Showing papers on "Combined forced and natural convection published in 2013"

The Relation between mantle dynamics and plate tectonics: A Primer

Abstract: We present an overview of the relation between mantle dynamics and plate tectonics, adopting the perspective that the plates are the surface manifestation, i.e., the top thermal boundary layer, of mantle convection. We review how simple convection pertains to plate formation, regarding the aspect ratio of convection cells; the forces that drive convection; and how internal heating and temperature-dependent viscosity affect convection. We examine how well basic convection explains plate tectonics, arguing that basic plate forces, slab pull and ridge push, are convective forces; that sea-floor structure is characteristic of thermal boundary layers; that slab-like downwellings are common in simple convective flow; and that slab and plume fluxes agree with models of internally heated convection. Temperature-dependent viscosity, or an internal resistive boundary (e.g., a viscosity jump and/or phase transition at 660km depth) can also lead to large, plate sized convection cells. Finally, we survey the aspects of plate tectonics that are poorly explained by simple convection theory, and the progress being made in accounting for them. We examine non-convective plate forces; dynamic topography; the deviations of seafloor structure from that of a thermal boundary layer; and abrupt plate-motion changes. Plate-like strength distributions and plate boundary formation are addressed by considering complex lithospheric rheological mechanisms. We examine the formation of convergent, divergent and strike-slip margins, which are all uniquely enigmatic. Strike-slip shear, which is highly significant in plate motions but extremely weak or entirely absent in simple viscous convection, is given ample discussion. Many of the problems of plate boundary formation remain unanswered, and thus a great deal of work remains in understanding the relation between plate tectonics and mantle convections.

Radiation effects on mixed convection about a cone embedded in a porous medium filled with a nanofluid

Abstract: The problem of steady, laminar, mixed convection boundary-layer flow over a vertical cone embedded in a porous medium saturated with a nanofluid is studied, in the presence of thermal radiation. The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis with Rosseland diffusion approximation. The cone surface is maintained at a constant temperature and a constant nanoparticle volume fraction. The resulting governing equations are non-dimensionalized and transformed into a non-similar form and then solved by Keller box method. A comparison is made with the available results in the literature, and our results are in very good agreement with the known results. A parametric study of the physical parameters is made and a representative set of numerical results for the local Nusselt and Sherwood numbers are presented graphically. Also, the salient features of the results are analyzed and discussed.

Experimental evidence of the atmospheric convective transport contribution to sessile droplet evaporation

Abstract: We investigate the contribution of the natural convective transport in the vapor phase on the evaporation rate of an evaporating sessile droplet. When comparing the experimental data with the quasi-steady diffusion-controlled evaporation model, an increasing deviation with substrate temperature that was attributed to the effect of the natural convection on the vapor field has been recently highlighted. To validate this analysis, we present experimental results obtained with two gravity levels: 1 g and μg. The contribution of the natural convection is analyzed with the Grashof number, and an empirical model is developed combining diffusive and convective transport.

Confinement-induced heat-transport enhancement in turbulent thermal convection.

Abstract: We report an experimental and numerical study of the effect of spatial confinement in turbulent thermal convection. It is found that when the width of the convection cell is narrowed, the heat-transfer efficiency increases significantly despite the fact that the overall flow is slowed down by the increased drag force from the sidewalls. Detailed experimental and numerical studies show that this enhancement is brought about by the changes in the dynamics and morphology of the thermal plumes in the boundary layers and in the large-scale flow structures in the bulk. It is found that the confined geometry produces more coherent and energetic hot and cold plume clusters that go up and down in random locations, resulting in more uniform and thinner thermal boundary layers. The study demonstrates how changes in turbulent bulk flow can influence the boundary layer dynamics and shows that the prevalent mode of heat transfer existing in larger aspect ratio convection cells, in which hot and cold thermal plumes are carried by the large-scale circulation along opposite sides of the sidewall, is not the most efficient way for heat transport.

Entropy generation of Cu–water nanofluid mixed convection in a cavity

Abstract: In this numerical work, mixed convection and entropy generation of Cu–water nanofluid and pure water in a lid-driven square cavity have been studied. Horizontal walls of the cavity are adiabatic and vertical walls have constant temperature but different values. The top wall has been considered as moving from left to right at a constant speed, U 0 . Rayleigh numbers of 10 4 , 10 5 and 10 6 and Reynolds numbers of 1, 10 and 100 have been considered. The results have shown that addition of nanoparticles to the base fluid affects the entropy generation, flow pattern and thermal behavior especially at higher Rayleigh and low Reynolds numbers. For pure fluid as well as nanofluid, increasing Reynolds number increases the average Nusselt number, linearly. The maximum entropy generation occurs in nanofluid at low Rayleigh number but high Reynolds number. The minimum entropy generation occurs in pure fluid at low Rayleigh and low Reynolds numbers. For the cases studied, at Rayleigh numbers greater than 10 5 , most of the entropy generation is due to heat transfer effects, thus the Bejan number converges to a constant value. A proper choice of Reynolds number is important, if enhanced heat transfer and minimum increased entropy generation is expected.

