Natural Convection Characteristics Research Articles

Experimental study of the natural convection characteristics of finned-tube sodium-to-air heat exchanger

Experimental study of the natural convection characteristics of finned-tube sodium-to-air heat exchanger

MODELING THE INFLUENCE OF NANOPARTICLES AND GYROTACTIC MICROORGANISMS ON NATURAL CONVECTION IN A HEATED SQUARE CAVITY USING ARTIFICIAL NEURAL NETWORK (ANN)

The phenomenon of natural convection, which is widely used in nature and engineering applications, is a current issue that can be encountered in every field of daily life. In this study, the natural convection characteristics of a complex liquid containing nanoparticles and gyrotactic microorganisms in a heated square cavity were investigated using artificial neural network approach. The Nusselt number, average Sherwood number of nanoparticles, and average Sherwood number of microorganisms were considered as natural convection parameters and an artificial neural network model was developed to estimate these values. The Lewis number, Brownian motion parameter, thermophoresis parameter, and buoyancy ratio parameter values were defined as input parameters in the network model, which has a multilayer perceptron architecture developed with a total of 24 datasets. There were 10 neurons in the hidden layer of the network model, which has a Bayesian regularization training algorithm. The outputs obtained from the network model were compared with the target values; in addition, the prediction performance of the model was extensively analyzed using various performance parameters. It was seen that the predicted values obtained from the neural network and the target values were in an ideal harmony. On the other hand, the value of the coefficient of determination for the network model was 0.99999% and the mean deviation rates were lower than -0.03%. The results of the study showed that the developed neural network model can predict the natural convection parameters discussed with high accuracy.

Investigation of compressible flow under low Mach number in an enclosed square cavity with a novel non-Boussinesq algorithm

Natural convection is widely observed in various scales of natural phenomena and industrial applications. The low Mach number (i.e., low-velocity) natural convection, especially under large temperature differences and significant density and pressure fluctuations, is of great research significance in industrial fields such as nuclear engineering. The Boussinesq approximation based on the incompressible Navier–Stokes (NS) equation is not fully descriptive due to the neglect of coupling effects among temperature, density, and pressure. As for numerical algorithms based on the compressible Navier–Stokes equation, they often suffer from high computational costs and convergence difficulties. In this paper, a novel numerical algorithm based on the decoupled and stabilized Lattice Boltzmann multiphase model with a complete physical description and clear conceptual framework is proposed. It couples the equation of state and the temperature equation, considering the full effects of gravity, pressure, and temperature-dependent density on flow disturbances, and it recovers the complete compressible NS equation. Taking the natural convection in an enclosed cavity as an example, the non-dimensional numbers governing the fluid system are identified by Buckingham π theorem; thus, a new thermal expansion number is proposed to connect the pressure effect. The accuracy and reliability of the numerical algorithm are validated by comparing it with standard benchmarks. On this basis, the proposed algorithm enables a unified physical description from low to high Rayleigh numbers and from small to large temperature differences. By analyzing the flow and heat transfer characteristics of natural convection under different Rayleigh numbers, temperature differences, and thermal expansion numbers, this study reveals the coupled physical mechanisms of low Mach number flow from small to large temperature differences, from low to high Rayleigh numbers and under different thermal expansion numbers.

Optimal characteristics of natural convection in a square porous-nanofluid-filled enclosure containing three tubes

