Study Of Turbulent Natural Convection Research Articles

Numerical Study of Turbulent Natural Convection in a Rectangular Enclosure with Local Heating and Cooling

Numerical Study of Turbulent Natural Convection in a Rectangular Enclosure with Local Heating and Cooling

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

Turbulent heat transfer in open cavities is relevant in several thermal engineering applications. However, the influence of thermal radiation has not been thoroughly studied yet. In this work, experimental and numerical results were obtained to analyze turbulent natural convection coupled with surface thermal radiation in a cubic open cavity. Two emissivities were considered (0.98 and 0.03). Experimental temperature profiles were obtained at six different depths and heights consisting of 14 thermocouples each. Five turbulence models were evaluated against experimental data. The radiative heat transfer model was solved with the discrete ordinate method. The validated model was used to analyze temperature fields and flow patterns in different open cavities configurations, which are presented and discussed. The effect of thermal radiation on experimental heat transfer coefficients was quantified. It was found that experimental heat transfer coefficients increase between 8.5% and 12.52% when the emissivity of the walls changes from 0.03 to 0.98. The temperature distribution in the vertical adiabatic walls changes significantly when emissivity increases from ε=0.03 to ε=0.98 and thus have a relevant influence on the flow patterns in the open cavity.

Numerical study of turbulent natural convection of nanofluids in differentially heated rectangular cavities

In this work we study numerically the turbulent natural convection of nanofluids (water + Al2O3/NTC/Cu) in rectangular cavities differentially heated. The objective is to compare the effect of the macro-structural aspect of the rectangular cavity and the effect of the types of nanofluids studied on the thermal exchange by turbulent natural convection in this type of geometry. Therefore, we have numerically treated the cases of these three nanofluids, for different particles volume fractions (0 ? ? ? 0.06) and for different form ratios of the rectangular cavity. The standard k-? turbulence model is used to take into account the effects of turbulence. The governing equations are discretized by the finite volume method using the power law scheme which offers a good stability characteristic in this type of flow. The results are presented in the form of streamlines and isothermal lines. The variation of the average Nusselt number is calculated as a function of the types of nanoparticles, of theirs particles volume fractions, for different form ratios of the cavity, and for different Rayleigh numbers. The results show that the average Nusselt number is greater as the form ratio is large and that the effect of the use of CNT in suspension in a water prevails for voluminal fractions and large Rayleigh numbers.

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.

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).

Turbulent natural convection combined with thermal surface radiation inside an inclined cavity having local heater

A numerical study of turbulent natural convection with thermal surface radiation inside an inclined square enclosure with a local heat source has been performed. The main attention is paid to the effect of the inclination angle on the fluid flow and heat transfer. Two-dimensional equations of conservation of mass, momentum and energy using the k–ε turbulence model have been solved by finite difference method. Localized heating has been simulated by a centrally located heat source on the bottom wall. The angle of inclination, changed from 0 to π, is used as a control parameter for heat transfer. A detailed numerical analysis has been conducted for a wide range of Rayleigh number of 108–1010 and surface emissivity 0≤ε˜≤0.9. The results show that a growth of the cavity inclination angle leads to a reduction of radiative Nusselt number. In general, it was found that the values of Rayleigh number, inclination angle and surface emissivity have significant effect on the temperature and stream function contours within the enclosure. Therefore, these parameters can be very good control parameters for fluid flow and heat transfer inside the cavity. The developed numerical method and obtained results can be widely used in different engineering problems, e.g. the simulation of air flow and heat transfer from heat-generating elements in power engineering. Moreover, the obtained results provide better technical support for development and research of electronic cooling systems.

Numerical study of turbulent natural convection in a cube having finite thickness heat-conducting walls

Three-dimensional transient natural convection in a cubic enclosure having finite thickness solid walls subject to opposing and horizontal temperature gradient has been investigated by a finite volume method. The turbulent flow considered into the volume is described mathematically by the 3D Reynolds averaged Navier–Stokes equations using the standard k–e model with wall functions, including the energy equation. The velocity and temperature distributions were calculated at fixed Prandtl number, Pr = 0.7 and different values of the Rayleigh number, thermal conductivity ratio and dimensionless time. Three-dimensional velocity and temperature fields, temperature profiles at middle cross-sections and average Nusselt numbers have been presented. It has been found that an insertion of the third coordinate for the conjugate problem leads to a decrease in the average Nusselt number by 5.8 % in conditions of a stationary heat transfer mode.

Experimental and numerical study of turbulent natural convection in an open cubic cavity

Study of natural convection in an open cubic cavity with side length of 1 m is presented. The experimental setup was built with the air as the heat transfer fluid. The vertical wall opposite to the aperture is subjected to uniform heat flux condition with the four different heat flux values in the range 55–333 W/m2, whereas the remaining walls were kept thermally insulated. The temperature at discrete locations inside the cavity was obtained which followed evaluation of heat transfer coefficient and Rayleigh number. The thermal and flow analysis in 3-D was based on the standard k–e turbulence model and implemented using CFD software Fluent 6.3. The spatial distribution for temperature, velocity and turbulent viscosity are determined and analyzed in the perspective of experimental observations. The experimentally determined ranges of Rayleigh number, Nusselt number and heat transfer coefficient are 1.66 × 1011–7.1 × 1011, 185.94–243.31 and 4.88–6.83 W/m2 K, respectively. The observed maximum difference between the experimental and numerical values for heat transfer coefficient and Nusselt number are 10.8 and 14 % respectively.