Density-driven Convection Enhanced by an Inclined Boundary: Implications for geological CO2 storage

Abstract: We experimentally examine dissolution-generated, density-driven convection with an inclined boundary in both a Hele-Shaw cell and in a porous medium. The convection, manifested by descending, dense fingers, is generated by a diffusive mixing of two liquids at the interface. We investigate the dynamics, widths, and wavelengths of the fingers and characterize the global convective transport for a wide range of permeabilities and tilt angles of the boundaries. Our results have implications for CO2 storage in a saline aquifer when brine saturated with CO2 produces a heavier mixture, which may result in an enhanced mass transfer by convection. Our measurements reveal a further enhancement of convection with inclined boundaries, which suggests that sloping formations provide improved sites for CO2 storage.

The effects of inclination angle and Prandtl number on the mixed convection in the inclined lid driven cavity using lattice Boltzmann method

Abstract: The laminar mixed convection in a two-dimensional rectangular inclined cavity with moving top lid is investigated using the double population thermal lattice Boltzmann method (LBM) at different values of the Richardson number, inclination angle and the Prandtl number. In this problem, velocity components are changed by both buoyancy forces and the inclination angle of the cavity. Comparison of the present results with other available data shows good agreement. As the results, the velocity and temperature profiles, the Nusselt number, streamlines and isotherms are presented and discussed. It is shown that the increase of Prandtl number enhances the heat transfer rate, especially at higher values of inclination angle and Richardson number. Moreover, the average Nusselt number at the upper limit of the considered range of the Richardson and Prandtl numbers variability increases by a factor of 9.

Scaling behaviour in Rayleigh–Bénard convection with and without rotation

Abstract: Rotating Rayleigh–Benard convection provides a simplified dynamical analogue for many planetary and stellar fluid systems. Here, we use numerical simulations of rotating Rayleigh–Benard convection to investigate the scaling behaviour of five quantities over a range of Rayleigh ( ), Prandtl ( ) and Ekman ( ) numbers. The five quantities of interest are the viscous and thermal boundary layer thicknesses, and , mean temperature gradients, , characteristic horizontal length scales, , and flow speeds, . Three parameter regimes in which different scalings apply are quantified: non-rotating, weakly rotating and rotationally constrained. In the rotationally constrained regime, all five quantities are affected by rotation. In the weakly rotating regime, , and , roughly conform to their non-rotating behaviour, but and are still strongly affected by the Coriolis force. A summary of scaling results is given in table 2.

"Experimental Investigation Of Mixed Convection With Water-Al2O3 & Hybrid Nanofluid In Inclined Tube For Laminar Flow"

Abstract: Two experiments were carried out.first to study mixed convection Al2O3 water nano fluidinside an inclined copper tube surface. The effects of nanoparticles concentration and power supply on the development of the thermal field are studied and discussed under laminar flow condition. Results show that the experimental heat transfer coefficient decreases slightly with an increase of particle volume concentration from 0 to 4%. Two new correlations are proposed to calculate the Nusselt number in the fully developed region for horizontal and vertical tubes volume concentrations up to 4%.and In second experimental work, a fully developed laminar convective heat through a uniformly heated circular tube using Al2O3-Cu/water hybrid nanofluid is presented. For this we synthesized Al2O3-Cu nanocomposite powder in a thermo chemical route that involves a hydrogen reduction technique and then dispersed the prepared hybrid nano powder in deionised water to form a stable hybrid nanofluid of 0.1% volume concentration. The convective heat transfer experimental results showed a maximum enhancement of 13.56% in Nusselt number at a Reynolds number of 1730 when compared to Nusselt number of water. The experimental results also show that 0.1% Al2O3-Cu/ water hybrid nanofluids have slightly higher friction factor when compared to 0.1% Al2O3/water nanofluid. The empirical correlations proposed for Nusselt number and friction factor are in good agreement with the experimental data.

Mixed Convection Radiative Flow of Maxwell Fluid Near a Stagnation Point with Convective Condition

Abstract: Effects of thermal radiation in mixed convection stagnation point flow over a moving surface subject to convective boundary conditions is addressed. Mathematical modeling is based upon constitutive equations of an incompressible Maxwell fluid. Nonlinear analysis is presented through implementation of homotopy analysis method. Numerical values of Local Nusselt number is computed and analyzed.

