Turbulent Natural Convection Research Articles

Analysis of natural convection in a representative cavity of a room considering oscillatory boundary conditions: An experimental and numerical approach

Natural convection heat transfer in cavities commonly occurs in various engineering problems. Specifically, the problem of a differentially heated cavity with oscillatory boundary conditions arises in the design of energy-saving strategies or energy evaluations within the building sector. This work addressed numerically and experimentally the natural convection in a full-scale room subjected to diurnal heating and nocturnal cooling. The experiment was instrumented with temperature sensors and an air velocity sensor, inspired by works reported in the literature. The numerical model involved solving the equations governing natural convection, considering turbulent bi-dimensional and three-dimensional flow in a transient state. This research allowed the selection of an appropriate turbulence model and evaluated the performance of the two-dimensional and three-dimensional approaches in simulations. Results showed that the k-omega SST model performed best in predicting velocity, while the standard k-omega model performed best in predicting temperature for the two-dimensional simulation, and the RNG k-epsilon model performed best for the three-dimensional simulation. Additionally, the three-dimensional simulation more accurately reproduced the chaotic fluctuations observed in experimental velocity data, demonstrating up to 30 % better estimation during periods of high velocities. The study identified the reversal of convective cells in the cavity, with an approximate duration of 2 h and 40 min in a real case. This study contributes to improving the understanding of turbulent natural convection in a differentially heated cavity with oscillatory boundary conditions. These findings have significant implications for various environmental and engineering applications, including the design of thermally comfortable buildings.

Dynamics of turbulent natural convection in a cubic cavity with centrally placed partially heated inner obstacle

Natural convection in a cavity with a partially heated obstacle at the center at the Rayleigh number Ra=1.46×109 is investigated using large eddy simulation (LES). The standard and dynamic Smagorinsky models, as well as the wall-adapting local eddy-viscosity model, are used for the subgrid scales, and the flow statistics are compared with recent experiments. The LES results obtained with different meshes show overall good agreement with the experiments as concerns the flow and heat transfer. Simulation with a non-ideal wall at the adiabatic side of the obstacle is also performed to explain the residual discrepancies observed in the unheated channel. Additional simulations performed with periodic conditions in the spanwise direction are very different from the full three-dimensional (3D) simulations, which demonstrate the significance of 3D effects in the cavity. In particular, periodic simulations show Tollmien–Schlichting kind waves in the transitional region, while the 3D cavity shows an early cross-flow transition to turbulence.

Turbulent natural convection in an air–water system with evaporation across the free surface

In this paper we report on direct numerical simulations of turbulent convection in a cuboidal pool that contains liquid water in the lower part and air in the upper one. The bottom wall is uniformly heated and natural convection is established in both phases, accompanied by water evaporation across the free surface of the water. The descent of the free surface due to evaporation is computed via a newly developed tracking algorithm based on the ghost-fluid method. We present results for three different cases which correspond to different pool heights. In all of them, the natural convection in the water lies in the soft-turbulence regime. Whereas in the gas, it lies in the laminar, transitional and soft-turbulence regimes, respectively. Our analysis focuses on the characteristics of the convective patterns in the two phases and the statistics of the various flow quantities of interest. According to our simulations, the flow in the water is organized in a single-roll Large-Scale Circulation (LSC). In the gas, it is organized in single or dual-roll LSCs, depending on the aspect ratio of the pool. Interestingly, the impingement points of the LSCs of the two phases at the free surface remain very close to one another, which is attributed to the continuity of the shear stresses across the free surface. Further, after the initial transient period, both the free-surface temperature and the evaporative mass flux are stabilized and remain almost constant, but they exhibit small-scale fluctuations in time due to turbulence. Also, the transport of water vapor in air has similar properties as the heat transport, and the ratio between the Sherwood and Nusselt numbers is very close to the Lewis number for air.

Моделирование турбулентной естественной конвекции на основе 2-жидкостного подхода

The paper provides mathematical modeling of turbulent natural air convection at a heated vertical plate based on a fairly recently developed two-liquid turbulence model. The considered problem, despite its relative simplicity, contains all the main elements characteristic of the currents near the wall due to buoyancy forces. A significant disadvantage of the RANS turbulence models used to solve such problems is that for their numerical implementation it is necessary to set the laminar-to-turbulent transition point, which must be determined experimentally. Thus, all RANS models are unable to describe a laminar-to-turbulent transition zone. Therefore, the main purpose of the work is to test the ability of the two-liquid turbulence model to describe the transition zone. Well-known publications have shown that the two-liquid model has high accuracy and stability, and is also able to adequately describe anisotropic turbulence. The turbulence model used in this work is supplemented with an additional thermal force, which can be ignored in many flows with forced convection. However, in the natural convection currents, it is this force that contributes to the transition of the flow regime. To validate the model, as well as to verify the computational procedure, the numerical results obtained are compared with the results of the well-known RANS turbulence models (the one-parameter Spalart-Allmaras (SA) model and the Reynolds stress transfer (RSM) model), as well as with the available experimental data. It is shown that the two-liquid model adequately reproduces the laminar-to-turbulent transition zone, and the numerical results obtained are in good agreement with experimental data.

