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This paper combines numerical and experimental to study the heat transfer by free convection for nanofluids of molten salt within a cavity with cylindrical shape which is heated at the bottom and releases heat at the top. The aim is to acquire the free convection laws for nanofluids of molten salt in a cylindrical cavity. It is found that after adding nanoparticles, the molten salts have lower viscosity, higher flow velocity and higher heat transfer rate, and therefore, the free convection heat transfer is enhanced. Meanwhile, nanofluids Nusselt number is larger than that of the molten salts at the same Rayleigh number. In addition, as the classical Garon's equation cannot provide good predictions of free convection heat transfer for the nanofluids of molten salt, this paper establishes a free convection equation for the nanofluids of molten salt in a cylindrical cavity. This equation is able to predict with an error less than 20% the experimental results. Therefore, the findings can provide a basis for engineering applications of nanofluids and for designing optimal systems of heat stockpile with a single tank. [ABSTRACT FROM AUTHOR]

Abstract The experimental study for the melting processes of phase change material (paraffin) inside a cylindrical container under different boundary conditions of the cylindrical outside surface was carried out. In the experiments paraffin was heated by the heating rod installed in the center of the cylindrical container. The interpolation method was employed to calculate phase front and liquid fraction during the melting process by the reading of the thermocouples in an array arrangement. Paraffin near the heating rod firstly melted due to heat conduction in the initial melting stage, and then the generation and intensification of natural convection led to the acceleration of phase changes, resulting in obvious temperature stratifications in the upper region of the cylinder container. Moreover, flat temperature phenomena are described and discussed in detail. The effect of different heating powers, boundary conditions and the temperatures of water bath on melting rates is evaluated as well. Highlights • Conduct paraffin melting experiments in cylindrical cavity with an electric heating rod. • Analyze the effect of different heating powers and boundary conditions on the melting rate. • Record the paraffin melting process. • Temperatures of the molten liquid show a significant stratification. • Lower undercooling or higher boundary temperature accelerate the melting process. [ABSTRACT FROM AUTHOR]

The effect of surface tension on laminar natural convection in a vertical cylindrical cavity filled with a weak evaporating liquid has been analyzed numerically. The cylindrical enclosure is insulated at the bottom, heated by a constant heat flux from the side, and cooled by a non-uniform evaporative heat flux from the top free surface having temperature-dependent surface tension. Governing equations with corresponding boundary conditions formulated in dimensionless stream function, vorticity, and temperature have been solved by finite difference method of the second-order accuracy. The influence of Rayleigh number, Marangoni number, and aspect ratio on the liquid flow and heat transfer has been studied. Obtained results have revealed that the heat transfer rate at free surface decreases with Marangoni number and increases with Rayleigh number, while the average temperature inside the cavity has an opposite behavior; namely, it growths with Marangoni number and reduces with Rayleigh number. [ABSTRACT FROM AUTHOR]

The impacts of wind speed, wind incident angle and cavity tilt angle on combined natural-forced convective heat loss of a fully open cylindrical cavity with constant heat flux were performed experimentally. Uncertainty analysis, temperature distribution on the bottom and side surfaces, the variation of the combined natural-forced convective heat loss Nusselt number Nu c with studied parameters were presented. The general and improved models for predicting the combined convective heat loss in cavity for engineering applications were provided. The results show that the temperature distribution on the bottom and side surfaces in the wind condition is not the same as that in the no-wind condition. The critical wind speed for minimizing the combined convective heat loss does not appear in the investigated range of the wind speed. Increasing the wind speed can weaken the effect of wind incident angle. The impact of cavity tilt angle becomes quite complicated due to the introduction of wind, implying that the wind condition should be considered when analyzing the influence of cavity tilt angle. Besides, the improved model has superior performance than the general model ascribing to the fact that some inherent physical mechanisms are considered. [ABSTRACT FROM AUTHOR]

Abstract: In the case of considering conduction heat loss through the insulation, a 3D numerical investigation of the combined natural convection and radiation heat losses in one side open cylindrical cavity with constant heat flux has been carried out. Numerical procedure is validated by our latest experimental measurements. Visualization results, i.e., temperature and velocity contours, velocity vector field are provided. The effects of tilt angle, aperture ratio, heat flux as well as surface emissivity on the convection heat loss Nusselt number and radiation heat loss Nusselt number have been quantitatively analyzed. Results show that natural convection heat loss Nusselt number is more sensitive to tilt angle and aperture size, except when the tilt angle is 90° (the opening facing vertically downward), at this point no matter how other parameters change, natural convection heat loss Nusselt number always fluctuates in a small value. While radiation heat loss Nusselt number increases with the increase of all the impact parameters to some extent. The radiation heat transfer weakens the natural convection in the cavity. When the total heat flux is constant, the percentages of three modes of heat losses show different variations with related parameters. In addition, separate empirical Nusselt number correlations for natural convection heat loss and radiation heat loss based on a large set of numerical data for a given range of parameters of practical interest have been proposed. [Copyright &y& Elsevier]