A numerical model was used to simulate natural convection heat transfer in a two-dimensional square enclosure filled with nanofluid and saturated with porous media. The enclosure contained three heated tubes with Rayleigh numbers (Ra) ranging from 102 to 3 × 104. The governing equations with Boussinesq approximation for natural convection were solved iteratively using the finite volume technique through ANSYS FLUENT-CFD commercial package. The Darcy–Forchheimer–Brinkman and Local Thermal Equilibrium models were applied for water and nanofluid flow across the porous zone. The numerical analysis was executed systematically for significant parameters that have a substantial impact on natural convection heat transfer, namely, aspect ratio (AR = 1.25 − 3.75), nanoparticle volume fraction (Φ = 0.25%, 0.5%, 0.7% and 1%), and the porosity of metal foam (ε = 0.3, 0.4, 0.5, 0.6 and 0.7). The aspect ratio was found to have a considerable impact on the averaged Nusselt number (Nuave ). For AR = 3.75, the value of Nuave for all Rayleigh numbers increased by approximately 5.22 times compared to AR = 1.25. Moreover, Nuave remained approximately constant as the concentration of alumina-water nanofluid increased from 0.25% to 1%. The novelty of this study lies in the multi-objective optimal design approach adopted by adopting DX-12 with the outcomes of ANSYS FLUENT-CFD. The averaged Nusselt number and convection heat transfer rate reached their optimal values by nearly 13 and 9 times, respectively, at the lowest possible porosity (ε = 0.3), the lowest possible nanofluid concentration (Φ = 0), and the highest possible aspect ratio value (AR = 3.75).

Natural convection of a reacting hybrid nanofluid in an open porous cavity bounded by vertical wavy walls

PurposeThe purpose of this study is to investigate the natural convection characteristics of a reacting hybrid nanofluid in an open porous cavity bounded by vertical wavy walls subject to an inclined magnetic field.Design/methodology/approachThe physical domain of the problem is constructed using coordinate transformations, and the equations are transformed accordingly. The resulting equations are then solved using finite difference method. Numerical results for the streamlines, isotherms and isoconcentration are illustrated with varying relevant parameters.FindingsWhatever the values of parameters, streamlines have two counter-rotating cells, and their intensities are the highest near the open end. Moreover, the maximum temperature and the minimum concentration are obtained in close proximity to the open end. The strength of streamlines is increased with increasing Rayleigh number, Frank-Kamenetskii number and Darcy number, whereas it is decreased with the increment of volume fractions of nanoparticles.Research limitations/implicationsThe limitations of this study are that the model is suitable for thermal equilibrium cases and constant thermo-physical properties, while the results can predict two-dimensional flow behaviors.Originality/valueTo the best of the authors’ knowledge, there is no study on the natural convection induced by a chemical reaction in an open cavity bounded by vertical wavy walls. The findings might be used to gather knowledge about the flow, energy and reactant distributions in an open space containing a chemical reaction.

Flow and heat transfer characteristics of natural convection in hydrothermal reactor with segmented heating

Hydrothermal reactor is very suitable to utilize low-grade heat especially concentrated solar energy. However, the heat flux generated by solar heating is usually non-uniform, which affects the natural convection inside the reactor. In this work, natural convection of near critical water in cylindrical hydrothermal reactor is investigated by numerical simulation. With constant total heat input into the cylinder, the sidewall is divided into two halves with different heat flux density. The effect of heating schemes and physical properties on flow structure, temperature distribution, fluid motion and wall heat transfer are analyzed. It is found that natural convection is dominated by a large vortex with high velocity when heat flux ratio is lower than 1, and is dominated by two independent weak vortices when heat flux ratio is higher than 1. In addition, the former case has more uniform vertical temperature distribution, stronger fluid flow and weaker wall heat transfer, while the latter case is in the opposite. When the temperature approaches the pseudocritical point, the flow structure remains qualitatively unchanged, but wall heat transfer is enhanced and fluid motion is generally weakened. The change of heat flux ratio significantly regulates fluid motion but slightly affects wall heat transfer. Besides, the regulating effect of heat flux ratio is weakened in near critical region.

Legendre wavelet operational matrix method for the analysis of thermal radiation effect on natural convection of a vertical plate embedded in a saturated porous medium

In this paper, the heat transfer characteristics of natural convection around a vertical flat surface embedded in a saturated porous medium are studied by taking into account the thermal radiation effect. With the aid of the similarity transformations, the governing equations of the problem are reduced to the system of ordinary differential equations over the infinite domain. Moreover, the Legendre wavelet operational matrix method (LWOMM) is hired to find the numerical solution to the considered problem. Initially, to ensure the efficiency of the proposed strategy, a third-order nonlinear boundary value problem having the exact solution is considered a test problem. The numerical findings elucidate that the solutions obtained by the LWOMM are in good agreement with the exact solution as compared with the solution obtained by the Haar wavelet operational matrix method, with the consumption of less computational CPU time. Also, the temperature and velocity profiles are discussed graphically.

