Turbulent Natural Convection Research Articles

Numerical simulation of turbulent natural convection around spheres

La mayor parte de las investigaciones que se han realizado sobre la convección natural han abordado su estudio en la región laminar y no se ha dirigido mucha atención a la convección natural turbulenta que suele presentarse cuando el número de Rayleigh supera un cierto valor crítico. Estudios recientes han mostrado que las correlaciones existentes para esta región suelen ser poco precisas. Por otra parte, el tratamiento numérico de este problema presenta dificultades relacionadas con el modelo de turbulencia usado, ya que se está en presencia de una capa límite laminar-turbulenta en un entorno laminar, por lo cual, el modelo laminar subestimará la transferencia de calor, mientras los modelos turbulentos la sobreestimarán. Modelos transicionales y otras opciones están disponibles, pero están diseñados para entornos turbulentos. El presente trabajo plantea la solución numérica mediante el programa computacional Ansys Fluent de la convección laminar turbulenta en torno a esferas sin modelo de turbulencia (modelolaminar) y usando un modelo de turbulencia k-ε para bajos números de Reynolds. El trabajo permitió caracterizar los números de Rayleigh a partir de los cuales empieza a ocurrir el cambio de capa límite laminar a turbulenta, así como la posición del desprendimiento de la capa límite. Los valores obtenidos para la transferencia de calor con el modelo laminar muestran muy buena concordancia con experimentos y correlaciones para bajos números de Rayleigh, pero pierde precisión a partir de cierto valor de Rayleigh, aspecto que pudo corregirse satisfactoriamente incorporando el modelo de turbulencia utilizado

Evaporation-driven turbulent convection in water pools

Abstract

Thermal analysis of phase change materials inside a square enclosure

Rising challenges in electric mobility and computing domain to control and condition the temperatures inside circuit boards and motor control drivers call for efficient and effective heat sink designs. Phase change materials come handy and have applications in conditioning temperatures in electronic devices. They prevent failures and increase their efficiencies at different temperatures and conditions. In the present study, the complex flow physics of phase change materials during melting is studied, inside a square enclosure with a heat source mounted at the bottom wall. The walls are assumed to be adiabatic, the material properties of heat source are that of silicon at 330K. The turbulent natural convection flow is modeled by the computational fluid dynamics (CFD) approach using Reynolds averaged Navier-stokes equation (RANS) with Lambremhorst k-ε turbulence model. A finite difference method is employed to discretize the governing equations. An in-house CFD code was computed for simulating the convective heat transfer characteristics. The flow physics of three different phase change materials (n-Octadecane, PEG900, and Paraffin (RT60)) have been analysed for a fixed Grashof number value of 103. The transient flow characteristics are examined by plotting the stream function, velocity, and temperature variation with time of the phase change materials.

Near wall Prandtl number effects on velocity gradient invariants and flow topologies in turbulent Rayleigh\u2013B\xe9nard convection

The statistical behaviours of the invariants of the velocity gradient tensor and flow topologies for Rayleigh–Bénard convection of Newtonian fluids in cubic enclosures have been analysed using Direct Numerical Simulations (DNS) for a range of different values of Rayleigh (i.e. Ra=10^7-10^9) and Prandtl (i.e. Pr=1 and 320) numbers. The behaviours of second and third invariants of the velocity gradient tensor suggest that the bulk region of the flow at the core of the domain is vorticity-dominated whereas the regions in the vicinity of cold and hot walls, in particular in the boundary layers, are found to be strain rate-dominated and this behaviour has been found to be independent of the choice of Ra and Pr values within the range considered here. Accordingly, it has been found that the focal topologies S1 and S4 remain predominant in the bulk region of the flow and the volume fraction of nodal topologies increases in the vicinity of the active hot and cold walls for all cases considered here. However, remarkable differences in the behaviours of the joint probability density functions (PDFs) between second and third invariants of the velocity gradient tensor (i.e. Q and R) have been found in response to the variations of Pr. The classical teardrop shape of the joint PDF between Q and R has been observed away from active walls for all values of Pr, but this behavior changes close to the heated and cooled walls for high values of Pr (e.g. Pr=320) where the joint PDF exhibits a shape mirrored at the vertical Q-axis. It has been demonstrated that the junctions at the edges of convection cells are responsible for this behaviour for Pr=320, which also increases the probability of finding S3 topologies with large negative magnitudes of Q and R. By contrast, this behaviour is not observed in the Pr=1 case and these differences between flow topology distributions in Rayleigh–Bénard convection in response to Pr suggest that the modelling strategy for turbulent natural convection of gaseous fluids may not be equally well suited for simulations of turbulent natural convection of liquids with high values of Pr.