Turbulent natural convection inside inclined enclosures: Effects of adiabatic and heat-conducting inserts

Natural convection phenomena are found in many engineering applications, ranging from building and refrigeration design to nuclear reactors. Such importance has prompted both experimental and numerical studies aiming at enhancing heat transfer characteristics. Turbulent natural convection in closed cavities, apparently simple, is a stepstone to such problems, which require mathematical and numerical tools capable of providing accurate results. This work is inserted within this framework, in which influences of insert size and type (adiabatic/heat-conducting), and cavity inclination angle are assessed for Rayleigh numbers 10 7 ≤ Ra ≤ 10 9 . The study shows that the insert is mostly detrimental to heat transfer owing to restriction to recirculation, irrespective of the cavity inclination angle. An operation envelope for the global Nusselt number for all cavity inclination angles, insert sizes and types, and Rayleigh numbers was also determined.

CFD Modeling and Analysis of a Heat-Generating Melt Pool Under External Vessel Cooling and Top Flooding

In some nuclear reactors, under accidental conditions, core debris forms a molten pool, which is later located in a core catcher. The core catcher proposed by the authors uses special refractory material to absorb enthalpy of corium so that temperatures are within 1500 K, which is possible to cool with side cooling and top flooding. Since performing a full-scale prototypic experiment is extremely challenging and complex because of the involvement of very high temperatures and the presence of radioactive materials, it is important to develop a Computational Fluid Dynamics (CFD) model capable of simulating coolability of the melt pool with the above cooling strategy. In the present work, a CFD model was developed for the above purpose and was benchmarked with experiments conducted under simulated conditions by the authors. The experiment involved the melting of about 25 L of sodium borosilicate glass at about 1473 K and cooling it in a scaled-down core catcher model. In the presence of decay heat inside the melt pool, turbulent natural convection plays an important role in the temperature distribution inside the melt pool and on the vessel walls. For this, we used different turbulence models. Comparisons among the Standard k-ε, Shear Stress Transport (SST) k-ω, and two-dimensional (2D) Large Eddy Simulation (LES) turbulence models show that SST k-ω and 2D LES turbulences are found to be in good agreement with the experimental results for the temperature distribution in the melt pool, and SST k-ω is found to be computationally less expensive than 2D LES. In general, the CFD model is capable of simulating heat transfer with phase changes inside the heat-generating melt pool. In view of this, the model can be further extended to include cooling of the melt pool in the prototype core catcher. The evolution of crust formation has been investigated in detail using a CFD model.

Effect of porous fin parameters on the turbulent natural convection heat transfer in the side-heated cavity containing the internal heat source

In order to study the effect of porous fin parameters on the natural convection heat transfer in the side-heated cavity containing the internal heat source, the influence of the structural parameters of porous fins (length of porous fins l, installation position of porous fins s, Installation angle of porous fins θ) and the physical property parameters of porous fins (effective thermal conductivity Ke, Darcy number Da) on the natural convection heat transfer in the cavity was numerically analyzed by arranging a porous fin on the hot wall. The effect of the coupling effect of the structure parameters of porous fins was analyzed by the response surface method, and it was found that the coupling effect of the installation position and installation angle of porous fins had the most significant effect on the convective heat transfer in the cavity, the combination of the structural parameters of porous fins with the optimal convective heat transfer effect is l = 0.053 H, s = 0.373 H, and θ = 93.21°, which increases the average Nusselt number of the hot wall by 38.72% compared with no-fin structure; The effect of Darcy number and effective thermal conductivity of porous fins on the average Nusselt number of the hot wall surface is analyzed. The results of the study can guide the study of solar radiation in buildings, the internal temperature control of automobiles, and the study of the thermal performance of electronic components.