Natural convection characteristics of nano-encapsulated phase change materials in a rectangular wavy enclosure with heating element and under an external magnetic field

Recently, nano-encapsulated phase change material (NEPCM) has received great attention due to its ability to absorb and release heat, making it a perfect choice for many engineering and industrial applications. The current work examines the natural convection of NEPCM confined in a cubic enclosure having two wavy sidewalls and under the effect of an external magnetic field. The enclosure is heated from below via a duct or cavity having a U shape; both wavy sidewalls experience cold temperatures, and the other walls are thermally insulated. The impacts of different operational and geometrical parameters on heat transfer inside the enclosure were numerically investigated. These parameters include the external magnetic field applied, Rayleigh number (Ra), sidewall wavenumber (N), Hartmann number (Ha), nanoparticle volume fraction (φ), and heat source position. Heat transfer within the enclosure was found to be most affected by the Ra and Ha numbers, while the other factors studied had little to no effect. The maximum local Nusslet number at Ra = 106 was four times greater than that at Ra = 103. As for the effect of the Ha number, increasing Ha from 0 to 100 decreased the average Nusselt number by 38 %.

Evaluation of non-Newtonian model to accurately predict high Rayleigh number natural convection characteristics using PIV experiment and CFD simulation

Non-Newtonian fluids exhibit variable viscosity particularly at relatively high Rayleigh numbers (Ra), resulting in the occurrence of a complex natural convection phenomenon. In this study, the flow and heat-transfer performances inside a square channel filled with carboxy methyl cellulose, which exhibits the shear-thinning behavior typical of non-Newtonian fluids, were investigated. A comparative study was conducted to develop a viscosity model that can accurately describe the thermal and flow performances. To achieve this objective, three different viscosity models were considered: Power-law, Cross, and Carreau models. A time-constant dependency study was conducted to determine the optimum time step since the numerical solution was highly sensitive to the selected time constant in the Carreau and Cross models. The heat-transfer performances were evaluated by analyzing the mean Nusselt number for the hot wall, whereas the flow performances were evaluated by analyzing the flow patterns obtained by numerical and experimental analyses using the nonintrusive particle image velocimetry technique (Ra = 3.20 × 105, 3.09 × 106, and 2.21 × 107). The mean Nusselt numbers obtained by numerical analysis using the Carreau model were approximately 1.76%, 1.23%, and 0.45% higher than those obtained experimentally. Therefore, the predictions afforded by the Carreau model were in excellent agreement with those obtained experimentally. The findings of the present experimental study will provide benchmarking data for future simulations involving shear thinning or pseudoplastic non-Newtonian fluid media.

MHD natural convection of Fe3O4- MWCNT/Water hybrid nanofluid filled in a porous annulus between a circular cylinder and Koch snowflake

