Turbulent Natural Convection Research Articles

Modifying a meshless method to solving κ−ε turbulent natural convection heat transfer

The conventional meshless local Petrov–Galerkin method is modified to enable the method to solve turbulent convection heat transfer problems. The modifications include developing a new computer code which empowers the method to adopt nonlinear equations. A source term expressed in terms of turbulent viscosity gradients is appended to the code to optimize the accuracy for turbulent flow domains. The standard [Formula: see text] transport equations, one of the most applicable two equation turbulent viscosity models, is incorporated, appropriately, into the developed code to bring about both versibility and stability for turbulent natural heat transfer applications. The amenability of the new developed technique is tested by applying the modified method to two conventional turbulent fluid flow test cases. Upon the obtained acceptable results, the modified technique is, next, applied to two conventional natural heat transfer test cases for their turbulent domain. Based on comparing the results of the new technique with those of the available experimental or conventional numerical methods, the proposed method shows good adaptability and accuracy for both the fluid flow and convection heat transfer applications in turbulent domains. The new technique, now, furthers the applicability of the mesh-free local Petrov-Galerkin (MLPG) method to turbulent flow and heat transfer problems and provides much closer results to those of the available experimental or conventional numerical methods.

Turbulent natural convection heat transfer from a vertical hollow cylinder suspended in air: A numerical approach

Numerical simulations have been conducted to predict turbulent natural convection flow field and temperature distribution around an isothermal vertical solid or hollow cylinder. Continuity, momentum and energy equations have been solved along with kinetic energy and specific dissipation rate equations in the turbulent range of Rayleigh number (Ra) spanning from 1010 to 1012 and length to diameter ratio of the cylinder (L/D) from 0.125 to 20. Average surface Nusselt number and non-dimensional mass flow rate of air through the hollow cylinder have been calculated and plotted with respect to the non-dimensional input parameters such as Ra and L/D. Nu for the inner surface of the hollow cylinder is marginally lesser than that of the outer surface of the hollow cylinder for all L/D and Ra. Nu for the outer surface increases monotonically with respect to L/D however, for the inner surface, it decreases up to L/D of 10 and beyond that, it starts rising. The temperature contours and velocity vector plots have been sketched to understand the heat transfer phenomenon in a better way. Correlations for Nu and mass flow rate for hollow cylinder have been proposed in the turbulent range which could be referred in steel industries, electronics industries and academic practices. Exploiting the developed correlation, the cooling curve (temp vs time) of a hollow cylinder at various Ra and L/D has been drawn which shows a cylinder with a higher Ra with high L/D requires more time to cool down than that of a lower Ra with low L/D. The cooling curve is very much helpful in reducing the idle time in loading and unloading operations of the hot products in steel industries.

Numerical study of laminar and turbulent natural convection from a stack of solid horizontal cylinders

Natural convection from a stack of isothermal solid horizontal cylinders has been investigated numerically in a three dimensional computational domain. Simulations were conducted in both laminar and turbulent flow regimes of Rayleigh number (Ra) spanning in the range (104≤Ra≤108) and (1010≤Ra≤1013), respectively. In the present study, the length to diameter ratio of the cylinders has been varied in the range 0.5–20. Three different stack arrangements were considered for the numerical simulations by arranging three, six and ten number of cylinders in a triangular manner. The present computational study is able to appraise very interesting thermo-buoyant plume structures around the stack of cylinders. The average Nusselt number (Nu) shows a positive dependence on Ra for all L/D. The average Nu for a stack of three-cylinders is marginally higher than that of six-cylinders followed by ten-cylinders. Furthermore, at a particular Ra, Nu is significantly higher for short cylinders (low L/D) and decreases with increase in L/D up to 10 or 15 and remain constant for long cylinders. In addition, the present numerical results are also compared with the stack of hollow cylinders. A new Nusselt number correlation has been developed for different stacks as a function of Ra and L/D, which would be useful to industrial practitioners and academic researchers.

