Showing papers on "Rayleigh number published in 2017"

Lattice Boltzmann method simulation for MHD non-Darcy nanofluid free convection

Abstract: Magnetohydrodynamic nanofluid convective flow in cubic porous enclosure is reported. Lattice Boltzmann Method is selected as mesoscopic approach. Brownian motion impact is taken into account via KKL model. Roles of Darcy number ( D a ) , Hartmann number ( H a ) , Rayleigh number ( R a ) , and Al 2 O 3 volume fraction ( ϕ ) are presented. Outputs are illustrated in forms of velocity contours, isokinetic energy, streamlines, isotherms and Nusselt number. Results indicate that temperature gradient over the hot surface augments with rise of Darcy numbers and ϕ while it reduces with augment of Lorentz forces. Nusselt number enhances with increase of buoyancy forces and permeability of porous media. Nanofluid motion enhances with augment of ϕ , D a , R a while it decreases with augment of H a .

Phase-change heat transfer in a cavity heated from below: The effect of utilizing single or hybrid nanoparticles as additives

Abstract: The present study deals with the effects of hybrid nanoparticles on the melting process of a nano-enhanced phase-change material (NEPCM) inside an enclosure. The bottom side of the cavity is isothermal at a hot temperature while the top wall is isothermal at a cold temperature and the left and right walls are insulated. The governing partial differential equations are first non-dimensional form and then solved using the Galerkin finite element method. Some of the dimensionless parameters are kept constant such as the Prandtl number, the Rayleigh number, the Stefan number and the ratio between the thermal diffusivity of the solid and liquid phases while the volume fraction of nanoparticles, the conductivity and viscosity parameters, and the Fourier number are altered. It is found out that increasing the values of the nanoparticles volume fraction, viscosity and conductivity parameters leads to significant variations in the solid-liquid interface for large values of Fourier number. Moreover, increasing the conductivity parameter and decreasing the viscosity parameter at the same time can cause an augmentation in the liquid fraction.

Natural convection of nanofluid inside a wavy cavity with a non-uniform heating: Entropy generation analysis

Abstract: Purpose The main purpose of this numerical study is to study on entropy generation in natural convection of nanofluid in a wavy cavity using a single-phase nanofluid model. Design/methodology/approach The cavity is heated non-uniformly from the wavy wall and cooled from the right side while it is insulated from the horizontal walls. The physical domain of the problem is transformed into a rectangular geometry in the computational domain using an algebraic coordinate transformation by introducing new independent variables ξ and η. The governing dimensionless partial differential equations with corresponding initially and boundary conditions were numerically solved by the finite difference method of the second-order accuracy. The governing parameters are Rayleigh number (Ra = 1000-100000), Prandtl number (Pr = 6.82), solid volume fraction parameter of nanoparticles (φ = 0.0-0.05), aspect ratio parameter (A = 1), undulation number (κ = 1-3), wavy contraction ratio (b = 0.1-0.3) and dimensionless time (τ = 0-0.27). Findings It is found that the average Bejan number is an increasing function of nanoparticle volume fraction and a decreasing function of the Rayleigh number, undulation number and wavy contraction ratio. Also, an insertion of nanoparticles leads to an attenuation of convective flow and enhancement of heat transfer. Originality The originality of this work is to analyze the entropy generation in natural convection within a wavy nanofluid cavity using single-phase nanofluid model. The results would benefit scientists and engineers to become familiar with the flow behaviour of such nanofluids, and will be a way to predict the properties of this flow for the possibility of using nanofluids in advanced nuclear systems, in industrial sectors including transportation, power generation, chemical sectors, ventilation, air-conditioning, etc.

Statistics of kinetic and thermal energy dissipation rates in two-dimensional turbulent Rayleigh–Bénard convection

Abstract: We investigate the statistical properties of the kinetic and thermal energy dissipation rates in two-dimensional (2-D) turbulent Rayleigh–Benard (RB) convection. Direct numerical simulations were carried out in a box with unit aspect ratio in the Rayleigh number range for Prandtl numbers and 5.3. The probability density functions (PDFs) of both dissipation rates are found to deviate significantly from a log-normal distribution. The PDF tails can be well described by a stretched exponential function, and become broader for higher Rayleigh number and lower Prandtl number, indicating an increasing degree of small-scale intermittency with increasing Reynolds number. Our results show that the ensemble averages and scale as , which is in excellent agreement with the scaling estimated from the two global exact relations for the dissipation rates. By separating the bulk and boundary-layer contributions to the total dissipations, our results further reveal that and are both dominated by the boundary layers, corresponding to regimes and in the Grossmann–Lohse (GL) theory (J. Fluid Mech., vol. 407, 2000, pp. 27–56). To include the effects of thermal plumes, the plume–background partition is also considered and is found to be plume dominated. Moreover, the boundary-layer/plume contributions scale as those predicted by the GL theory, while the deviations from the GL predictions are observed for the bulk/background contributions. The possible reasons for the deviations are discussed.

Roughness-Facilitated Local 1/2 Scaling Does Not Imply the Onset of the Ultimate Regime of Thermal Convection.

Abstract: In thermal convection, roughness is often used as a means to enhance heat transport, expressed in Nusselt number Yet there is no consensus on whether the Nusselt vs Rayleigh number scaling exponent (Nu∼Ra^{β}) increases or remains unchanged Here we numerically investigate turbulent Rayleigh-Benard convection over rough plates in two dimensions, up to Ra≈10^{12} Varying the height and wavelength of the roughness elements with over 200 combinations, we reveal the existence of two universal regimes In the first regime, the local effective scaling exponent can reach up to 1/2 However, this cannot be explained as the attainment of the so-called ultimate regime as suggested in previous studies, because a further increase in Ra leads to the second regime, in which the scaling saturates back to a value close to the smooth wall case Counterintuitively, the transition from the first to the second regime corresponds to the competition between bulk and boundary layer flow: from the bulk-dominated regime back to the classical boundary-layer-controlled regime Our study demonstrates that the local 1/2 scaling does not necessarily signal the onset of ultimate turbulence