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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]

In search of efficient energy utilization and superior thermal performance, the present study is aimed at understanding the positional effect of discrete heaters and coolers on a cylindrical thermal system. This system undergoes a magneto-thermal convective process using Cu-Al 2 O 3 H 2 O hybrid nanofluid. Four options are investigated by altering the position of four heater and cooler segments mounted centrally on the wall of four quadrants of the cylinder. The equations of motion and energy are solved in nondimensionalized form by employing the finite element technique. The effects of operating parameters on the thermal transport are investigated systematically for a fixed nanoparticle volume fraction (φ) and a range of the Rayleigh number (signifying the buoyant forces), Hartmann number (signifying the magnetic field strength), and the magnetic field inclination (γ). The study shows that heat transfer, as well as entropy production, is markedly influenced by the heater-cooler positions and flow-controlling parameters. Among all the studied configurations, two cases show the optimum value of the Nusselt number. The cross heater-cooler arrangement affirms as the most effective heating-cooling option to obtain enhanced heat transfer. Higher heat transfer corresponds to greater entropy production. Higher magnetic field intensity dampens the heat transfer rate and thermodynamic irreversibility. The innovative identification of superior cross-wise heating-cooling over the usual side-side or bottom-top heating-cooling is the major outcome of this study. The experimental hybrid nanofluid properties are considered for this study. [Display omitted] [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]

Convective heat transfer from an isothermal hot cylindrical cavity due to a turbulent round jet impingement is investigated numerically. Three-dimensional turbulent flow is considered in this work. The Reynolds stress second order turbulence model with wall standard treatment is used for the turbulence predictions the problem parameters are the jet exit Reynolds number, ranging from 2·104 to 105 and the normalized impinging distance to the cavity bottom and the jet exit Lf, ranging from 4 to 35. The computed flow patterns and isotherms for various combinations of these parameters are analyzed in order to understand the effect of the cavity confinement on the heat transfer phenomena. The flow in the cavity is divided into three parts, the area of free jet, and the area of the jet interaction with the reverse flow and the semi-quiescent flow in the region of the cavity bottom. The distribution of the local and mean Nusselt numbers along the cavity walls for above combinations of the flow parameters are detailed. Results are compared against to corresponding cases for impinging jet on a plate for the case of the bottom wall. The analysis reveals that the average Nusselt number increases considerably with the jet exit Reynolds number. Finally, it was found that the average Nusselt number at the stagnation point could be correlated by a relationship in the form Nu = f(Lf, Re). [ABSTRACT FROM AUTHOR]

The turbulent heat transfer by a confined jet flowing inside a hot cylindrical cavity is investigated numerically in this paper. This configuration is found in several engineering applications such as air conditioning and the ventilation of mines, deadlock, or corridors. The parameters investigated in this work are the Reynolds number (Re, 20,000≤Re≤50,000) and the normalized distance Lf between jet exit and the cavity bottom (Lf, 2≤Lf< 12). The numerical predictions are performed by finite volume method using the second order one-point closure turbulence model (RSM). The Nusselt number increases and attains maximum values at stagnation points, after it decreases. For an experimental test case available in the literature Lf= 8, the numerical predictions are in good agreement. Processes of heat transfer are analyzed from the flow behavior and the underlying mechanisms. The maximum local heat transfer between the cavity walls and the flow occurs at Lf= 6 corresponding to the length of the potential core. Nusselt number at the stagnation point is correlated versus Reynolds number Re and impinging distance Lf; [Nu0 =f(Re,Lf)] [ABSTRACT FROM AUTHOR]

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]

Vortex breakdown plays a central role in the performance of countless rotating machinery applications, many of which contain thermal gradients either inadvertently or by design. The effect of thermal gradients on vortex breakdown and further flow development in a cylindrical domain with a rotating bottom plate is examined using the Generalized Integral Transformation Technique (GITT) with a streamfunction-only formulation. A thermal gradient is imposed in the axial direction, such that the buoyancy forces oppose the base flow driven by the rotation of the lower plate, i.e. the temperature difference acts to stabilize the flow. The hybrid numerical-analytical approach is shown to accurately capture vortex breakdown phenomena for a variety of conditions involving single, double and triple recirculation bubbles. The buoyancy forces – expressed in terms of the Richardson number (Ri) – act to suppress vortex breakdown in all cases examined and led to a series of flow transitions with increasing Ri, characterized by the appearance of a stratified structure with multiple fluid layers. These flow transitions have a significant impact on the overall performance of the system. The torque coefficient decreases with Ri, compared to the base (isothermal) case following an empirical power law relationship, which is independent of Reynolds number, aspect ratio or number of fluid layers present. Flow stratification suppresses the transport of angular momentum; azimuthal velocity is shown to decline exponentially in the regions where layering occurs, accompanied by a sharp reduction in the Nusselt number, as fluid layers act to insulate the upper plate. • Vortex breakdown dynamics is analyzed for the flow in cylindrical enclosures. • Integral transform solution is obtained for the governing equations. • Effect of mixed-convection is verified on the bubble characteristics. [ABSTRACT FROM AUTHOR]