Turbulent natural convection coupled with surface radiation in tall cavities heated by a linear heat flux and cold isothermally

Turbulent natural convection coupled with surface radiation in tall cavities heated by a linear heat flux and cold isothermally

A novel interpolation-MLP coupled reconstruction method for turbulent natural convection thermal environment reconstruction of MW-class offshore wind turbine nacelles

Localized overheating in the nacelle due to heat loss in high-power offshore wind turbines of 10 MW or more, where localized overheating affects the operation of other components, it is particularly important to visualize the thermal environment in the nacelle accurately and in real time in order to control the start-up and shutdown of cooling methods. However, the complexity of the high turbulence intensity (Rayleigh number (Ra) > 109) thermal environment in the nacelle of high-power units leads to significant errors in existing visualization techniques. Therefore, this paper presents a novel coupled interpolation-MLP reconstruction technique based on a data-driven deep neural network. Based on the interpolation method as primary reconstruction, a secondary reconstruction neural network model is obtained by training the thermal environment parameters (static pressure P, tangential velocity U, normal velocity V and temperature T) of specific computing nodes in the nacelle. The model is used to predict the values of thermal environment parameters at each time point for computing nodes with large gradients. An improved PSO algorithm is used to arrange the positions of a defined number of sensors to save sensor resources during the measurement process and to and ensure computing stability. The above improvements to the traditional interpolation reconstruction algorithms have led to an increase in the accuracy of the thermal environment reconstruction.

Experimental Study and Analysis of Turbulent Natural Convection at Isothermal Vertical Plate

Experimental Study and Analysis of Turbulent Natural Convection at Isothermal Vertical Plate

Scaling high Rayleigh number natural convection boundary layer statistics: A vertical water tunnel experiment

A high Rayleigh number natural convection boundary layer adjacent to the vertical heated wall was investigated at a large-scale facility. The global Rayleigh number (Rax) measured by the temperature difference between the wall and ambient water and the distance from the bottom of the heated wall reached 1013. Experimental results confirm that the global Nusselt number Nux is scaled by power 1/3 of Rax, which is similar to the well-known asymptote of the previously achieved Rax. The velocity field obtained using particle image velocimetry (PIV) indicated that the buoyancy-dominant outer-layer scaling suggested by Wells and Worster [A geophysical-scale model of vertical natural convection boundary layers, J. Fluid Mech. 609, 111–137 (2008)] was not only applicable to scale the lower-order velocity statistics but was also valid as a reasonable measure of the spatial correlation, probability distribution, and quadrant contribution features in the outer layer. The dynamical behavior of fluid motion captured by PIV supported a robust momentum transfer to positive wall-normal direction, which was sustained by the Q1 and Q3 quadrants. In addition, merging the existing literature and current data suggested that near-wall function can be applied at a moderate Rax; the universality of this wall-function model was confirmed around z×∼0.7 (where z× represents the near-wall scaled wall-normal distance, Kiš and Herwig [The near wall physics and wall functions for turbulent natural convection, Int. J. Heat Mass Transfer 55, 2625–2635 (2012)]). At a larger buoyancy regime, it was expected to follow a canonical boundary layer flow.

Improvement of the photovoltaic panel cooling by natural air ventilation

AbstractThe operating temperature increase in solar photovoltaic modules is a major problem, which affects their performance negatively. This numerical study introduces the effect of natural convection air‐cooling on the performance a novel hybrid photovoltaic/thermal collector. The standard k‐ε turbulence model is implemented for describing the turbulent natural convection. For improving the heat transfer rate of a novel photovoltaic/thermal collector system (PV/T), a sloped entrance channel (SEC) and an aluminum flat plate (AFP) were installed. The effects of SEC geometry parameters, the number of AFP and their distance to the photovoltaic module (PV) on the average Nusselt number are investigated. In addition, the effect of the PV/T system tilt angle and Rayleigh number on the thermo‐hydrodynamic flow behavior has been discussed. The results indicated that the heat transfer rate of the new hybrid PV/T system was improved by up to 20%, compared to the reference case without an aluminum flat plate. In addition, increasing the tilt angle of the PV/T system boosts airflow velocity, particularly for high Rayleigh number values (with and without AFP). Furthermore, several correlations, as functions of Rayleigh numbers and inlet channel dimensions, have been developed to predict the average Nusselt number.

