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Showing papers on "Convection published in 2018"


Journal ArticleDOI
TL;DR: A novel approach to convective parameterization based on machine learning is presented, using an aquaplanet with prescribed sea surface temperatures as a proof of concept to show that neural networks trained on a high-resolution model in which moist convection is resolved can be an appealing technique to tackle and better represent moist convections in coarse resolution climate models.
Abstract: Modeling and representing moist convection in coarse-scale climate models remains one of the main bottlenecks of current climate simulations. Many of the biases present with parameterized convection are strongly reduced when convection is explicitly resolved (in cloud resolving models at high spatial resolution ~ a kilometer or so). We here present a novel approach to convective parameterization based on machine learning over an aquaplanet with prescribed sea surface temperatures. The machine learning is trained over a superparameterized version of a climate model in which convection is resolved by an embedded 2D cloud resolving models. The machine learning representation of convection, called Cloud Brain (CBRAIN) replicates many of the convective features of the superparameterized climate model, yet reduces its inherent stochasticity. The approach presented here opens up a new possibility and a first step towards better representing convection in climate models and reducing uncertainties in climate predictions.

301 citations


Journal ArticleDOI
TL;DR: In this paper, the steady laminar MHD mixed convection boundary layer flow of a SiO2-Al2O3/water hybrid nanofluid near the stagnation point on a vertical permeable flat plate is analyzed.

184 citations


Journal ArticleDOI
TL;DR: In this paper, water base nanofluid flow over a wavy surface in a porous medium of spherical packing beds is investigated, and the results illustrate that convection heat transfer is improved by nanoparticles concentration but reduces when fluid attract to porous walls.

180 citations


Journal ArticleDOI
TL;DR: In this paper, the improvement of nanofluid heat transfer inside a porous cavity by means of a non-equilibrium model in the existence of Lorentz forces has been investigated by employing control volume based finite element method.
Abstract: In the present article, the improvement of nanofluid heat transfer inside a porous cavity by means of a non-equilibrium model in the existence of Lorentz forces has been investigated by employing control volume based finite element method Nanofluid properties are estimated by means of Koo-Kleinstreuer-Li The Darcy-Boussinesq approximation is utilized for the nanofluid flow Roles of the solid-nanofluid interface heat transfer parameter Nhs, Hartmann number Ha, porosity e, and Rayleigh number Ra were presented Outputs demonstrate that the convective flow decreases with the rise of Nhs, but it enhances with the rise of Ra Porosity has opposite relationship with the temperature gradient

165 citations


Journal ArticleDOI
TL;DR: In this paper, a three-dimensional numerical model was developed to investigate the fluid flow and heat transfer behaviors in multilayer deposition of plasma arc welding (PAW) based wire and arc additive manufacture (WAAM).

155 citations


Book
09 Sep 2018

151 citations


Journal ArticleDOI
TL;DR: Numerical simulations of turbulent convection in fluids at different Prandtl number levels suggest a scale separation and thus the existence of a simplified description of the turbulent superstructures in geo- and astrophysical settings.
Abstract: Turbulent Rayleigh-Benard convection displays a large-scale order in the form of rolls and cells on lengths larger than the layer height once the fluctuations of temperature and velocity are removed. These turbulent superstructures are reminiscent of the patterns close to the onset of convection. Here we report numerical simulations of turbulent convection in fluids at different Prandtl number ranging from 0.005 to 70 and for Rayleigh numbers up to 107. We identify characteristic scales and times that separate the fast, small-scale turbulent fluctuations from the gradually changing large-scale superstructures. The characteristic scales of the large-scale patterns, which change with Prandtl and Rayleigh number, are also correlated with the boundary layer dynamics, and in particular the clustering of thermal plumes at the top and bottom plates. Our analysis suggests a scale separation and thus the existence of a simplified description of the turbulent superstructures in geo- and astrophysical settings.

150 citations



Journal ArticleDOI
TL;DR: In this paper, the effects of the number of fins and their length on heat transfer enhancement and entropy generation are scrutinized using a two-phase approach, and results indicate that adding porous fins with a high Darcy number improves heat transfer while fins with low Darcy numbers can weaken the convection and decline Nusselt number.