We studied in this paper the phenomena of double-diffusive convection in a cylindrical enclosure filled with a porous medium saturated with a Newtonian fluid. The porosity of the porous media is variable from the walls to the bulk and follows an exponential law. The enclosure is heated from below and two mass concentrations C0, C1 are applied respectively to the surfaces (bottom, top) of the cylinder. The vertical walls are rigid, impermeable and adiabatic. An extended law of Brinkman-Forchheimer (EBFD) describes the fluid flow occurring in the porous layers by using the Boussinesq approximation. The mass concentration follows the basic Fick law, and the model chosen to describe the heat transfer is based on the approximation of one temperature. The heat and mass flow are controlled by the dimensionless numbers such as Rayleigh, Lewis, Darcy, Prandtl number that appear by dimensionless of the system of equations. The finites differences method is used to solve numerically the problem. The study focused on the effect of the dimensionless numbers on the concentrations and on the rate of heat transfers profiles in the overall Nusselt number, while considering the porosity uniform or non-uniform. The numerical code developed can be used for various industrial processes involving the phenomenon of natural convection. [ABSTRACT FROM AUTHOR]

Abstract: An experimental investigation using electrically heating method was carried out to explore the effects of surface boundary condition, tilt angle and heat flux on heat loss of a fully open cylindrical cavity. Three cases were examined: (i) only the bottom surface heated, (ii) only the side surface heated, and (iii) all surfaces heated. For all cases, the surfaces were heated with constant heat flux and the cavity rotates from 0° (the opening facing horizontally) to 90° (the opening facing vertically downwards) at intervals of 30° in addition to 45°. From the three cases, it is concluded that temperatures of bottom surface fluctuate in a small region, and side surface temperatures decrease with increasing position departure from bottom surface. The natural convection heat loss is sensitive to the tilt angle in comparison with the radiation and conduction heat losses. The heat loss characteristics are also dependent on the surface boundary conditions to some extent. When the total heat flux keeps constant, Nusselt numbers for determining the natural convection heat loss and the total heat loss reduce with increasing inclination. However, the inclination has little effect on the Nusselt number for determining the radiation heat loss. In addition, the empirical correlations of the natural convection, radiation and total heat loss Nusselt numbers versus the Grashof number, tilt angle and ambient temperature were proposed. [Copyright &y& Elsevier]

Theoretical and numerical study of the phenomenon of natural convection in a cylindrical enclosure filled by a non-Darcy porous media and saturated by a Newtonian fluid. The porosity of the media considered is variable and approximated by an exponential function based on an empirical law and equal to the unity near the walls due to the effects of the walls. The hydrodynamic flow in the porous medium is governed by Forchheimer-Brinkman extension of Darcy model (EBFD) with non-uniform porosity. The dimensionless equations reveal some important dimensionless numbers controlling this phenomenon such as the thermal Rayleigh number, Ra, the number of Darcy, Da, the number of Prandtl, Pr, the aspect ratio, RA, and the porosity, ε. The results are discussed graphically. The heat exchange traduced by Nusselt number is also considered in the study. The results obtained under the limiting conditions were found to be in good agreement with the existing ones. The established numerical code can also be used in various other industrial applications. [ABSTRACT FROM AUTHOR]

The objective of this work is to establish a numerical code in order to study the natural convection in a cylinder heated from bellow and filled with a porous media which has a variable porosity. A generalized non-Darcian porous medium model for natural convective flow has been developed taking into account the viscous forces within the fluid, The porosity of the medium is assumed to vary exponentially with distance from the wall. The dimensionless equations reveal some important dimensionless numbers controlling this phenomenon such as the thermal Rayleigh number, Ra, the number of Darcy, Da, the number of Prandtl, Pr, the aspect ratio, RA, and the porosity, ε. It is found that the variable porosity has an effect on the flow and the heat transfer rate. The established numerical code can also be used in various other industrial applications. [ABSTRACT FROM AUTHOR]

Abstract: The numerical simulation of laminar natural convection in a vertical cylindrical cavity is proposed. The cavity is insulated at the bottom, laterally heated at a uniform heat flux and cooled by a non uniform evaporative heat flux at the top surface obeying the Hashemi–Wesson''s law. The equations of mass conservation, momentum and energy are resolved with finite-volume method in fully implicit form. The influence of the characteristic parameters (103 ≤ Ra ≤105, Pr =2, 1/3≤ Al ≤1) on the thermal and dynamic behavior at steady state is analysed and discussed. In particular, the study describes the solutions at Ra =105 and Pr =2 for various aspect ratios. The evaporative heat flux is computed for all the parameters and it is found that its maximum is localized near the lateral wall whereas its minimum is at the free surface center. [Copyright &y& Elsevier]