Numerical simulation of the particle wall mass transfer rates on rough surfaces confining turbulent natural convection flows

We performed numerical simulations of the particle mass transfer rates on the horizontal and vertical thermally active rough walls confining a turbulent natural convection flow. We considered the air flow conditions in a cubical cavity (L=1.22m, Ra=5.4·108, Pr=0.71) with two opposed pairs of consecutive walls at different temperatures, for which measurements of the particle deposition velocities are reported in the literature. The simulations were carried out by solving the momentum, thermal energy and particle mass transfer equations using a series of two-dimensional computational cell units to determine spatial evolution and development of the particle concentration boundary layer in the vicinity of hot and cold rough walls. Different flow conditions, wall orientations, sizes of roughness elements, particle diameters and temperature increments between the wall and the bulk fluid, typically found in indoor environments, have been considered and consequently, results can be used to estimate the particle deposition rates in real scenarios. At large mass transfer particle Péclet numbers, corresponding to spherical particles with diameters of 0.5–1 μm, the wall mass transfer rates of particles on rough textures typically increase by a factor of about 3 in comparison with the deposition on smooth surfaces. The thermophoretic effect significantly decreases the particle deposition on hot surfaces and increases it on cold surfaces, especially at large particle Péclet numbers. The numerical predictions of the deposition velocities are in good agreement with the measurements reported in the literature for the same flow conditions considered.

Simulation of Turbulent Natural Convection in Photovoltaic Solar Panels Based on the Spalart–Allmares (SA) Turbulence Model

Simulation of Turbulent Natural Convection in Photovoltaic Solar Panels Based on the Spalart–Allmares (SA) Turbulence Model

Modeling and simulation of cryogenic propellant tank pressurization in normal gravity

The self-pressurization process of cryogenic propellant storage under a normal gravity condition often involves turbulent mixing in the liquid region and complex flow fields in the vapor region caused by heat leak into a cryogenic tank. Over time, the cryogenic tank pressurizes due to a temperature increase in the vapor phase and the heat and mass transfer across the liquid–vapor interface. A nodal simulation approach was employed to study the long term transients of tank self-pressurization. In this paper, mechanistic models and correlations were implemented to simulate the self-pressurization of Multipurpose Hydrogen Test Bed experiments with a commercial nodal code, namely, Systems Improved Numerical Differencing Analyzer with Fluid Integrator (SINDA/FLUINT). A lump number sensitivity study on the prediction of tank pressure and temperature was performed to determine the number of lumps appropriate for a 50% initial liquid fill level. Uniform heat flux boundary conditions were implemented on both phases during the tank self-pressurization with an estimated heat leak provided by NASA. Numerical simulation showed that the predicted pressure profiles of the 50% and 90% fill level cases agreed well with experimental data. In the liquid region, temperatures were primarily uniform and close to saturation due to turbulent natural convection mixing. Findings showed the current model overestimated the temperature profiles in the vapor region when assuming heat conduction transfer.

LES of natural convection along a side-heated vertical wall in a water cavity applied for the scale of Small Modular Reactor ([formula omitted

The passive safety concept of Small Modular Reactors (SMR) is based on the transfer of residual heat from the reactor to a water pool. Since the height of the reactor vessel reaches approximately 15 m, this leads to a strong heat exchange between the wall and the water by the way of natural convection. The maximum Rayleigh number (Ra) reaches 1014 or 1015. Reliable heat transfer correlations exist only up to Ra≈1012, still along with uncertainties in the extrapolation to higher Rayleigh number values. To improve the understanding of natural convection at high Rayleigh number values, the turbulent natural convection boundary layer (TNCBL) along a 15 m high, side-heated vertical wall in a water tank is simulated by employing the Large Eddy Simulation method with the CEA in-house code Triocfd (Angeli et al., 2015). The numerical results in terms of heat transfer correlation, mean temperature and mean velocity, second moment variables show a reasonable agreement with available experimental data in the range of Ra < 1012. It is observed in the simulation that the powers of the heat transfer correlations in the laminar (Nuy=0.56Gry∙Pr0.25) and the turbulent regimes (Nuy=0.114Gry∙Pr1/3) correspond well to the classical regimes at Ra<2×1012. A slight deviation from the classical exponent (1/3) is found at higher Rayleigh number (2×1012 <Ra<1.8×1015), namely Nuy=0.114(Gry∙Pr)0.33 . Through the analysis of turbulent kinetic energy budget, it is found that turbulent contribution from shear stress increases continuously with the increase of Rayleigh number. However, the turbulence contribution from buoyancy keeps increasing until Ra≈6.6×1013, then decreases gradually at higher Rayleigh number. The behavior of the maximum buoyant force shifts slowly from the inner boundary layer to the outer boundary layer may indicate the potential transition of the buoyancy-dominated regime to the shear stress dominated regime, thus confirming previous theoretical prediction (Wells Andrew and Worster, 2008). The increasing scale of the streak-like turbulent structures with the increase of Rayleigh number are clearly demonstrated through visualization process.