Turbulent natural convection combined with entropy generation in a nanofluid cavity with non-uniformly heated side walls

The two-dimensional turbulent natural convection and entropy generation within an alumina-water nanoliquid chamber in the presence of sinusoidal temperature profiles at vertical walls is calculated. The considered process has been modelled using the RANS (Reynoldsaveraged Navier–Stokes) approach for turbulent regime and Boussinesq approximation for the buoyancy force. The control equations written using the non-dimensional stream function, vorticity and temperature variables combined with parameters of standard κ-ε turbulence model have been solved by the finite difference method with non-uniform computational mesh. The special algebraic transformation has been used for an introduction of the non-uniform mesh in physical domain. The influence of the nano-sized particles concentration and Rayleigh number on the nanofluid circulation and energy transport patterns has been examined.

Numerical investigations of turbulent natural convection heat transfer within a wind turbine nacelle operating in hot climate

The Algerian Sahara is characterized by severe climate conditions where temperature reaches high levels with large variations during day and season. Nacelles of wind turbines operating in this region are subjected to the overheating problem, in particular the electromechanical components becoming less effective as they heat up during use. In order to maintain an appropriate temperature of the air within the nacelle, an active cooling system has to be set up to reject the generated heat towards the external environment and the resulting heat transfer must be properly controlled. The present work investigates numerically the impact of turbulent natural convection heat transfer on the nacelle thermal behavior. Reynolds-Averaged Navier–Stokes and energy equations have been considered. ANSYS FLUENT code has been employed to solve the resulting mathematical model. Temperature and velocity profiles within the 2D and 3D-configurations of the nacelle have been presented and discussed. Variations of the average temperature inside the nacelle and the required cooling capacity are determined and thoroughly discussed.

Moisture transfer by turbulent natural convection

We present an experimental and numerical study of natural convection with moist air as convecting fluid. By simplifying the system as two-component convection, an experimental method is proposed for indirectly measuring the moisture transfer rates in buoyancy-driven flows. We verify the results using direct numerical simulations. It is found that the non-dimensionalized transfer rates for both sensible heat ($Nu_{T}$) and water vapour ($Nu_{e}$) are essentially determined by a generalized Grashof number $Gr$ (the ratio of combined buoyancy generated by the imposed temperature and vapour pressure gradients to viscous force), and are only weakly dependent on the buoyancy ratio $\unicode[STIX]{x1D6EC}$ (the ratio of buoyancy induced by temperature variation to that due to vapour pressure variation). Moreover, we show that the full set of control parameters $\{Gr,\unicode[STIX]{x1D6EC},Pr,Sc\}$ is more suitable than other choices for characterizing the two-component system under investigation. As a special case, the Schmidt number dependence for passive scalar transport rates in buoyancy-driven flows is also deduced.

Laminar and turbulent natural convection from a stack of thin hollow horizontal cylinders: A numerical study

Three dimensional natural convection from a stack of thin hollow horizontal cylinders has been investigated numerically in both laminar and turbulent regimes of Rayleigh number (Ra) spanning in the range 104 to 108 and 1010 to 1013, respectively. In the present study, the length to diameter ratio (L/D) of the cylinders is considered in the range 0.5–20. Thin horizontal hollow cylinders of three, six, and ten in numbers are arranged in a triangular manner to form three different types of stacks of same length scale. The full Navier-Stokes equation along with the energy equation are solved to predict the flow pattern and heat loss from the cylinders. The present computational study is able to capture very interesting buoyancy plume structures around the stack of short and long hollow cylinders. The average Nusselt number (Nu) shows a positive dependence on Ra for all L/D. In addition, Nu for a stack of three cylinders is found to be marginally higher than the stack of six cylinders followed by ten cylinders. Empirical correlations are developed for Nu as a function of Ra and L/D, which would be beneficial to academic researchers as well as practicing engineers.