Numerical modeling of heat losses in a line focusing solar compound parabolic concentrator with planar absorber

In this work, a three-dimensional steady state thermal model is developed for a compound parabolic concentrator of 2× concentration considering the effects of surface radiation and wall conduction. The performance of the compound parabolic concentrator is modeled to obtain the convective and radiative heat loss under the influence of different parameters such as absorber temperature, absorber emissivity, ambient temperature, external heat loss coefficient, aspect ratio and concentrator tilt. The turbulent natural convection in the concentrator is modeled using standard two equation k-ε turbulent model with enhanced wall treatment. For the surface radiation inside the concentrator, discrete ordinate radiation model is used. The thermo physical properties of air inside the cavity are assumed to be constant except the density which is modeled as incompressible ideal gas. The conduction at the walls is accounted by setting suitable wall thickness for reflectors, glass cover and end walls in the solver. This work clearly demonstrates the need for a three dimensional numerical simulation and coupled natural convection and radiation to exactly reproduce the velocity fields as would be obtained in experiments. The convective Nusselt number, radiative Nusselt Number and thermal stratification are obtained from the parametric study. This study on estimation of heat losses in compound parabolic concentrator is applicable to both solar thermal and solar photo-voltaic collectors with a view of modifying the concentrator design to optimize its thermal performance.

Study of turbulent natural convection coupled with thermal radiation in a vertical cavity having a large form factor

AbstractIn this study, a numerical simulation study of turbulent natural convection coupled with thermal radiation in a vertical cavity differentially heated and filled with air assumed as a transparent fluid was carried out. The cavity has a variable form factor which can reach large values. The vertical walls are subjected to constant temperatures (Tc and Tf), whereas the horizontal walls are assumed adiabatic. The flow inside the cavity is turbulent and turbulence was modeled by using the model, and to take into account of the radiative transfer, the discrete ordinate model (DO) was introduced. To solve the different equations, Ansys‐Fluent software based on the finite volume method was used. Some numerical results obtained for the Rayleigh number value of 1011 have been validated by some existing results in the theory. It is found that the thermal radiation has a significant influence on the flow structure and temperature variation where the flow becomes reinforced. It accelerates the airflow inside the cavity and gives the formation of significant velocity and temperature gradients along the walls of the cavity. Taking into account of the surface, thermal radiation is essential in the correct evaluation of temperature in the cavity.

Experimental study of turbulent air natural convection in open-ended vertical parallel plates under asymmetric heating conditions

In this study, we investigated the turbulent air natural convection phenomena between open-ended vertical parallel plates under asymmetric heating conditions. The rectangular air channel consisted of two insulating walls and two heated walls: one of the vertical walls is heated to maintain uniform wall temperature conditions at various temperatures from 50 to 500 °C, and the other wall is indirectly heated by thermal radiation from the directly heated wall. We measured both wall temperature distributions in the vertical direction and the air temperature profiles with the horizontal direction at certain heights.The results showed that when both walls are heated asymmetrically, reverse flow was absent at the outlet of the channel, as confirmed by a simple visualization method. In addition, we confirmed the practical result of a deviation between the mean air temperature and the measured air temperature at the center of the channel. From the results of the wall temperature distributions and the air temperature profiles, we suggest a reasonable flow behavior, which is the generation of two different recirculation flow patterns that are not affected by changes in the thermal boundary. In addition, we compared the ratio between the convective and radiative heat rates on the directly heated wall. Based on the experimental results, we analyzed the natural convection heat transfer between vertical parallel plates and we proposed two correlations. The heat transfer correlation agreed well with the correlation for turbulent natural convection on a single infinite vertical plate. The correlating equation related to the induced flow rate was presented as a modified Rayleigh number to estimate the flow rate.

Coupling turbulent natural convection-radiation-conduction in differentially heated cavity with high aspect ratio