135 citations


Journal ArticleDOI
TL;DR: In this article, the authors present knowledge of double-diffusive convection at low Prandtl number obtained using direct numerical simulations, in both the fingering regime and the oscillatory regime.
Abstract: This work reviews present knowledge of double-diffusive convection at low Prandtl number obtained using direct numerical simulations, in both the fingering regime and the oscillatory regime. Particular emphasis is given to modeling the induced turbulent mixing and its impact in various astrophysical applications. The nonlinear saturation of fingering convection at low Prandtl number usually drives small-scale turbulent motions whose transport properties can be predicted reasonably accurately using a simple semi-analytical model. In some instances, large-scale internal gravity waves can be excited by a collective instability and eventually cause layering. The nonlinear saturation of oscillatory double-diffusive convection exhibits much more complex behavior. Weakly stratified systems always spontaneously transition into layered convection associated with very efficient mixing. More strongly stratified systems remain dominated by weak wave turbulence unless they are initialized into a layered state. The eff...

134 citations


Journal ArticleDOI
TL;DR: In this article, the authors review the current understanding of moist orographic convection and its regulation by surface exchange processes, including large-scale moistening and ascent, positive surface sensible and latent heat fluxes, and differential advection.
Abstract: This paper reviews the current understanding of moist orographic convection and its regulation by surface-exchange processes. Such convection tends to develop when and where moist instability coincides with sufficient terrain-induced ascent to locally overcome convective inhibition. The terrain-induced ascent can be owing to mechanical (airflow over or around an obstacle) and/or thermal (differential heating over sloping terrain) forcing. For the former, the location of convective initiation depends on the dynamical flow regime. In “unblocked” flows that ascend the barrier, the convection tends to initiate over the windward slopes, while in “blocked” flows that detour around the barrier, the convection tends to initiate upstream and/or downstream of the high terrain where impinging flows split and rejoin, respectively. Processes that destabilize the upstream flow for mechanically forced moist convection include large-scale moistening and ascent, positive surface sensible and latent heat fluxes, and differential advection in baroclinic zones. For thermally forced flows, convective initiation is driven by thermally direct circulations with sharp updrafts over or downwind of the mountain crest (daytime) or foot (nighttime). Along with the larger-scale background flow, local evapotranspiration and transport of moisture, as well as thermodynamic heterogeneities over the complex terrain, regulate moist instability in such events. Longstanding limitations in the quantitative understanding of related processes, including both convective preconditioning and initiation, must be overcome to improve the prediction of this convection, and its collective effects, in weather and climate models.

Journal ArticleDOI
TL;DR: In this article, the phase change melting enhancement in a latent heat thermal energy storage (LHTES) unit by arranging the internal double-fin length was investigated, and two schemes for double fin setting in unequal length were proposed.

Journal ArticleDOI
TL;DR: In this article, the authors developed, tested and utilized a three-dimensional heat transfer and fluid flow model of wire arc additive manufacturing (WAAM) to calculate temperature and velocity fields, deposit shape and size, cooling rates and solidification parameters.

Journal ArticleDOI
TL;DR: In this article, a TiO 2 -ethylene glycol nanofluid flow over a porous stretching sheet in presence of non-uniform generation or absorption of heat and convective boundary condition is investigated.

Journal ArticleDOI
TL;DR: In this article, numerical and experimental investigations have been done on the plate heat exchanger using hybrid nanofluid (Al2O3+MWCNT/water) at different concentration to investigate its effect on heat transfer and pressure drop characteristics.

Journal ArticleDOI
TL;DR: In this article, a numerical study of conjugate convective heat transfer in systems containing phase change materials with copper heat dissipating profile and the constant volumetric heat generation source was carried out.

Journal ArticleDOI
TL;DR: In this paper, the authors quantify heat transfer by rheological modeling of the pressure drop data in the process to generate a general Nusselt number-Graetz number correlation.
Abstract: The fused filament fabrication (FFF) process is similar to classic extrusion operations; solid polymer is melted, pressurized, and extruded to produce an object. At this level of investigation, it appears no new science or engineering is required. However, FFF has heat transfer limitations that are unique to it, due to its small throughput, not encountered in contemporary polymer processing, negating the use of present-day correlations or heuristics. Here, we quantify heat transfer by rheological modeling of the pressure drop data in the process to generate a general Nusselt number–Graetz number correlation. This is the first time the pressure has been measured in the die (nozzle) during normal printing that we accomplished by monitoring the power used to drive the hot end. Ultimately, we find that fouling within the region used to melt/soften the polymer significantly reduces the heat transfer rate.


