Increasing Rayleigh Number Research Articles

Numerical Analysis of the Improving Thermal Energy Efficiency of Taylor-couette Flow

This paper deals with adimensionnal analysis of natural convection in a horizontal cylindrical annulus. The inner cylinder is isothermally heated and rotates with an angular velocity Ω, however the outer one is kept cold and motionless. The gap between cylinders is defined by an adimensional radius ratio f. The numerical study was carried out using COMSOL Multiphysics. The effects of Rayleigh number ranging from 102 to 106, radius ratio and rotation velocity on the flow pattern and the thermal behavior in the annulus are then elaborated. Particular attention is paid to the effect of different parameters on the local Nusselt numbers on the inner and outer cylinders, the mean Nusselt number and the energy efficiency of the process. Results show that the mean Nusselt number increases with the increase of Rayleigh number. However, it decreases with the increase of the radius ratio f because of the narrowing of the annulus. The results prove also that the heat transfer rate drops with the rise of rotation velocity. Finally, it was found that the energy efficiency achieved its maximum for lower Rayleigh numbers Ra=103, and lower rotation velocities.

Controls on the present-day dynamic topography predicted from mantle flow models since 410 Ma

SUMMARYMantle convection induces dynamic topography, the lithosphere's surface deflections driven by the vertical stresses from sublithospheric mantle convection. Dynamic topography has important influences on a range of geophysical and geological observations. Here, we studied controls on the Earth's dynamic topography through 3-D spherical models of mantle convection, which use reconstructed past 410 Myr global plate motion history as time-dependent surface mechanical boundary condition. The numerical model assumes the extended-Boussinesq approximation and includes strongly depth- and temperature-dependent viscosity and phase changes in the mantle. Our results show that removing the chemical layer above the core–mantle boundary (CMB) and including depth-dependent thermal expansivity have both a limited influence on the predicted present-day dynamic topography. Considering phase transitions in our models increases the predicted amplitude of dynamic topography, which is mainly influenced by the 410 km exothermic phase transition. The predicted dynamic topography is very sensitive to shallow temperature-induced lateral viscosity variations (LVVs) and Rayleigh number. The preservation of LVVs significantly increases the negative dynamic topography at subduction zones. A decrease (or increase) of Rayleigh number increases (or decreases) the predicted present-day dynamic topography considerably. The dynamic topography predicted from the model considering LVVs and with a Rayleigh number of 6 × 108 is most compatible with residual topography models. This Rayleigh number is consistent with the convective vigor of the Earth as supported by generating more realistic lower mantle structure, slab sinking rate and surface and CMB heat fluxes. The evolution of the surface heat flux pattern is similar to the long-term eustatic sea level change. Before the formation of Pangea, large negative dynamic topography formed between the plate convergence region of Gondwana and Laurussia. The predicted dynamic topography similar to that of present-day has already emerged by about 262 Ma. Powers for degrees 1–3 dynamic topography at 337 and 104 Ma which correspond to times of higher plate velocities and higher surface heat fluxes are larger.

Erratum to: MHD natural convection and thermal radiation of diamond–water nanofluid around rotating elliptical baffle inside inclined trapezoidal cavity

In this work, we study the natural convection and thermal radiation heat transfer of diamond–water nanofluid around rotating elliptical baffle inside inclined trapezoidal cavity under Lorentz forces effect in the presence of uniform heat generation/absorption. The trapezoidal cavity lateral walls are uniformly cooled, and the top and bottom walls are kept adiabatic. The interior rotating elliptical baffle is uniformly heated. Our numerical study is realized by the software Comsol Multiphysics based on the on the finite element method. The results extracted show that an increment on the convection heat transfer is reached by increasing Rayleigh number, the radiation parameter and the elliptical baffle horizontal radius; however, the increment of Hartmann number reduces it. The effect of cavity inclination on the convection heat transfer depends on the baffle position. The presence of uniform heat generation increases the temperature field inside cavity and improves the convection flow, but the presence of uniform heat absorption reduces it. Using nanoparticles with higher shape factor improves more the convection heat transfer.