In this paper, the effect of a several combined heat transfer processes: turbulent natural convection, surface radiation and conduction, are investigated numerically in an air-filled three-dimensional (3D) differentially heated tall cavity. The objectives of this work is to analyse the effect of coupled of different heat exchanges on flow structures of turbulent natural convection, and to distinguish the most realistic thermal boundary conditions of internal walls providing thus more accurate results.The cavity is parallelepiped and extended with sides length 0.076 × 2.18 × 0.52 m. A temperature gradient between the lateral walls is subjected of ΔT=19.9°C., inducing a turbulent flow due to the high Rayleigh number (RaH = 2.03 × 1010). A three dimensional unsteady numerical simulations are performed by the finite volume method using the Shear Stress Transport (SST k-ω) model and using the P-1 thermal radiation model.The effect of surface thermal radiation on heat transfer and dynamic structure is examined through a different values of internal surface emissivity of ε = [0, 0.6] and the air is assumed non-participating medium. Also, different previously works concluded that the heat conduction of passive walls must be considered for more accurate prediction. However, the heat conduction is considered in the likewise direction by imposing a temperature distribution along adiabatic walls. In the present study, the thickness of horizontal walls is specified in order to consider the normal heat conduction as a new approach in physical modelling of an enclosed cavity.The present numerical results are compared to available experimental databases for the same flow configuration. A numerical predictions show that the emissivity of the internal walls influences essentially the flow structures close to the horizontal walls, without any effects on the flow in the core of the cavity. On the other hand, somewhat accurate numerical predictions are obtained with combined different heat transfer along internal walls of differentially heated cavity.

Numerical analysis of heat source surface emissivity impact on heat transfer performance in a rectangular enclosure at high Rayleigh numbers

Turbulent natural convection and surface radiation in a square cavity with a heat source have been investigated by the finite difference method. A parametric study has been carried out to examine the effect of surface emissivity of both cavity walls and heater surfaces on fluid flow and heat transfer. Detailed results were discussed regarding the flow and temperature distributions and average Nusselt number profiles. The developed computational code has been verified for several benchmark problems. The results show that the average convective Nusselt number decreases slightly with surface emissivity. The surface radiation modifies the flow pattern and temperature fields.

Numerical simulation of turbulent natural convection in an enclosure with a curved surface heated from below

Natural circulation, which is employed in the design of a small modular reactor, influences the removal of decay heat from the upper header of a pressure vessel under severe accident conditions. This phenomenon has been emphasized for a better understanding of passive safety systems. In this paper, a Reynolds-averaged turbulence model is used to simulate turbulent natural convection in a rectangular enclosure with a curved surface heated from below. The simulation results are first compared with the Direct Numerical Simulation (DNS) of a differential heated cavity obtained from the literature. A good agreement demonstrated the ability of the employed computational fluid dynamics (CFD) approach to predict turbulent natural convection flow and its induced heat transfer. Consequently, the turbulent natural convection formed in a rectangular enclosure heated by a curved surface was investigated. The ensemble average was employed to evaluate the local Nusselt number distribution quantitatively as well as the velocity and temperature distributions. Their dependency was also evaluated based on a systematic CFD simulation that covers a wide range of Rayleigh numbers. Least square regression analysis was employed to propose a correlation to predict the averaged Nusselt number as a function of the Rayleigh number. The influence of the thermal boundary conditions and the discrete heater was also determined.

Thermal fluctuations and boundary layer properties of turbulent natural convection inside open cavities of different dimensions heated from below

We report the experimental measurement of the temperature fluctuation in the vicinity of different zones of the thermal boundary layer in water-filled open cubic cavities heated from below and open at the top. The experiments are performed on the cubic cavity of aspect ratio 1 and lateral dimension 30 mm; the results of our previously reported open cubic cavities of aspect ratio 1 and lateral dimensions (120 mm and 240 mm) are also considered here. The transient nature of the temperature has been measured from the temperature–time series recorded across the central axis of the cavity at different vertical positions z from the heated bottom plate. The Prandtl number and Rayleigh number ranges reported in this paper are 4 ≤ Pr ≤ 6 and 105 ≤ Ra ≤ 109, respectively. The different basic statistical properties, of temperature fluctuation such as mean temperature, root mean square, and probability density function, are studied and discussed. The power-law of power spectral density of the temperature fluctuations at different regions of the thermal boundary layer is studied, and the different roles of rate are compared with the previously established theories and models. The validity criteria for the Oberbeck–Boussinesq approximation are fulfilled. The trend of the dimensionless Nusselt number (Nu) representing the global convective heat transfer is obtained and discussed. We also study the variation in Raδth−1 3 for the heat transfer representation in the range of 0.04–0.24, where δth is the boundary layer thickness.