Prandtl number effect on thermal behavior in volumetrically heated pool in the high Rayleigh number region

The in-vessel retention by external reactor vessel cooling (IVR-ERVC) is a severe accident management strategy to hold or mitigate the molten radioactive material in the reactor pressure vessel (RPV). Highly turbulent natural convection is one of the most important phenomenon determining the thermal behavior of the molten pool. In the safety evaluation of IVR-ERVC, thermal behavior of the corium pool has been determined using experimental correlations. Property differences between the experimental simulant and the corium have been considered negligible. The effect of property differences represented by Prandtl number (Pr) on the thermal behavior of oxide pool is numerically investigated. The algebraic heat flux model (AFM) is used to simulate the turbulent natural convection and is validated with the BALI experiment. The Pr effect on the flow characteristics and the thermal behavior is investigated in the high Rayleigh number region. The present results show that the upward heat transfer tends to increase at the same Ra′ in the case of the molten pool which is lower Pr fluid than the simulant material.

Lattice Boltzmann Modeling of Natural Convection in a Large-Scale Cavity Heated From Below by a Centered Source

Turbulent natural convection in a large-scale cavity has taken a great attention thanks to its importance in many engineering applications such as building. In this work, the lattice Boltzmann method (LBM) is used to simulate turbulent natural convection heat transfer in a small room of housing heated from below by means of a heated floor. The ceiling and the four vertical walls of the room are adiabatic except for a portion of one vertical wall. This portion simulates a glass door with a cold temperature θc = 0. The cavity is filled by air (Pr = 0.71) and heated from below with uniformly imposed temperature θh = 1. The effects of the heat source length (Lr) and Rayleigh number (Ra) on the flow structure and heat transfer are studied for ranges of 0.2 ≤ Lr ≤ 0.8 and 5 × 106 ≤Ra ≤ 108. The heat transfer is examined in terms of local and mean Nusselt numbers. The results show that an increase in Rayleigh number or in heat source length increases the temperature in the core of the cavity. The flow structure shows that turbulent natural convection regime is fully developed for Ra = 108. Correlations for mean Nusselt number as a function with Ra for different values of Lr are expressly derived.

Effects of radiation on turbulent natural convection in channel flows

A computational study has been carried out to analyse complex interaction of radiation with turbulent natural convective flow of dry and humid air in open-ended channels. Transient flow simulations are undertaken in the channel with one uniformly heated wall and adiabatic side walls for different values of emissivity of active walls with and without participating medium. To adequately present turbulence and radiation, a computational model included large eddy simulations for the turbulent flow coupled with discrete ordinates method for radiation transfer. Spectral line-based weighted-sum-of-grey-gases for the absorption properties of water vapour has been adopted. Complex three-dimensional vortical structures are identified which directly affect the temperature distribution on the heated wall. Including wall to wall radiation resulted in significant changes in the heat transfer, reaching 14 °C temperature drop at the hot wall with wall emissivity of 0.9. Mixing and cooling rates in this case were increased by up to 25%. Including gas radiation for the humid air with the water vapour molar fraction of 0.02 corresponding to saturated conditions at inlet temperature of 25 °C did not have a significant effect on the mean flow and temperature values comparing with wall to wall radiation. However, turbulent statistics have changed significantly resulting in a delayed transition to turbulence near the active wall of the channel and increased turbulent activity near the cold wall. The model developed in the present study is also applicable in fire management, where the aim is to reduce the damage that occurs when a PV module is exposed to high temperatures.

Thermal design optimization of passive cooling capability in a dry-storage system by adding wall undulations or semi-circular fins

