number are computed and examined.

Effect of uniform suction on nanofluid flow and heat transfer over a cylinder

Abstract: The aim of the present paper is to study the nanofluid flow and heat transfer over a stretching porous cylinder. The effective thermal conductivity and viscosity of the nanofluid are calculated by KKL (Koo–Kleinstreuer–Li) correlation. In KKL model, the effect of Brownian motion on the effective thermal conductivity is considered. The governing partial differential equations with the corresponding boundary conditions are reduced to a set of ordinary differential equations with the appropriate boundary conditions using similarity transformation, which is then solved numerically by the fourth-order Runge–Kutta integration scheme featuring a shooting technique. Numerical results for flow and heat transfer characteristics are obtained for various values of the nanoparticle volume fraction, suction parameter, Reynolds number and different kinds of nanofluids. Results show that inclusion of a nanoparticle into the base fluid of this problem is capable to change the flow pattern. It is found that Nusselt number is an increasing function of nanoparticle volume fraction, suction parameter and Reynolds number.

Two-Phase Simulation of Nanofluid Flow and Heat Transfer in an Annulus in the Presence of an Axial Magnetic Field

Abstract: In this study, the effects of magnetic field on nanofluid flow, heat, and mass transfer between two horizontal coaxial cylinders are studied using a two-phase model. The effect of viscous dissipation is also taken into account. By using the appropriate transformation for the velocity, temperature, and concentration, the basic equations governing the flow, heat, and mass transfer are reduced to a set of ordinary differential equations. These equations subject to the associated boundary conditions are solved numerically using the fourth-order Runge–Kutta method. The effects of Hartmann number, Reynolds number, Schmidt number, Brownian parameter, thermophoresis parameter, Eckert number, and aspect ratio on flow, heat, and mass transfer are examined. Results show that the Nusselt number has a direct relationship with the aspect ratio and Hartmann number but it has a reverse relationship with the Reynolds number, Schmidt number, Brownian parameter, thermophoresis parameter, and Eckert number.

Thermal radiation and slip effects on MHD stagnation point flow of nanofluid over a stretching sheet

Abstract: Present model is devoted for the stagnation point flow of nanofluid with magneto-hydrodynamics (MHD) and thermal radiation effects passed over a stretching sheet. Moreover, we have considered the combined effects of velocity and thermal slip. Condition of zero normal flux of nanoparticles at the wall for the stretched flow phenomena is yet to be explored in the literature. Convinced partial differential equations of the model are transformed into the system of coupled nonlinear differential equations and then solved numerically. Graphical results are plotted for velocity, temperature and nanoparticle concentration for various values of emerging parameters. Variation of stream lines, skin friction coefficient, local Nusselt and Sherwood number are displayed along with the effective parameters. Final conclusion has been drawn on the basis of both numerical and graphs results.

Convective heat transfer in MHD slip flow over a stretching surface in the presence of carbon nanotubes

Abstract: In the present study, thermal conductivity and viscosity of both single-wall and multiple-wall Carbon Nanotubes (CNT) within the base fluids (water, engine oil and ethylene glycol) of similar volume have been investigated when the fluid is flowing over a stretching surface The magnetohydrodynamic (MHD) and viscous dissipation effects are also incorporated in the present phenomena Experimental data consists of thermo-physical properties of each base fluid and CNT have been considered The mathematical model has been constructed and by employing similarity transformation, system of partial differential equations is rehabilitated into the system of non-linear ordinary differential equations The results of local skin friction and local Nusselt number are plotted for each base fluid by considering both Single Wall Carbon Nanotube (SWCNT) and Multiple-Wall Carbon Nanotubes (MWCNT) The behavior of fluid flow for water based-SWCNT and MWCNT are analyzed through streamlines Concluding remarks have been developed on behalf of the whole analysis and it is found that engine oil-based CNT have higher skin friction and heat transfer rate as compared to water and ethylene glycol-based CNT

Statistical optimization of microchannel heat sink (MCHS) geometry cooled by different nanofluids using RSM analysis

Abstract: In this work, an analytical investigation of the heat transfer for the microchannel heat sink (MCHS) cooled by different nanofluids (Cu, Al2O3, Ag, TiO2 in water and ethylene glycol as base fluids) is studied by the porous media approach and the Galerkin method and results are compared with numerical procedure. Response surface methodology (RSM) is applied to obtain the desirability of the optimum design of the channel geometry. The effective thermal conductivity and viscosity of the nanofluid are calculated by the Patel et al. and Khanafer et al. model, respectively, and MCHS is considered as a porous medium, as proposed by Kim and Kim. In addition, to deal with nanofluid heat transfer, a model based on the Brownian motion of nanoparticles is used. The effects of the nanoparticles volume fraction, nanoparticle type and size, base fluid type, etc., on the temperature distribution, velocity and Nusselt number are considered. Results show that, by increasing the nanoparticles volume fraction, the Brownian movement of the particles, which carries the heat and distributes it to the surroundings, increases and, consequently, the difference between coolant and wall temperature becomes less.

Effects of Al2O3–Cu/water hybrid nanofluid on heat transfer and flow characteristics in turbulent regime

Abstract: In this paper, forced convection of a turbulent flow of pure water, Al2O3/water nanofluid and Al2O3–Cu/water hybrid nanofluid (a new advanced nanofluid composited of Cu and Al2O3 nanoparticles) through a uniform heated circular tube is numerically analyzed. This paper examines the effects of these three fluids as the working fluids, a wide range of Reynolds number (10 000 ≤ Re ≤ 10 0000) and also the volume concentration (0% ≤ ϕ ≤ 2%) on heat transfer and hydrodynamic performance. The finite volume discretization method is employed to solve the set of the governing equations. The results indicate that employing hybrid nanofluid improves the heat transfer rate with respect to pure water and nanofluid, yet it reveals an adverse effect on friction factor and appears severely outweighed by pressure drop penalty. However, the average increase of the average Nusselt number (when compared to pure water) in Al2O3–Cu/water hybrid nanofluid is 32.07% and the amount for the average increase of friction factor would be 13.76%.