Buoyancy Driven Turbulent Heat Transfer Characteristics in Enclosure Filled with Phase Change Materials

The exponentially growing computing capabilities of electronic circuits require effective and efficient heat sink designs to control the temperature inside the system. To achieve the desired temperature for efficient working of electronic devices, phase change materials are widely used. In the present study we discuss the complex flow physics of a phase change material filled in a cubical enclosure in the turbulent flow regime with a local heater mounted on the enclosure bottom wall. The thermal properties of local heat source are that of silicon at a working temperature of 330 K. The walls of the enclosure other than the heat source are considered as adiabatic. The turbulent natural convection flow is modeled by the computational fluid dynamics (CFD) approach using Reynolds averaged Navier-stokes equation (RANS) with Lambremhorst k-ε turbulence model. A finite difference method is used to discretize the governing equations and an in-house CFD code is developed for simulating the turbulent characteristics. The flow physics of three different phase change materials (n-Octadecane, PEG900, Paraffin (RT60)) has been analyzed with Groshof number being fixed for all three phase change materials. The transient flow characteristics are investigated by plotting the stream function, velocity and temperature contours of the phase change materials..

Simulation of Fire with a Gas Kinetic Scheme on Distributed GPGPU Architectures

The simulation of fire is a challenging task due to its occurrence on multiple space-time scales and the non-linear interaction of multiple physical processes. Current state-of-the-art software such as the Fire Dynamics Simulator (FDS) implements most of the required physics, yet a significant drawback of this implementation is its limited scalability on modern massively parallel hardware. The current paper presents a massively parallel implementation of a Gas Kinetic Scheme (GKS) on General Purpose Graphics Processing Units (GPGPUs) as a potential alternative modeling and simulation approach. The implementation is validated for turbulent natural convection against experimental data. Subsequently, it is validated for two simulations of fire plumes, including a small-scale table top setup and a fire on the scale of a few meters. We show that the present GKS achieves comparable accuracy to the results obtained by FDS. Yet, due to the parallel efficiency on dedicated hardware, our GKS implementation delivers a reduction of wall-clock times of more than an order of magnitude. This paper demonstrates the potential of explicit local schemes in massively parallel environments for the simulation of fire.

Numerical Study of the Turbulent Natural Convection in a Trapezoidal Cavity: Influence of the Inclination of the Lateral Walls

In this paper, we study the heat transfer in turbulent natural convection in a two- dimensional cavity with a trapezoidal section and isoscales filled out of air with as height H =2.5 m. In these conditions, the side walls are differentially heated while the horizontal walls are adiabatic. The k-ε turbulence model with a small Reynolds number was integrated in our calculation code. The governing equations of the problem were solved numerically by the commercial CFD code Fluent; which is based on the finite volume method and the Boussinesq approximation. The elaborated model is validated from the experimental results in the case of the turbulent flow in a square cavity. Then, the study was related primarily to the influence of the slope of the side walls of the cavity on the dynamic behavior and the heat transfer within the cavity.

Combined Effect of Turbulent Natural Convection and Radiation from a Horizontal Cylinder

The present study deals with a computational investigation on the effect of surface radiation for a buoyancy-induced flow and heat transfer from a horizontal cylinder suspended in air. The numerical simulations have been performed in the turbulent regime by changing the Rayleigh number from to , surface emissivity of the cylinder surface within the ranges of 0.2–1, and considering temperature dependent fluid properties. The closure issues in the Reynolds-averaged Navier–Stokes equation have been taken care of by the standard model in conjunction with the standard wall function to resolve the near-wall turbulence. The discrete ordinate method has been implemented for solving the radiation equations. The effects of the and surface emissivity on the heat transfer have been depicted graphically to provide a more detailed insight into the physical phenomena through the normalized temperature contours and the velocity vectors. The relative strength of the radiation has been calculated and compared with the strength of the convection under different operating conditions. The relative magnitude of the radiation diminishes with an increase in the at a particular surface emissivity for the range of parameters considered in this study. In addition to that, an empirical correlation for the average Nusselt number has been proposed that will be helpful for industrial and academic purposes.

Computational fluid dynamics and experimental study of turbulent natural convection with surface thermal radiation in a cubic enclosure

This paper reports a computational fluid dynamics and experimental study to analyze the effect of surface thermal radiation on the turbulent natural convection in a closed cubic cavity. Experimental and numerical results are compared for low and high wall emissivities. Experimental temperature profiles were obtained at six different depths and heights consisting of 14 thermocouples each. Several turbulence models were evaluated against experimental data. It was found that renormalized [Formula: see text]-[Formula: see text] and standard [Formula: see text]-[Formula: see text] turbulence models present the best agreement with the experimental data for emissivities of walls of 0.98 and 0.03, respectively. Thus, the numerical results of temperature fields and flow patterns were obtained with these models. From the results, it was found that the effect of thermal radiation on experimental heat transfer coefficients is significantly, increased between 48.7% ([Formula: see text]) and 50.16% ([Formula: see text]), when the emissivity of the walls increases from 0.03 to 0.98. Therefore, the radiative exchange should not be neglected in heat transfer calculations in cubic enclosures, even if the temperature difference between heated wall and cold wall is relatively small (between 15 and 30[Formula: see text]K).