Coupled heat and mass transfer in shallow caves: Interactions between turbulent convection, gas radiative transfer and moisture transport

Understanding and predicting the microclimate of shallow caves is a key issue for the conservation of parietal art. In order to determine the dominant mechanisms of heat transfer in a configuration close to that of painted caves, we performed numerical simulations of a parallelepiped cavity whose dimensions and depth are of the order of 10m. This simple geometry allowed us to use a detailed model including turbulent natural convection, gas radiation, along with vapor transport and latent heat fluxes resulting from condensation and evaporation on the walls.Gas radiation increases the flow circulation in the cavity through wall–gas radiative exchanges and significantly modifies the wall radiative flux. Conversely, the wall conductive flux remains unchanged (a non trivial behavior reported in the literature about the differentially heated cavity). In the energy balance at the walls, the radiative flux overcomes the conductive flux. However, the addition of conduction and latent heat fluxes, both driven by convection, prevails over radiation in some regions of the cavity.Heat and mass fluxes are maximum in areas of the cavity roof where the distance from the ground is the shortest. Due to the asymmetry induced by the inversion of the vertical temperature gradient twice a year, net condensation resulting in limestone dissolution is expected in these areas, whereas the other regions of the cave undergo net evaporation resulting in limestone deposition. The orders of magnitude of the condensation flux (a few microns per day) and of the retreat velocity of the wall (a few tenth of a micron per year) are in line with the field data available in the literature.

Large-scale motions in a turbulent natural convection boundary layer immersed in a stably stratified environment

This study investigates the coherence of turbulent fluctuations in a turbulent vertical natural convection boundary layer immersed in a stably stratified medium (turbulent buoyancy layer). A turbulent buoyancy layer of a fluid having a Prandtl number of $0.71$ at a Reynolds number of $800$ is numerically simulated using direct numerical simulation. The two-point correlations reveal that the streamwise velocity fluctuations are coherent over large streamwise distances, with the length scale of the streamwise coherence being greater than the boundary layer thickness. This is due to large-scale motions (LSMs), similar to the LSMs observed in canonical wall-bounded turbulence despite the stark differences in flow dynamics. Both high-speed (positive) and low-speed (negative) streamwise velocity fluctuations form LSMs, with their streamwise length scales increasing with increasing wall-normal distance. High-speed LSMs are composed of upwash flow with high temperatures, while low-speed LSMs are composed of downwash flow with low temperatures. Both high-speed and low-speed LSMs meander appreciably in the streamwise direction, with the degree of meandering being correlated with the sign of the spanwise velocity fluctuations. The LSMs exhibit coherence across significant wall-normal distances and contribute significantly to the turbulence production in the outer layer. Examining the one-dimensional energy spectra of the turbulent buoyancy layer shows that the LSMs are the dominant energy-containing motions, implying that the length scale of the energy-containing range is of the order of boundary layer thickness. Notably, wall-normal velocity, spanwise velocity and buoyancy fluctuations do not form LSMs with streamwise length scales comparable to streamwise velocity fluctuations.

Uncertainty quantification of a spent fuel refrigeration pool utilizing a [formula omitted] turbulence reduced order model

In this article, the uncertainty of a three-dimensional turbulent natural convection transient is quantified in a geometry representing an idealized spent fuel pool. The assessment is carried out through the creation of a surrogate fast model. This is build utilizing Proper Orthogonal Decomposition and Galerkin projection applied to a K-ϵ turbulent Navier–Stokes formulation. We discuss the uncertainties created by a uncertain heat release of the spent fuel elements. We consider the hypothesis of deviations that follow the normal distribution around a certain nominal value, with different standard deviations and sample size. The expected results and its uncertainty are henceforth computed by Monte–Carlo method, calculating hundreds of solutions of the Initial Value Problem with the surrogate model.

Computational Analysis of Turbulent Natural Convection in Water Filled Tall Enclosure

In this study, the turbulent buoyancy driven fluid flow and heat transfer in a differentially heated rectangular enclosure filled with water is quantified numerically. The two dimensional governing differential equations are discretized using the finite volume method. SIMPLE algorithm is employed to obtain stabilized solution for high Rayleigh numbers by a computational code written in FORTRAN language. A parametric study is undertaken and the effect of Rayleigh numbers (1010 to 1014), the aspect ratio (30, 40 and 50), and the tilt angle (10o to 170o ) on fluid flow and heat transfer are investigated. The results of the adopted model in the present work is compared with previously published results and a qualitative agreement and a good validation is obtained. Results show that the fluid circulation and temperature fields are strongly affected by the enclosure tilt angle and Rayleigh Number.