Melting Heat Transfer in the Stagnation Point Flow of Powell–Eyring Fluid

Abstract: This paper looks at the influence of melting heat transfer in stagnation point flow of Powell–Eyring fluid toward a linear stretching sheet. The mathematical modeling is characterized by conservation laws of mass, linear momentum, and energy. Appropriate similarity transformations are employed for the reduction of partial differential systems into the ordinary differential systems. Series solutions to the resulting problems are presented. Variations of embedded parameters into the derived problems are graphically illustrated. The skin-friction coefficient and the Nusselt number are computed and examined.

Mixed convective flow of a dusty fluid over a vertical stretching sheet with non‐uniform heat source/sink and radiation

Abstract: Purpose – The purpose of this paper is to study the problem of two‐dimensional unsteady mixed convective flow a dusty fluid over a stretching sheet in the presence of thermal radiation and space‐dependent heat source/sink.Design/methodology/approach – The equations governing the fluid flow and temperature fields for both the fluid and dust phases are reduced to coupled non‐linear ordinary differential equations by using a suitable set of similarity transformations. Numerical solutions of the resulting equations are obtained using the well known RKF45 method.Findings – The numerical results are benchmarked with previously published studies and found to be in excellent agreement. Finally, the effects of the pertinent parameters which are of physical and engineering interest on the flow and heat transfer characteristics are presented graphically and in tabulated form.Originality/value – The problem is relatively original as the dusty fluid works for this type of problem are lacking.

Numerical simulation of natural convection in a square enclosure filled with nanofluid using the two-phase Lattice Boltzmann method.

Abstract: Considering interaction forces (gravity and buoyancy force, drag force, interaction potential force, and Brownian force) between nanoparticles and a base fluid, a two-phase Lattice Boltzmann model for natural convection of nanofluid is developed in this work. It is applied to investigate the natural convection in a square enclosure (the left wall is kept at a high constant temperature (TH), and the top wall is kept at a low constant temperature (TC)) filled with Al2O3/H2O nanofluid. This model is validated by comparing numerical results with published results, and a satisfactory agreement is shown between them. The effects of different nanoparticle fractions and Rayleigh numbers on natural convection heat transfer of nanofluid are investigated. It is found that the average Nusselt number of the enclosure increases with increasing nanoparticle volume fraction and increases more rapidly at a high Rayleigh number. Also, the effects of forces on nanoparticle volume fraction distribution in the square enclosure are studied in this paper. It is found that the driving force of the temperature difference has the biggest effect on nanoparticle volume fraction distribution. In addition, the effects of interaction forces on flow and heat transfer are investigated. It is found that Brownian force, interaction potential force, and gravity-buoyancy force have positive effects on the enhancement of natural convective heat transfer, while drag force has a negative effect.

Natural convection in horizontal pipe flow with a strong transverse magnetic field

Abstract: Linear stability analysis and direct numerical simulations are conducted to analyse mixed convection in a liquid metal flow in a horizontal pipe with imposed transverse magnetic field. The pipe walls are electrically insulated and subject to constant flux heating in the lower half. The results reveal the nature of anomalous temperature fluctuations detected in earlier experiments. It is found that, at the magnetic field strength far exceeding the laminarization threshold, the natural convection develops in the form of coherent quasi-two-dimensional rolls aligned with the magnetic field. Transport of the rolls by the mean flow causes high-amplitude, low-frequency fluctuations of temperature.

Heat transport in bubbling turbulent convection

Abstract: Boiling is an extremely effective way to promote heat transfer from a hot surface to a liquid due to numerous mechanisms, many of which are not understood in quantitative detail. An important component of the overall process is that the buoyancy of the bubble compounds with that of the liquid to give rise to a much-enhanced natural convection. In this article, we focus specifically on this enhancement and present a numerical study of the resulting two-phase Rayleigh–Benard convection process in a cylindrical cell with a diameter equal to its height. We make no attempt to model other aspects of the boiling process such as bubble nucleation and detachment. The cell base and top are held at temperatures above and below the boiling point of the liquid, respectively. By keeping this difference constant, we study the effect of the liquid superheat in a Rayleigh number range that, in the absence of boiling, would be between 2 × 106 and 5 × 109. We find a considerable enhancement of the heat transfer and study its dependence on the number of bubbles, the degree of superheat of the hot cell bottom, and the Rayleigh number. The increased buoyancy provided by the bubbles leads to more energetic hot plumes detaching from the cell bottom, and the strength of the circulation in the cell is significantly increased. Our results are in general agreement with recent experiments on boiling Rayleigh–Benard convection