Lattice Boltzmann Modeling for Natural Convection in Power-Law Fluids Within a Partially Heated Square Enclosure

Abstract This paper reports the numerical investigations for steady-state natural convection in power-law fluids inside a square enclosure embedded with bottom discrete heaters. The lattice Boltzmann method (LBM) is employed to model the flow and heat transfer phenomenon at different combinations of power-law index, Rayleigh number, and heat source length for a constant Prandtl number. The buoyancy force is incorporated in the collision term of the LBM through Boussinesq approximation. Simulation outcomes are furnished using streamlines, temperature contours, velocity profiles, and variation of heat transfer on the nonadiabatic walls to probe natural convection phenomena. The results indicate that the temperature and the flow fields in the enclosure are symmetric about the vertical centerline. The detailed physical interpretations have been provided for the reported results. Further, the increase in the power-law index means a rise in viscosity and a decrease in thermal energy transport for other constant parameters. The outcomes also specify that the intensity of circulation and heat transfer enhances with the increase of Rayleigh number and size of the localized heater. Finally, though, a rise in the size of the confined heat source enhances the rate of total thermal transport, it does not change the trend of fluid flow and local heat transfer rate.

Natural Convection Heat Transfer from Arrays of Different Fin Geometry.(Dept.M)

Natural convection heat transfer from vertical fin arrays of different fin geometry is experimentally investigated. Number, thickness and height of fins are kept constant while base to ambient temperature difference is varied. Experiments are carried out for vertical and horizontal fin array base plate. The Raleigh number is ranged from 2x103 to 8x103. Results showed that the average heat transfer coefficient is greatly affected by the base-to-ambient temperature difference and less affected by the fin geometry and base plate orientation. Over the tested range of Rayleigh number, and with vertical base plate arrays, the straight fin array offered the highest heat transfer coefficient (by a factor of 15% and 10% more than that of unsymmetrical and symmetrical corrugated sin array respectively). An enhancement in the heat transfer coefficient by a factor of about 10 % for the straight fin array more than that of horizontal base plate is also observed and that enhancement is reduced to about 4 and 2% for unsymmetrical and symmetrical corrugated fin array respectively. The effect of both orientation and fin geometry seems to be diminished with the increase of Rayleigh number.

Effect of non-uniform diameter and fractal dimension of Al2O3 nanoparticle on double-diffusion in tilted enclosure

The present numerical work examines the effect of variable fractal dimension and non-uniform diameter of Al2O3 nanoparticle on heat and mass transfer due to natural convection in tilted square enclosures. Enclosures are filled with Al2O3/H2O nanofluid and under constant heat and mass fluxes boundary conditions. The Navier-Stokes, energy, and concentration equations are solved numerically by finite difference method. Xu’s model [1] and Jang’s model [2] have been used to compute effective thermal conductivity and dynamic viscosity, respectively. Present work validated with published work experimentally and numerically. Influence of Al2O3 nanoparticle’s non-uniform diameter, fractal dimension, and the ratio of minimum to maximum diameter are illustrated in the form of streamlines, heatlines, and masslines in addition to average Nusselt and Sherwood numbers at different enclosure tilt angles. Heat transfer increases with the decrement in mean nanoparticle diameter, and maximum at tilt angle of θ=45∘ is observed for all Rayleigh numbers. At weak buoyancy effect (Ra=5×103) and minimum mean nanoparticle diameter (dp=1nm), maximum of 30% enhancement in overall heat transfer is observed at tilt angle, θ=45∘ and nanoparticle volume fraction, ϕ=5%. At strong buoyancy effect (Ra=1×105), maximum of 66% enhancement in overall heat transfer is observed at same nanoparticle conditions and tilt angle. An increase in Rayleigh and Lewis number decreases the tilt angle at which maximum mass transfer occurred. Mass transfer is maximum in the range of mean nanoparticle diameter of dp=25−50 nm.

Fe3O4-Water Nanofluid Free Convection within an Inclined 2D Rectangular Enclosure Heated by Solar Energy Using Finned Absorber Plate

This work investigates a hydrodynamic problem involving the Fe3O4-Water nanofluid. The novelty of this investigation lies in the fact that the nanofluid free convection is evaluated within a specific rectangular enclosure having a finned absorber plate as the top wall, heated by solar energy. The fins below the absorber plate permit to enhance heat transfer towards the nanofluid. A numerical simulation is carried out in order to predict the influence of Rayleigh number, nanofluid layer position, enclosure inclination angle, and absorber plate fins height on the nanofluid flow (in terms of streamlines and velocity magnitude) and heat transfer (in terms of temperature and Nusselt number divided by a certain thermal conductivity ratio). Numerical results show a nanofluid buoyancy enhancement and a temperature distribution homogenization, when the Rayleigh number increases, all the more important and pushed to the right area of the enclosure, as the inclination angle of the enclosure is higher. For relatively low fin heights, the nanofluid buoyancy enhancement is all the more important and pushed to the right area of the enclosure as the inclination angle is high. As the fin height increases, the temperature distribution becomes more homogenous.