Thermal design investigation of a novel liner inner wall surface geometry alteration of a ventilated concrete cask used for spent nuclear fuel storage is carried out through three-dimensional Computational Fluid Dynamics (CFD) simulations. The present study reveals that the new proposal returns values for the convective heat transfer coefficient that are almost three orders of magnitude higher than the base case. Hence, proving that even making such a rather simple alterations to the smooth walls geometry, i.e. adding undulations or semi-circular fins, can result in a significant convective heat transfer enhancement, therefore, further reducing the risks of structural damages caused by the dry-storage system overheating. Before conducting the design optimization study mentioned above, two validation test cases have been considered in order to examine the Reynolds Averaged Navier-Stokes (RANS) models’ capability to predict the flow under buoyancy driven and low turbulence levels such is the case in the passively cooled dry cask system. The test cases considered in the validation study sections are the experimental measurements of Betts and Bokhari for turbulent natural convection in an enclosed tall cavity and the temperature measurements in concrete-shielded spent fuel storage cask system reported by McKinnon. The models selected for the validation study are namely the k-ω SST model, the v2-f model, the low-Reynolds k-ε model, and the recently formulated elliptic blending k-ε model. It has been found that out of these models, the low-Reynolds k-ε model was found to provide a good combination in returning both; physically realistic and conservative solution. The present validation using well-resolved RANS simulations has shown that a more quantitative and detailed comparison of the predictions is required in order to clearly highlight the discrepancies between the models’ predictions in terms of velocity, turbulent kinetic energy and, in particular, convective heat transfer coefficient profiles. The commonly adopted methodology in limiting the validation process to temperature profiles comparison only, would cover up these differences as the temperature variation is affected by the combined effects of the convective and the radiation heat transfer modes.

Heat and Moisture Transfer in a Rectangular Cavity Partially Filled with Hygroscopic Porous Media

This work deals with turbulent natural convection heat and moisture transfer with thermal radiation in a rectangular cavity partially filled with hygroscopic porous medium. The governing equations for the momentum and heat transfer in both free fluid and hygroscopic porous media and moisture content transfer in hygroscopic porous medium were solved by the finite element method. Comparisons with experimental and numerical results in the literature have been carried out. Effects of thermal radiation, Rayleigh number on natural convection and heat transfer in both free fluid and porous medium and moisture content transfer in porous medium were analyzed. It was found that surface thermal radiation can significantly change the temperature and moisture content fields in the regions of free flow and hygroscopic porous medium. With the increase in Rayleigh number, the temperature of porous medium at the interface increased slightly, and the magnitude of moisture change becomes smaller.

Temperature drop and gelatinization characteristics of waxy crude oil in 1000 m3 single and double-plate floating roof oil tanks during storage

Single and double-plate floating roof oil tanks are two types of widely used floating roof oil tanks in petrochemical industry. However, the differences in flow and hear transfer characteristics of waxy crude oil inside these two tanks have been studied insufficiently. Finite volume method is employed in this research to study the temperature drop and gelatinization processes of waxy crude oil as well as the differences in single and double-plate floating roof tanks. Based on a comprehensive consideration of the atmosphere, soil, floating roof oil tank as well as the tank structure and the variations of the waxy crude oil state and rheological behavior, general physical and mathematical models are established. In the model, wax precipitation and gelatinization processes of waxy crude oil are described by the enthalpy-porous media method. Non-Newtonian behavior is described by the Power law equation. Turbulent natural convection is described by the LES method. SIMPLE algorithm is employed to couple pressure and velocity. Taking 1000 m3 single and double-plate floating roof oil tanks as examples, the evolution of oil temperature and flow behavior is studied and the variations of the gel oil thickness and heat flux are analyzed. Moreover, the differences between these two tanks are also discussed. Results show that due to the structure difference of the tank roof, the temperature drop rates are 0.018 °C/h and 0.007 °C/h respectively in single and double-plate floating roof tanks in the case of this research. Secondly, for the growth of gel oil on tank bottom, in double-plate floating roof tank, gel oil thickness keeps growing and fluctuating, while in single-plate tank, the original gelled oil layer disappears firstly and then increases gradually. Thirdly, although for both tanks, tank roof is the main part of heat dissipation towards the atmosphere, the maximum heat fluxes are respectively over 2.0 kW and 0.4 kW for single and double-plate floating roof tanks, and the total average heat fluxes respectively are 1.49 kW and 0.59 kW.