Mathematical modelling of conjugate laminar and turbulent heat transfer in a cavity: Effect of a vertical glazed wall

A numerical analysis of the laminar and turbulent natural convection combined with surface thermal radiation in a square cavity with a glazed wall is presented. The enclosure is composed of one left vertical isothermal wall, two horizontal adiabatic walls, and one right vertical partially semitransparent wall (clear glass sheet). Different values of the aspect ratio of the right vertical glazed wall (A=0.15,0.25,0.5,0.75,1.0) are analyzed, and the incoming solar irradiation (G) over the glass is varied from 0 to 750 W/m2 at intervals of 150 W/m2. Three different case studies were considered based on the position of the clear glass sheet on the right wall: (a) located at bottom side/Case 1 (C1), (b) located at the middle side/Case 2 (C2), and (c) located at the upper side/Case 3 (C3). The numerical solution was carried out using an in-house code based on the finite volume method. The numerical code was validated against experimental data for the turbulent natural convection problem. A comparison between the predicted and measured dimensionless vertical velocity, temperature profiles and local and average Nusselt numbers are presented, obtaining good agreement. Results show that convective heat transfer increases from about 70 to 250 as the aspect ratio increases from 0.15 to 1.0, for all solar radiation intensities, and all cases of turbulent flow. Also, radiative heat transfer decreases as the aspect ratio and solar radiation increase, which implies that it could become lower than convective heat transfer for some aspect ratio values set. When a laminar flow regime is considered, lower average convective and radiative heat transfer rates are obtained regarding the case of turbulent flow. In general, they are between 2.0<Nuconv<7.5 and 2.0<Nurad<13.5 for all cases under analysis. Finally, a set of correlations for the total Nusselt number were obtained for turbulent conjugate heat transfer.

Numerical experiment of turbulent transfer of thermal energy using the immersed boundary method and adaptive mesh refinement

This paper presents the mathematical modeling and numerical simulations of turbulent flows with thermal convection. Cartesian numerical meshes are used for non-Cartesian problems thanks to the dynamic and thermal immersed boundary methods. Turbulence modeling was performed using the LES (Large Eddy Simulation) methodology with dynamic Smagorinsky subgrid-scale modeling. All of the simulations were carried out using a computational code that was developed at home (MFSim - Multiphysic Simulator). Two kinds of thermal immersed boundary conditions were proposed, implemented, and validated. To exemplify the functionalities of this computational tool, simulations of turbulent natural convection were performed on a horizontal cylinder. Consistent qualitative results were obtained, which reveal the dynamic structures characterizing the transition to turbulence. The corresponding statistical results compare well with the reference results that have been published by other authors.

Heat transfer and boundary layer analyses of laminar and turbulent natural convection in a cubical cavity with differently heated opposed walls

The buoyancy-driven flow in a cubical cavity with differently heated opposed walls has been chosen to investigate the laminar and turbulent heat transfer in enclosed domains. This type of configuration contains the essential physics existing in flows where transport between a fluid and an adjacent solid surface is controlled by the existence of boundary layers. The dependence of the wall heat transfer on the buoyancy force over the range Ra=105 to Ra=5.4×108 has been investigated, for air (Pr=0.7), using Direct Numerical Simulations. Under the assumption of mixed convection flow within the boundary layer produced by the combined effect of buoyancy and forced convection due to the large-scale flow, we assessed the validity of classical 2D boundary layer solutions in this flow configuration. The resulting characterization of the near-wall flow allows to derive new semi-analytical models for wall transfer applications in enclosed cavities.

CFD-based analysis of entropy generation in turbulent double diffusive natural convection flow in square cavity

The present study concerns the problem of natural and double diffusive natural convection inside differentially heated cavity filled with a binary mixture composed of air and carbon dioxide (CO2). Temperature and CO2 concentration gradients are imposed on both perpendicular left and right walls. Simulations have been performed using the CFD commercial code ANSYS Fluent by solving continuity, momentum, energy and species diffusion equations. Numerical results obtained have been compared to data from the literature for both natural convection thermosolutal cases under laminar and turbulent regimes. For turbulent runs the RNG k-ε model has been selected. A good agreement has been noted between the different types of data for both cases for Rayleigh number ranging between 103 and 1010 and buoyancy ratio between -5 and +5. Entropy generation rates due to thermal, viscous and diffusive effects have been calculated in post processing for all cases.