Convection in viscoplastic fluids in rectangular cavities at lateral heating

The paper is devoted to the study of convective motions of viscoplastic fluids in closed two-dimensional rectangular domains with different aspect ratios under lateral heating. The problem was solved numerically using the ANSYS Fluent software. The Herschel-Bulkley model was chosen for describing the rheological behavior of the fluid. Under certain rheological parameters, this model was transformed into a Newtonian fluid model, whose behavior was also simulated as a limiting case. Based on the calculated results, the dependences of the maximum value of the stream function in the cavity on the Rayleigh number are plotted. It was found that at small Rayleigh numbers the intensity of motion is close to zero. At a certain value of the Rayleigh number there is a sharp increase in the intensity of motion, and a further increase in the Rayleigh number causes an almost linear growth of the maximum value of the stream function. For each of the considered ratios of cavity sides, the threshold values of the Rayleigh number, at which a sharp increase in the intensity of motion of liquid is observed, were determined. The obtained values of Rayleigh numbers turned out to be close to the threshold values of the Rayleigh number for the Bingham fluid found in the works of other authors earlier. The fields of the stream function and the square root of the second invariant of the viscous stress tensor were obtained for different values of the Rayleigh number and different aspect ratios. The scenarios of rearrangement of quasisolid motion zones with increasing Rayleigh number were compared with the available results for the Bingham fluid.

NATURAL CONVECTION HEAT TRANSFER INSIDE HORIZONTAL CIRCULAR ENCLOSURE WITH TRIANGULAR CYLINDER AT DIFFERENT ANGLES OF INCLINATION

Numerical simulation study by using Fluent CFD software has been carried out to investigate the steady. laminar natural convective heat transfer for air formed by heated inner equilateral triangular cylinder inside cold circular enclosure. The triangular cylinder was located at center of enclosure with four radius ratio RR=1.5, 2, 2.5, and 3; and four inclination angles, θ=0o (the base of triangle cylinder at bottom), 20o, 40o and 60o (the base of triangle at top). All numerical calculations were performed in the range of Rayleigh number extends from 103 to 107. The fluid and temperature fields were represented in the form of streamlines and isotherms. Results show that, As Rayleigh number increases, the streamlines become more concentrated next to the walls and the center of vortex displays upward towards the central plane of enclosure. Also, there is no effect for the inclination angle of triangular cylinder on the peak values of local Nusselt number along the inner surface of circular cylinder except the positions of these peaks due to turning of thermal plumes.

Natural convection from perforated vertical fins with different hole diameters

Natural convection is a physical mechanism that is mostly benefited in cooling of electronics. Due to the variety of the geometrical and operational parameters, industrial and scientific studies are continuing for better performance. One of the focuses is on perforations of heat sink fins as a passive flow control technique. This work experimentally investigates a sinusoidal wavy fin heat sink after fins were perforated with two different hole diameters. Heat sink was heated by using an electrical resistance for six different heating powers. The temperature at the heater-heat sink interface was measured with the aid of four thermocouples. Transient and steady temperature values were measured and then recorded by means of a data-logger. The details of the experimental setup are given alongside of visuals. It is desired to state some assessment and evaluations about the experimental setup. Related literature studies are also summarized in the introduction section. Heat transfer, Rayleigh and Nusselt numbers were calculated and compared with each other and parameters by means of 2D plot graphics. The time for reaching steady state is changing between 1.5 to 2 hours. Average wall temperature changes linearly with heating power. Average wall temperature values are between 300 and 370 Kelvin. Nusselt number increases with increasing Rayleigh number as expected. While Rayleigh number changes between 500,000 to 5,000,000 Nusselt number approximately increases from 18 to 25. 3 mm perforations yield better results in terms of Nusselt numbers, which means better thermal convection. Surface temperature trends are similar according to power increments. Considering experimental uncertainty, general performances of the two heat sinks are found similar. Some remarks for the future work are given in conclusion section.The original article was published on 31 December 2018 https://dergipark.org.tr/en/pub/ijeat/issue/42280/505116