Study of flow field, heat transfer, and entropy generation of nanofluid turbulent natural convection in an enclosure utilizing the computational fluid dynamics‐artificial neural network hybrid method

AbstractIn this study, the turbulent natural convection of Ag‐water nanofluid in a tall, inclined enclosure has been investigated. The main objective of this study is finding the optimized angle of the enclosure with operational boundary condition in cooling from ceiling utilizing the computational fluid dynamics‐artificial neural network (CFD‐ANN) hybrid method, which has not been noticed in previous studies. To achieve this, we proposed two approaches. First, the simulations have been done with a deviation angle of 0 to 90° by using water and Ag‐water nanofluid. And second, a new prediction approach is proposed based on radial basis function artificial neural networks (RBF‐ANN) to predict the mean Nusselt number and entropy generation with the variation of Rayleigh numbers, deviation angles, and volume fractions as inputs. The results from the first approach indicate that the Rayleigh number has a considerable function in the determination of optimized angle. The results from the second approach, which used the first approach simulation results as training data set, could predict the mean Nusselt number and entropy generation with 1.4577e−022 and 1.552e−015 mean square error, respectively. Moreover, a new set of data for Rayleigh numbers, deviation angles, and volume fractions were used to test the performance of the prediction model, which shows promising and superior prospects for RBF‐ANN.

Numerical study of turbulent magnetohydrodynamic convection of molten sodium with variable properties in a square cavity

In this study, the finite volume method is used to simulate the turbulent natural convection in a square partitioned cavity. In this paper a fluid-flow with Pr = 0.01 and Ra = 106 and 107 in the presence of a magnetic field is investigated. The fluid properties are function of temperature. A parametric study is carried out using following parameters: non-dimensional different partition position from 0.2 to 0.6, non-dimensional different partition height from 0.1 to 0.4, and different Hartmann numbers from 0 to 200. It is found that Nusselt number is a decreasing function of partition height, Hp, and Hartmann number and the non-dimensional position of partitions, Dp, affects on streamlines and isotherms. It is observed that at Ra = 106 and Dp = 0.6 the Nusselt number is maximum and as Hartmann number increases the Nusselt number tends to a constant number. Also at Ra = 107 and Dp = 0.4 the variation of mean Nusselt number for different partition heights is more different than the other cases. Also the Nusselt number at Hp = 0.4 is nearly half for Dp = 0.4 in comparison with the other cases.

Numerical simulation of turbulent natural convection of nanofluid with thermal radiation inside a tall enclosure under the influence of magnetohydrodynamic

AbstractIn the present study, the natural convective heat transfer in the turbulent flow of water/CuO nanofluid with volumetric radiation and magnetic field inside a tall enclosure has been numerically investigated. The thermophysical properties of nanofluid have been considered variable with temperature and the effects of Brownian motion of nanoparticles have been considered. The main objective of this work is an investigation of the effect of using water/CuO nanofluid and presence of magnetic field on turbulent natural convection in three types of enclosures (vertical, inclined, and horizontal) by considering the volumetric radiation. The governing equations on turbulent flow domain under the influence of the magnetic field and by considering the combination of volumetric radiation and natural convection have been solved by a coupled algorithm. For validating the present research, a comparison has been carried out with the laminar natural convection flow under the influence of the magnetic field and radiation effects and also, the natural turbulent convection flow of previous studies and a proper coincidence has been achieved. The results indicated that by increasing volume fraction and Hartmann number the average Nusselt number enhances and reduces, respectively. By adding 1% CuO nanoparticles to the base fluid, heat transfer improves from 10.59% to 17.05%. However, by increasing the volume fraction from 1% to 4%, heat transfer improves from 1.35% to 4.90%. By increasing Hartmann number from 0 to 600, heat transfer reduces from 9.29% to 22.07%. Also, the results show that the ratio of deviation angle of the enclosure to the horizontal surface has considerable effects on heat transfer performance. Therefore, in similar conditions, the inclined enclosure with a deviation angle of 45° compared to the vertical and horizontal enclosure has better thermal performance.