In this work, the numerical investigation was conducted for the MHD natural convection and entropy generation characteristics of water-based hybrid nanofluid (Fe3O4-MWCNT) in a porous annulus between a cooled circular cylinder and a heated Koch snowflake subjected to a uniform magnetic force. The novelty of this work is presented by the special shape and different studied positions of the hot barrier. The governing equations are explained by employing the Finite Element Method. The impacts of nanoparticle volume fraction (φ = 2 %, 3%,4%, and 5 %), Rayleigh number (Ra = 103 to 106), Hartman number (Ha = 0,25,50,100), Darcy number (Da = 10-2, 10-3, 10-4, and 10-5), and the position of the Koch snowflake (four cases) on the distributions of isotherms, streamlines, average Nusselt number (Nuavg) as well as on total entropy generation and Bejan number are thoroughly examined. The computational outcomes indicate that increasing the Ra number is possible by changing the temperature between the hot and cold sources. By increasing this parameter, the buoyancy force of the fluid is strengthened. As the Da number increases, the penetration of the flow cross-section in the cavity increases, and the flow circulates in the cavity with less depreciation due to the buoyancy force. Applying Lorentz force, if not in the direction of natural flow, causes the flow velocity to be depleted and fluid flow in the cavity to be facilitated. With increasing Ra number, the application of Lorentz force with different intensities becomes important due to the importance of fluid circulation in the cavity. Applying a Lorentz force with a higher Ha number at high Ra numbers reduces heat transfer due to junction balance and flow separation on hot and cold surfaces. In the lower Ra numbers, the Bejan number tends to 1, indicating a significant increase in the temperature gradient in the cavity and the expansion of the thermal boundary layer at low fluid velocities. At the highest Ra, increasing Ha from 0 to 100 decreases Nuavg by 50 %, while decreasing Da from 10−2 to 10−5 reduces Nuavg by 70 %.

A numerical study on the natural convective heat loss of an isothermal upward-facing cylindrical cavity

• Uncovered the influences of surface temperature, tilt angle and aperture area. • Developed an empirical correlation with high prediction accuracy. • Proposed a strategy that reduces the air velocity to control convective heat loss. The development of second-stage concentrators and lens systems facilitates the application of upward-facing cavities in solar energy utilization. To design a cavity with high efficiency and low cost, convective heat loss must be determined then controlled. Taking the commonly used isothermal cylindrical cavities as the object, natural convective heat loss characteristics were analysed numerically. The influences of surface temperature, tilt angle and aperture ratio were elaborated. Results show that the natural convective heat loss keeps increasing with increasing aperture ratio. As the tilt angle reduces, the natural convective heat loss increases before attaining a maximum at about −30° after which it drops. In short, the natural convective heat loss is proportional to the air velocity, the higher the air velocity, the larger the heat loss. A strategy controlling the air velocity is thus proposed to control the convective heat loss. Unlike isoflux conditions, a clockwise vortex forms inside the cavity for the tilt angle less than −30°. Large prediction errors are presented for using previous correlations to predict the natural convection heat loss of upward-facing cylindrical cavities. Hence, a novel empirical correlation based upon the influence characteristics of studied factors and with high accuracy is developed.

Natural convective nanofluid flow characteristics with Brownian motion effect in an annular space between confocal elliptic cylinders

The principal aim of this work is to study numerically the natural convection in an annular space between confocal elliptic cylinders. The Brownian motion was added based on the thermal conductivity model and the simulation was carried out using finite volume control method. Various nanofluids with different volume concentrations were undertaken ranging from 2% to 6% and various Rayleigh numbers were tested for different particles. The isotherms and stream function are presented and it was clearly seen that by increasing the Rayleigh number, the heat transfer rate increases on the domain. On the other hand it was noticed that the heat transfer enhancement is less sensitive to the nanoparticles when the Brownian motion was considered

Numerical study on long-term passive heat removal of EPRHR cooling water tank (CWT) using heat pipe heat exchanger

The EPRHR in the HPR1000 adopts natural circulation to meet the heat removal requirements of the containment for 72-hours under accident conditions. To reduce the evaporation depletion of water in CWT and enhances the passive cooling capability of the EPRHR, a long-term passive cooling scheme based on a heat pipe heat exchanger is proposed for CWT in this paper. Experimental study and numerical simulation are used to study the natural convection characteristics in the CWT. The results show that the scheme can take away 9 MW heat load as the average water temperature (Tave) is below 90 °C. The heat removal ability of the tube bundle is weakened row by row and the maximum difference of heat removal ability between tube bundles is 23%. The position of the tube bundles affects the Tave, and when the dimensionless layout factor (γ) = 3/4, Tave (346.34 K) is the smallest.