Numerical investigation on the nanofluids heat transfer inside a porous inclined cavity with wavy boundary

In the present work, a numerical study on the free-convective heat transfer in a porous media cavity with a wavy boundary was carried out. The validation was done by comparing the results with the experimental data. The cavity inclination angle, material of nanofluid, nanoparticles volume fraction, the Rayleigh number, and porosity of the medium are the parameters which are investigated in this study. Results suggested that, due to the thermophysical properties of Cu particles in water, the heat transfer rate was increased for Cu-Water nanofluid in comparison to Al 2 O 3 -Water nanofluid, while the heat transfer rate decreased by increasing the volume fraction of nanoparticles. Numerical results showed that the Rayleigh number has significant effect on the heat transfer rate so that increase in the Rayleigh number from 100 to 10 000 increased the averaged Nusselt number between 2 to 3 times. The effect of porosity on heat transfer proved that the convective heat transfer rate increased with increasing the porosity of the porous medium. The effect of inclination angle of cavity on the heat transfer rate suggested that the optimum angle of cavity causing the highest heat transfer rate from wavy wall is 45°.

A hybrid scheme of single relaxation time lattice Boltzmann and finite volume methods coupled with discrete ordinates method for combined natural convection and volumetric radiation in an enclosure

This paper is focused on the application of hybrid Single relaxation time lattice Boltzmann and finite volume methods in conjunction with discrete ordinates method to simulate coupled natural convection and volumetric radiation in differentially heated enclosure, filled with an absorbing, emitting and non-scattering gray medium. In this work, the velocity and temperature fields are calculated using lattice Boltzmann and finite volume methods respectively, whereas the radiative term is computed by the discrete ordinates method. This study is carried out for Pr = 0.71, a Rayleigh number range of 103 ≤ Ra ≤ 106, an optical thickness with values 0 ≤ τ ≤ 100, a Planck number ranging in 0.001≤ Pl ≤ 100 and an aspect ratio varying between 0.5 ≤ Ar ≤ 2. Results are presented in terms of streamlines, isotherms, velocity profiles and average Nusselt number. Based on the obtained results, it can be concluded that the presence of volumetric radiation is noteworthy. Its effect, as a function of Rayleigh number and the radiative properties, yields significant changes on the behavior of streamlines and isotherms. In the taller enclosure, the increase of average total Nusselt number with increasing Rayleigh number is less significant than that in the case of the shallow enclosure.

Numerical Study on Natural Convection and Fluid Flow Inside a Tilted Wavy Enclosure (Dept.M)

Natural convection heat transfer and fluid flow is investigated, numerically, inside a tilted wavy walled enclosure. Simulating solar air heater, flat ceiling is considered as cover (cold temperature) of solar air heater and wavy wall as absorber hot temperature). The low describing equations are presented in, 1wo dimensional in Cartesian coordinates, dimensionless form by introducing appropriate dimensionless independent and dependent variables. The dimensionless form of the governing equations is solved by using the finite divided difference technique. Solving these equations, the temperature distribution and the hydrodynamic flow field in the enclosure are predicted and accordingly, local and average Nusselt number can be calculated. The effects of system parameters on the natural convection heat transfer inside enclosure are simulated. These parameters are Rayleigh number (5x103 ≤Ra≤ 106), the characteristic height ratio of the charnel (1 ≤ A≤ 5) , characteristic geometric ratio (1 ≤ L ≤ 5) and the inclination angle (10° ≤ o ≤ 60°) on heat transfer performance are investigated considering air as working fluid. The numerical results show that, the character of clockwise rotating vortex is found to be, strongly, depending on problem parameters. Heat transfer is increased with increasing Rayleigh number and characteristic geometric ratio. To suppress the natural convection heat loss effectively, A should be equal to 2, L larger than I and o between 20° and 40°.

Numerical simulation of free convection of MHD non-Newtonian nanofluid within a square wavy enclosure using Meshfree method

In this article, flow and heat transfer of nanofluids inside a wavy square enclosure filled with non-Newtonian (shear thinning) nanofluid under magnetic field has been simulated numerically. Single-phase model is used. The governing equations have been solved numerically using element free Galerkin method. The results are obtained for isotherms, streamlines, and average Nusselt number for various values of Hartmann number, Rayleigh number, nanoparticles volume fraction, and power-law index. Here, the main objective is to explore the effect of power-law index, Rayleigh number, Hartmann number, and volume fraction on average Nusselt number. It is found that in the absence of magnetic field, Nusselt number drops on increasing the value of power-law index whereas in the presence of magnetic field, heat transfer rate increases with increase in power-law index. With the increase in Rayleigh number and volume fraction of nanoparticles, the heat transfer rate increase in all cases. This type of problem has a direct application in the coolant systems, solar collector where the structure used is wavy in order to increase the rate of heat flow. Here, EFGM is efficiently applied for simulation due to irregular domain, which creates a novelty in the work.