Numerical study of turbulent natural convection in a cube having finite thickness heat-conducting walls

Numerical study of turbulent natural convection in a cube having finite thickness heat-conducting walls

Study of Turbulent Natural Convection in Vertical Storage Tubes for Supercritical Thermal Energy Storage

Heat transfer to storage fluid is a critical subject for any thermal energy storage system. The poor thermal conductivity of the storage medium may lead to insufficient heat transfer and may impair the functionality of the system. Supercritical thermal energy storage systems benefit from turbulent natural convection that dominates the heat transfer mechanism and compensates for the low thermal conductivity of the storage fluids. A computational model is validated and adopted to study the buoyancy-driven flow in vertical storage tubes and the effect of the aspect ratio of the vertical storage tubes on the charge time is investigated.

Study of turbulent natural convection in a tall differentially heated cavity filled with either non-participating, participating grey and participating semigrey media

Turbulent natural convection in a tall differentially heated cavity of aspect ratio 5:1, filled with air under a Rayleigh number based on the height of 4.5·1010 is studied numerically. Three different situations have been analysed. In the first one, the cavity is filled with a transparent medium. In the second one, the cavity is filled with a semigrey participating mixture of air and water vapour. In the last one the cavity contains a grey participating gas. The turbulent flow is described by means of Large Eddy Simulation (LES) using symmetry-preserving discretizations. Simulations are compared with experimental data available in the literature and with Direct Numerical Simulations (DNS). Surface and gas radiation have been simulated using the Discrete Ordinates Method (DOM). The influence of radiation on fluid flow behaviour has been analysed.

Numerical Study Of Turbulent Natural Convection In An Enclosure With Localized Heating From Left Side

Numerical Study Of Turbulent Natural Convection In An Enclosure With Localized Heating From Left Side

Computational study of turbulent natural convection in a side-heated near-cubic enclosure at a high Rayleigh number

A computational study of turbulent natural convection in a side-heated near-cubic enclosure at a high Rayleigh number ( Ra=4.9×10 10) is performed, aimed at gaining a better insight into the flow pattern, particularly in the corner regions. Two types of thermal boundary conditions are applied at the horizontal walls: adiabatic and isothermal. Also, two kinds of lateral vertical walls are studied, corresponding to different experimental approximations of adiabatic conditions: the first by insulation and the second by imposing a stratified wall heating. The latter conditions ensure better flow two-dimensionality, with the temperature stratification on the vertical walls close to that expected in the parallel mid-plane. Computations are performed with both a two-dimensional (2D) and three-dimensional (3D) code using a low-Reynolds-number differential second-moment stress/flux closure and the related k– ε model (KEM) simplification. The numerical computations show that the second-moment closure (SMC) is better in capturing thermal three-dimensionality effects and strong streamline curvature in the corners. The KEM, however, still provides reasonable predictions of the first moments away from the corners.

A direct numerical simulation of natural convection between two infinite vertical differentially heated walls scaling laws and wall functions

A direct numerical simulation has been performed for the case of a natural convection flow between two differentially heated vertical walls for a range of Rayleigh numbers (5.4×10 5< Ra<5.0×10 6). The simulation data are compared with experimental data of Dafa’Alla and Betts [Experimental study of turbulent natural convection in a tall cavity, Exp. Heat Transfer 9 (1996) 165–194] and the agreement is found to be acceptable. Given the numerical data we consider the scaling behaviour of the mean temperature, the mean velocity profile and of the profiles of various turbulence statistics. Point of departure is the approach proposed by George and Capp [A theory for natural convection turbulent boundary layers next to heated vertical surfaces, Int. J. Heat Mass Transfer 22 (1979) 813–826] who have formulated scaling relationships valid, respectively, in the near-wall inner layer and in the outer layer in the centre region of the channel. Matching of the scaling relationships in the overlap between the inner and outer region leads to explicit expressions which can be used as wall functions in computational procedures. The DNS data confirm the results of George and Capp for the scaling of the mean temperature profile. For the mean velocity profile our DNS data support another scaling in terms of a defect law for the velocity gradient in the inner layer. The scaling of George and Capp is also found to apply to the Reynolds stress, the temperature variance and the temperature fluxes. However, the velocity variances again seem to follow a different scaling.

EXPERIMENTAL STUDY OF TURBULENT NATURAL CONVECTION IN A TALL AIR CAVITY

Abstract This article describes an experimental study of turbulent natural convection in an air cavity with an aspect ratio of 28.6. The study provides data for the velocities and temperatures at a Rayleigh number based on cavity width of 0.83 × 106, which is useful in validating computational and theoretical results. A laser Doppler anemometer was used for velocity measurements, while fine thermocouples were used for temperature measurements. Heal transfer rates and heat losses from the cell are evaluated. Further, the experimental data in the central portion of the tall cavity have been averaged to simulate an antisymmetric, Boussinesq, fully developed flow in an infinitely tall air cavity. Causes of asymmetry of the data, including radiation absorption, are also assessed.

Velocity and temperature measurements in a natural convection boundary layer along a vertical flat plate

A turbulent natural convection boundary layer along a vertical flat plate in air is investigated with hot-wire techniques. Profile measurements of mean velocity, mean temperature, and tubulent quantities, which are obtained well into the wall region, clarify some characteristics peculiar to the turbulent natural convection boundary layer and different in various respects from those in forced convection. The results presented include important knowledge concerning the concept of the viscous sublayer, the availability of the conventional analogy between heat and momentum transfer, Reynolds stress, and turbulent heat fluxes. The reliability of the measurements is verified through theoretical considerations. Also, the scaling of the turbulent natural convection boundary layer is discussed from the experimental standpoint in terms of mean velocity, mean temperature, and velocity and temperature fluctuation intensities. The information obtained in the present study will be very useful for the theoretical study of turbulent natural convection using turbulence models.