Numerical study on the flow and heat transfer characteristics of natural convection in CFETR cryostat

The cryostat in the China Fusion Engineering Testing Reactor (CFETR) provides vacuum and low temperature operating conditions for the superconducting magnet system. A high pressure in the cryostat can increase the Rayleigh number (Ra) and cause the natural convection within the continuous hypothesis. A three-dimensional model, using incompressible ideal gas model and realizable k-ε model, is built to simulate the natural convection between the superconducting magnet system and the thermal shield within the temperature range of 4.5-80 K. The distributions of velocity field and heat flux on different surfaces of superconducting magnet system are obtained under different Rayleigh number conditions. The results show that the areas with large natural convection velocity mainly locate near the Central Solenoid (CS) coils and at the bottom of vacuum chamber. When the Rayleigh number ranges from 104 to 1010, the maximum velocity of helium gas decreases, while the heat flux on walls of superconducting magnets increases rapidly, and the heat flux on walls of PF-1, CS and TF coils is more affected by pressure, which needs to be considered in designing the safety system.

Investigation of natural convection characteristics in the molding chamber of a 3-D printer cooled by thermoelectric cooling modules

Refrigeration using thermoelectric effect is a hot spot and frontier issue at present for its advantages of simple structure, no refrigerant, and no moving parts. This paper aims at characterizing the natural convection characteristics of air in the molding chamber of a 3-D printer that is cooled by thermoelectric cooling modules (TECMs). The distributions of the temperature and velocity field in the molding chamber are obtained, and the influences of the number of the TECMs and their arrangement modes on the magnitude and the spatial difference of temperature and velocity are analyzed. It is found that the Case 2 arrangement mode that all the TECMs are located on the top of the chamber is characterized as the optimal mode with the lowest final average air temperature and the smallest temperature non-uniformity coefficient of the molding chamber. Based on the optimal arrangement mode, a computational formula that correlates the final average air temperature of molding chamber with the volume of the molding chamber and the number of the TECMs is proposed. The new correlation can reproduce all of the 104 data points within error band of ±1.0 °C, among which 76.9% of the data can be predicted within error band of ±0.5 °C. The conclusions of this paper can provide a reference for the application of 3-D printing technology and the design of temperature control scheme of low-temperature molding chamber.

Natural convection characteristics of honeycomb fin with different hole cells for battery phase-change material cooling systems

As a nature production the lightweight honeycomb is used as a fin to increase the contact area with the PCM and improve its overall thermal conductivity. In this paper the effects of different honeycomb core shapes, sizes, and arrangements on melting under natural convection were investigated to determine the optimal honeycomb cell parameters. In this study, transient simulations based on VOF and enthalpy-porosity methods are developed, and the results show that compared with heat conduction, the melting time under natural convection conditions can be reduced. By analyzing different shapes, it is discovered that the melting effects of triangles, trapezoids, rectangles, and rhombuses are better than that of hexagons, the triangular melting time is reduced by 23.1%. The orthogonal experiments show that the ranking order of the effects on melting time and natural convection is Ra > Shape > LHR. Furthermore, the optimal honeycomb hole core parameters are obtained from the analysis of a single factor. Compared with the hexagonal honeycomb fin, a battery's PCM with an optimal triangular honeycomb fin under natural convection exhibits a 23.3% increase in terms of temperature decrease.

Natural Convection of Reacting Chemical in Square Enclosure with Circular Cylinder

Natural convection characteristics of a reacting chemical are presented considering a circular cylinder at different locations in a square cavity. The problem is formulated providing a detailed description of the transformations. To end, the physical domain is changed to the computational domain using a suitable coordinate transformation. We then solve the equations employing the finite difference method. The magnitude of stream function and maximum temperature are in general substantially augmented owing to the increase of horizontal, vertical, or diagonal distance between the centers of the circular cylinder and the square enclosure. But in the case of changing the vertical or diagonal locations the above quantities for a cylinder down from the horizontal centerline are found to be higher than those for a cylinder up from it. For higher values of the Lewis number (), Rayleigh number (), and buoyancy force parameter (), magnitude of stream function increases, whereas maximum temperature decreases. Contrary to this, the Frank-Kamenetskii number () strongly enhances the strength of stream function and the maximum temperature. From this investigation, one can achieve knowledge to formulate a problem for more complex geometry. In addition, the results might be used to improve the shortcomings of present technologies or to invent a new state-of-the-art technology.