Melting process modeling of Carreau non-Newtonian phase change material in dual porous vertical concentric cylinders

In this paper a numerical simulation of the melting process of Carreau non-Newtonian phase change material inside two porous vertical concentric cylinders included constant temperatures of the inner and outer walls, represented by Th and Tc, respectively. Half of the void between the two pipes is filled with copper porous media and paraffin wax as a phase change material. The governing equations are converted into a dimensionless form and are solved using the finite element method. The enthalpy porosity theory is applied to simulate the phase change of phase change material while the porous media follow to the Darcy law. Outcomes are shown and compared in terms of the streamline, isotherm, melting fraction and mean Nusselt numbers. The solid liquid interface location and the temperature distribution are predicted to describe the melting process. The effects of the Carreau index, porosity and non-dimensional parameters such as Stefan number, Darcy number, and Rayleigh number are analyzed. Our results indicate a good agreement between this study and the previous investigations. The results show that an increase in Rayleigh number, Stefan number, and Darcy number, increases the melting volume fraction and reduces the melting time. Also, the time of melting non-Newtonian phase change material decreases when Carreau index and porosity decrease.

MHD natural convection and thermal radiation of diamond–water nanofluid around rotating elliptical baffle inside inclined trapezoidal cavity

In this work, we study the natural convection and thermal radiation heat transfer of diamond–water nanofluid around rotating elliptical baffle inside inclined trapezoidal cavity under Lorentz forces effect in the presence of uniform heat generation/absorption. The trapezoidal cavity lateral walls are uniformly cooled, and the top and bottom walls are kept adiabatic. The interior rotating elliptical baffle is uniformly heated. Our numerical study is realized by the software Comsol Multiphysics based on the on the finite element method. The results extracted show that an increment on the convection heat transfer is reached by increasing Rayleigh number, the radiation parameter and the elliptical baffle horizontal radius; however, the increment of Hartmann number reduces it. The effect of cavity inclination on the convection heat transfer depends on the baffle position. The presence of uniform heat generation increases the temperature field inside cavity and improves the convection flow, but the presence of uniform heat absorption reduces it. Using nanoparticles with higher shape factor improves more the convection heat transfer.

Numerical Investigation of the nanofluid mixed convection on two layers enclosure with rotating cylinder: High Darcy Number Effects

The present study, numerically investigated of mixed convection in a square enclosure with two layers, with Al2O3–water nanofluid (upper layer) and nano-porous medium (lower layer) with an adiabatic rotating circular cylinder at the center of the enclosure. The top and bottom walls are assumed adiabatic, while the left sidewall is heated, and the right sidewall kept cooled. Numerically, COMSOL code based on the Galerkin finite element method used for solved the dimensionless governing equations. The non-dimensional parameters that used in this study are: Rayleigh number (Ra) ranged from 103 up to 106, Darcy number (Da) equal to 10−3, the angular rotational velocity (Ω) ranged (0 and 6000), the solid volume fraction (ɸ=0.06), and the inner circular cylinder radius as (R = 0.2). The results showed that when Rayleigh numbers increase, a noticeable increase in the flow intensity and the steep temperature gradient, while the values increase when the cylinder rotates. The value of the local Nusselt number was high in the upper half of the cavity. The effect of the cylinder rotates is greater on the value of the local Nusselt number when using the low Rayleigh Numbers.