Journal ArticleDOI
TL;DR: In this article, the authors analyzed the large-scale superstructures of turbulent Rayleigh-Benard convection in fluids at different Prandtl number ranging from 0.005 to 70 and for Rayleigh numbers up to $10^7.
Abstract: Turbulent Rayleigh-Benard convection displays a large-scale order in the form of rolls and cells on lengths larger than the layer height once the fluctuations of temperature and velocity are removed. These turbulent superstructures are reminiscent of the patterns close to the onset of convection. They are analyzed by numerical simulations of turbulent convection in fluids at different Prandtl number ranging from 0.005 to 70 and for Rayleigh numbers up to $10^7$. For each case, we identify characteristic scales and times that separate the fast, small-scale turbulent fluctuations from the gradually changing large-scale superstructures. The characteristic scales of the large-scale patterns, which change with Prandtl and Rayleigh number, are also found to be correlated with the boundary layer dynamics, and in particular the clustering of thermal plumes at the top and bottom plates. Our analysis suggests a scale separation and thus the existence of a simplified description of the turbulent superstructures in geo- and astrophysical settings.

Journal ArticleDOI
TL;DR: In this article, the influence of heat generation/absorption and volume fraction on the entropy generation and MHD combined convection heat transfer in a porous enclosure filled with a Cu-water nanofluid are studied numerically with of partial slip effect.
Abstract: In this work, the influences of heat generation/absorption and nanofluid volume fraction on the entropy generation and MHD combined convection heat transfer in a porous enclosure filled with a Cu–water nanofluid are studied numerically with of partial slip effect. The finite volume technique is utilized to solve the dimensionless equations governing the problem. A comparison with already published studies is conducted, and the data are found to be in an excellent agreement. The minimization of entropy generation and the local heat transfer according to various values of the controlling parameters are reported in detail. The outcome indicates that an augmentation in the heat generation/absorption parameter decreases the Nusselt number. Also, when the volume fraction is raised, the Nusselt number and entropy generation are reduced. The impact of Hartmann number on heat transfer and the Richardson number on the entropy generation and the thermal rendering criteria are also presented and discussed.

Journal ArticleDOI
TL;DR: In this article, the authors presented a three-dimensional, numerical thermo-hydrodynamic and second low analysis of nanofluid flow inside a square duct equipped with transverse twisted-baffles.

Journal ArticleDOI
TL;DR: In this article, the co-effect of the inclination angle, heater configuration, and nanofluid and porous media on heat transfer enhancement has been investigated with detailed investigation of the decision variables effect (Rayleigh number, Darcy number, inclination angle and volume fraction of Cu nanoparticles).

Journal ArticleDOI
TL;DR: In this article, a novel cross-fin heat sink consisting of a series of long fins and perpendicularly arranged short fins was proposed to enhance natural convective heat transfer, which is based on overcoming internal thermal fluid-flow defects in a conventional plate-fin Heat Sink.

Journal ArticleDOI
TL;DR: The main physical implication of the results is that both momentum and temperature layers are thinned with strong magnetic fields, and upper branch solutions are more cooled leading to higher heat transfer rates as compared to the lower branches.

Journal ArticleDOI
TL;DR: In this paper, a mathematical relation for two dimensional flow of magnetite Maxwell nanofluid influenced by a stretched cylinder is established to visualize the stimulus of Brownian moment and thermophoresis phenomena on Maxwell fluid.