Performance of second-moment differential stress and flux models for natural convection in an enclosure

In this paper the relative performance of two different differential stress and flux models for computing natural convection in an enclosure is examined. One model is the low-Reynolds number differential stress and flux model and the other model is a recently developed elliptic blending based differential stress and flux model. The primary emphasis of the study is placed on investigating the accuracy of the both models for the turbulent natural convection in an enclosure. Both models are used to predict turbulent natural convections in a square cavity with hot and cold side walls and conducting top and bottom walls (Ra = 1.58 × 109). The relative performance between both models is examined through comparing their solutions with the experimental data. It is shown that both models predict the vertical mean velocity component and mean temperature well. It is also shown that the low-Reynolds number differential stress and flux model slightly under-predicts the turbulent kinetic energy and vertical turbulent heat flux. It is also noted that the implementation of the elliptic blending second moment model is easier since this model does not contain any wall-related parameters and damping functions.

Turbulent natural convection combined with surface thermal radiation in a square cavity with local heater

PurposeThe purpose of this paper is to conduct a numerical analysis of transient turbulent natural convection combined with surface thermal radiation in a square cavity with a local heater.Design/methodology/approachThe domain of interest includes the air-filled cavity with cold vertical walls, adiabatic horizontal walls and isothermal heater located on the bottom cavity wall. It is assumed in the analysis that the thermophysical properties of the fluid are independent of temperature and the flow is turbulent. Surface thermal radiation is considered for more accurate analysis of the complex heat transfer inside the cavity. The governing equations have been discretized using the finite difference method with the non-uniform grid on the basis of the special algebraic transformation. Turbulence was modeled using the k–ε model. Simulations have been carried out for different values of the Rayleigh number, surface emissivity and location of the heater.FindingsIt has been found that the presence of surface radiation leads to both an increase in the average total Nusselt number and intensive cooling of such type of system. A significant intensification of convective flow was also observed owing to an increase in the Rayleigh number. It should be noted that a displacement of the heater from central part of the bottom wall leads to significant modification of the thermal plume and flow pattern inside the cavity.Originality/valueAn efficient numerical technique has been developed to solve this problem. The originality of this work is to analyze unsteady turbulent natural convection combined with surface thermal radiation in a square air-filled cavity in the presence of a local isothermal heater. The results would benefit scientists and engineers to become familiar with the analysis of turbulent convective–radiative heat transfer in enclosures with local heaters, and the way to predict the heat transfer rate in advanced technical systems, in industrial sectors including transportation, power generation, chemical sectors and electronics.

Electronic component cooling inside switch cabinets: combined radiation and natural convection heat transfer

The thermal conditions inside switch cabinets for industrial applications without cooling devices are equally influenced by natural convection and radiation heat transfer. In this work the open source library OpenFOAM is applied and enhanced, in order to simulate the temperature field of the air inside the cabinet. Turbulent natural convection heat transfer is modeled using Menter’s SST model. Radiation heat transfer is modeled using a surface-to-surface model, whereas the view factors are computed based on a Monte Carlo algorithm. Numerical results obtained with different flow models are compared with measurements and correlations from literature for a cavity and a Rayleigh-Benard test case. The Radiation model is validated against comparative numerical data. Finally, the models are applied to calculate the temperature field of a switch cabinet test rig. These numerical results are compared with experimental data.

Large Eddy Simulation of turbulent natural convection in an inclined tall cavity

To a better understanding of buoyancy-driven flows in enclosed tall cavity, Large Eddy Simulations are performed with an original solver, which combines fourth-order compact scheme and parallel computing. The direct numerical simulation of the natural convection flow between two heated parallel plates are provided to validate the present numerical method. Then, the mean and turbulent statistics of the buoyancy-driven in a tall cavity at the moderate Rayleigh number are computed. The results of the vertical configuration flow are successfully compared with the available experimental data. Then, the statistics are discussed with respect to the inclination angle of the tall cavity. Especially, the occurrence of convection rolls in the cavity inclined to the horizontal is shown, as well as their roles in the deterioration of the thermal stratification.