Natural convection heat transfer in corrugated annuli with H2O-Al2O3 nanofluid

The present work includes a numerical study of natural convection heat transfer in symmetrical and unsymmetrical corrugated annuli filled with H2O-Al2O3 nanofluid. In this study, higher and lower temperatures were kept constant at inner and outer cylinders of the annulus; respectively. Eight mathematical models with an aspect ratio of 1.5 were developed to find the best model giving the highest heat transfer rates. The stream-vorticity formulation in curvilinear coordinates was used to solve the governing equations of heat transfer and fluid motion. The influences of Rayleigh number. ((103≤ Ra ≤106) and volume fraction of nanoparticles. (0≤ϕ≤0.25) on isotherms, streamlines, local and average Nusselt numbers on the inner and outer cylinder were investigated. The results show that the heat transfer rate is significantly increased with an increase in nanoparticles volume fraction and Rayleigh number. The activity of the heated surface is increased with an increase in the undulation number, but the flow motion tends to be most difficult in the spaces between active undulation walls. Moreover, the heat transfer rates in unsymmetrical annuli are relatively higher than the rates in the symmetrical annuli. There are no evident changes in isotherms with an increase in the nanofluid volume fraction. Correlations for the mean Nusselt number on the inner and outer walls of annulus were deduced as a function of Rayleigh number and nanoparticles volume fraction for eight models with an accuracy range of 8–15 %.

Non-Darcian Bénard convection in eccentric annuli containing spherical particles

In this paper, a numerical investigation of natural convection in a porous medium confined by two horizontal eccentric cylinders is presented. The cylinders are impermeable to fluid motion and retained at uniform different temperatures. While, the annular porous layer is packed with glass spheres and fully-saturated with air, and the cylindrical packed bed is under the condition of local thermal non-equilibrium. The mathematical model describing the thermal and hydrodynamic phenomena consists of the two-phase energy model coupled by the Brinkman-Forchheimer-extended Darcy model under the Boussinesq approximation. The non-dimensional derived system of formulations is numerically discretised and solved using the spectral-element method. The investigation is conducted for a constant cylinder/particle diameter ratio (Di/d) = 30, porosity (ε) = 0.5, and solid/fluid thermal conductivity ratio (kr) = 38.6. The effects of the vertical, horizontal and diagonal heat source eccentricity (−0.8 ⩽e⩽0.8) and the annulus radius ratio (1.5 ⩽RR⩽ 5.0) on the temperature and velocity distributions as well as the overall heat dissipation within both the fluid and solid phases, for a broad range of Rayleigh number (104 ⩽ Ra ⩽ 8 ×107). The results show that uni-cellular, bi-cellular and tri-cellular flow regimes appear in the vertical eccentric annulus at the higher positive eccentricity e = 0.8 as Rayleigh number increases. However, in the diagonal eccentric annulus, the multi-cellular flow regimes are shown to be deformed and the isotherms are particularly distorted when Rayleigh number increases. In contrast, in the horizontal eccentric annulus, it is found that whatever the Rayleigh number is only an uni-cellular flow regime is seen. In addition, it is shown that the fluid flow is always unstable in the diagonal eccentric geometry at e = 0.8 for moderate and higher Rayleigh numbers. However, it loses its stability in the vertical eccentric geometry only at two particular cases, while it is always stable in the horizontal eccentric geometry, for all eccentricities and Rayleigh numbers.

Parallel direct numerical simulation and analysis of turbulent Rayleigh–Bénard convection at moderate Rayleigh numbers using an efficient algorithm

Direct numerical simulation of turbulent Rayleigh-Bénard convection up to Rayleigh number 108 is performed using a fully-implicit, non-dissipative, discrete kinetic energy-conserving algorithm and a parallel flow solver based on it. The algorithm is especially suitable for simulating low-Mach number, variable density/viscosity, transitional and turbulent flows with or without heat transfer. Furthermore, since it does not rely on the Boussinesq assumption, large temperature differences and high Rayleigh numbers can be handled without loss of accuracy, unlike the pure incompressible ones. It is first shown that the algorithm is able to predict the evolution of thermally-driven instability to turbulent regime and all the characteristics of turbulent convection accurately, using low- and high-order turbulent statistics and various secondary diagnostics derived. Then, effects of increasing Rayleigh numbers on the development of the instability are analyzed in detail. Additionally, Nusselt-Rayleigh scaling properties are studied and a scaling relation is provided. Results show that Rayleigh-Bénard convection at relatively high Rayleigh numbers, corresponding to a boundary layer-dominated regime and little beyond it to a bulk-dominated regime, is characterized by weakening thermal fluctuations, thinning thermal boundary layers, increasing vertical velocity fluctuations and decreasing skewness. It is also observed that the turbulent heat flux dominates the heat transfer. Finally, the corresponding Nusselt-Rayleigh scaling relation is predicted as Nu=0.132Ra0.297.