Transition of natural convection of liquid metal in an annular enclosure

Natural convection of a low-Prandtl-number fluid in an annular enclosure with a square cross section and radius ratio of the annulus of 0.5 is numerically studied using a finite-difference method in the cylindrical coordinate system. The fluid in the annular enclosure is heated from an inner vertical wall and cooled from an opposing vertical outer wall, both isothermally, whereas the two horizontal walls are adiabatic. Characteristics of the present natural convection are investigated in detail, with one of the annular computational domains divided into several equal parts. Three-dimensional (3D) regular oscillatory flow appears for a range of Rayleigh numbers when the computational domain angle is set to 90° or less. It is revealed that both the average Nusselt number and kinetic energy obtained from 3D flow are smaller than those obtained from a two-dimensional (2D) computation for the same Rayleigh number. When considered for an annular domain divided into 20 equal parts, flow transition of natural convection occurs gradually on the order of 2D steady, 3D regular steady, 3D regular oscillatory, and 3D irregular oscillatory flows, as the Rayleigh number increases. Most of the 3D regular oscillatory flows exhibit half-period symmetry in time and space. Both steady and oscillatory disturbance components coexist during the growing process from an initial state. In a fully developed stage of the 3D steady flow, only the steady disturbance component survives, whereas in that of the 3D regular half-symmetric oscillatory flow, only the oscillatory disturbance component survives.

NUMERICAL SIMULATION ON NATURAL CONVECTION AND HEAT TRANSFER OF SUPERCRITICAL WATER IN A SIDEWALL HEATED CYLINDER

In some reactors related to supercritical water technology, such as supercritical water gasification, natural convection is a factor to be considered because of the high thermal compressibility of supercritical water. In this paper, a numerical method is applied to analyze the natural convection and heat transfer characteristics of supercritical water in a sidewall heated cylinder. In order to match the actual situation, an inlet and an outlet are added to the bottom and the top of the cylinder, respectively. The quasi-DNS method is used for numerical simulation, and the thermodynamic properties of supercritical water are calculated by IAPWS-IF97 equations. The flow structure and temperature distribution inside the cylinder are analyzed, and it is found that the flow boundary layer state will affect wall temperature and heat transfer. A heat transfer correlation of supercritical water natural convection is proposed. The Nusselt number has a power relationship with the Rayleigh number with an exponent 0.248, and the average Nusselt number with turbulent boundary layer will be 21.8% higher than that with laminar boundary layer under the present working conditions. Finally, factors affecting wall heat transfer are discussed according to the findings.

Local thermal non-equilibrium (LTNE) effects on thermal-free convection in a nanofluid-saturated horizontal elliptical non-Darcian porous annulus

In this investigation, a comprehensive and accurate numerical analysis of the local thermal non-equilibrium effects on the natural convection characteristics in a horizontal elliptical porous annulus saturated with nanofluid has been carried out using finite volume technique. The internal surface is isothermally heated while the external one is fixed at uniform lower temperature. The nanofluid phase and the solid phase of the porous structure are out of local thermal equilibrium situation. The heat and the hydrodynamics equations in their dimensionless form under Darcy–Brinkman Forchheimer model were solved computationally by the finite volume technique using the standard SIMPLER algorithm. The results have been examined for various porous medium properties under the LTNE condition. The standard 0.05 significance level was used to identify the local thermal equilibrium within the system. According to the obtained results, the higher the Darcy number and the smaller the non-dimensional heat transfer coefficient for the solid/nanofluid interface, the modified thermal conductivity ratio and the media’s porosity, the higher are the LTNE effects. In addition, the spatial distribution contours of the LTNE sources within the porous annulus were schematized.