Journal ArticleDOI
TL;DR: In this article, the authors carried out numerical investigations of turbulent Rayleigh-Benard convection over rough conducting plates and found that roughness does not always mean a heat-transfer enhancement, but in some cases it can also reduce the overall heat transport through the system.
Abstract: Rough surfaces have been widely used as an efficient way to enhance the heat-transfer efficiency in turbulent thermal convection. In this paper, however, we show that roughness does not always mean a heat-transfer enhancement, but in some cases it can also reduce the overall heat transport through the system. To reveal this, we carry out numerical investigations of turbulent Rayleigh–Benard convection over rough conducting plates. Our study includes two-dimensional (2D) simulations over the Rayleigh number range and three-dimensional (3D) simulations at . The Prandtl number is fixed to for both the 2D and the 3D cases. At a fixed Rayleigh number , reduction of the Nusselt number is observed for small roughness height , whereas heat-transport enhancement occurs for large . The crossover between the two regimes yields a critical roughness height , which is found to decrease with increasing as . Through dimensional analysis, we provide a physical explanation for this dependence. The physical reason for the reduction is that the hot/cold fluid is trapped and accumulated inside the cavity regions between the rough elements, leading to a much thicker thermal boundary layer and thus impeding the overall heat flux through the system.

Journal ArticleDOI
TL;DR: In this article, a three-dimensional numerical model of water-cooled PV/T system with cooling channel above PV panel was built to analyze the influences of mass flow rate, cooling channel height, inlet water temperature and solar radiation intensity on heat transfer characteristics of cooling channel and performance of system.

Journal ArticleDOI
TL;DR: In this paper, a physical model coupled with heat transfer and fluid flow was developed to investigate the thermofluid field of molten pool and its effects on SLM process of Inconel 718 alloy, in which a heat source considering the porous properties of powder bed and its reflection to laser beam is used.
Abstract: A physical model coupled with heat transfer and fluid flow was developed to investigate the thermofluid field of molten pool and its effects on SLM process of Inconel 718 alloy, in which a heat source considering the porous properties of powder bed and its reflection to laser beam is used. The simulation results showed that surface tension caused by temperature gradient on the surface of molten pool drives to Marangoni convection, which makes fluid flow state mainly an outward convection during SLM process. Marangoni convection includes convective and conductive heat flux, both of them have effects of on molten pool shape, but the effect of convective heat flux is dominant because its magnitude is one order larger than that of conductive heat flux. The convective heat flux accelerates the flow rate of the molten metal, benefits to heat dissipation. The convective heat flux makes the molten pool wider, while the conductive heat flux makes comparably the molten pool deeper and wider. Furthermore, heat accumulation caused by multiple scanning increases convection and conduction heat flux resulting in the increase of the width and depth of the molten pool, but no change of dominant role of convective heat flux to the shape of the molten pool.

Journal ArticleDOI
TL;DR: In this paper, the authors proposed an investigation on the application of Al 2 O 3 -water nanofluid within a PV/T panel in order to assess the potential to improve the performance of the device.

Journal ArticleDOI
Abstract: The significance of an externally applied magnetic field and an imposed negative temperature gradient on the onset of natural convection in a thin horizontal layer of alumina-water nanofluid for various sizes of spherical alumina nanoparticles (e.g., 30nm, 35nm, 40nm, 45nm) and volumetric fractions (e.g., 0.01, 0.02, 0.03, 0.04) is explored and analyzed numerically in this paper. The generalized Buongiorno's mathematical model with the simplified Maxwell's equations and the Oberbeck-Boussinesq approximation were adopted to simulate the two-phase transport phenomena, in which the Brownian motion and thermophoresis aspects are taken into account. Moreover, the rheological behavior of alumina-water nanofluid and related flow are assumed to be Newtonian, incompressible and laminar. Based on the linear stability theory, the perturbed partial differential equations (PDEs) of magnetohydrodynamic convective nanofluid flow are firstly simplified formally using the normal mode analysis technique and secondly converted to a generalized eigenvalue problem considering more realistic boundary conditions, in which the thermal Rayleigh number is the associated eigenvalue. Additionally, the resulting eigenvalue problem was solved numerically using powerful collocation methods, like Chebyshev-Gauss-Lobatto Spectral Method (CGLSM) and Generalized Differential Quadrature Method (GDQM). Furthermore, the thermo-magneto-hydrodynamic stability of the nanofluidic system and the critical size of convection cells are highlighted graphically in terms of the critical thermal Rayleigh and wave numbers, for various values of the magnetic Chandrasekhar number, the volumetric fraction and the diameter of alumina nanoparticles.