Begell House Inc.
Computational Thermal Sciences: An International Journal
CTS
1940-2503
4
4
2012
NUMERICAL STUDY OF HEAT AND FLUID FLOW PAST A CUBICAL PARTICLE AT SUBCRITICAL REYNOLDS NUMBERS
283-296
Kay
Wittig
CIC Virtuhcon, Department for Energy Process Engineering and Chemical Engineering, Technische Universitat Bergakademie Freiberg, Fuchsmuhlenweg 9, 09596 Freiberg, Germany
Andreas
Richter
CIC Virtuhcon, Technische Universitat Bergakademie Freiberg, Fuchsmuhlenweg 9, 09599 Freiberg, Germany
Petr A.
Nikrityuk
CIC Virtuhcon, Department for Energy Process Engineering and Chemical Engineering, Technische Universitat Bergakademie Freiberg, Fuchsmuhlenweg 9, 09596 Freiberg, Germany
This work is devoted to a numerical investigation into heat and fluid flow past a particle of a cubical shape. Additionally, it is a continuation of the recent publication by Richter and Nikrityuk (Int. J. Heat Mass Transfer, vol. 55, no. 4, pp. 1343−1354, 2012a), where the authors performed numerical calculations of drag forces and heat transfer coefficients for nonspherical particles in laminar flows with an angle of attack of zero. In this work we focus on the influence of the angle of attack on flow past a cubical particle. Due to the asymmetric flow, three-dimensional calculations were carried out based on the immersed boundary (IB) method in continuous forcing mode; see the review by Mittal and Iaccarino (Ann. Rev. Fluid Mech., vol. 37, pp. 239−261, 2005). In order to substantiate the results, additional calculations were made based on a so-called conventional computational fluid dynamics (CFD) solver using body-fitted meshes. Based on the present analysis of numerical results obtained for a cubical particle, new correlations for both the drag coefficient (cD) and the Nusselt number (Nu) were developed. In addition to the Prandtl and Reynolds numbers, both correlations incorporate the angle of attack. A significant influence of the particle orientation on the characteristics cD and Nu was observed. The accuracy of the closures developed for cD and Nu is discussed, comparing the relations developed with published models.
THE USE OF TRANSPORT APPROXIMATION AND DIFFUSION-BASED MODELS IN RADIATIVE TRANSFER CALCULATIONS
297-315
Leonid A.
Dombrovsky
Joint Institute for High Temperatures, 17A Krasnokazarmennaya Str., Moscow,
111116, Russia; Tyumen State University, 6 Volodarsky Str., Tyumen, 625003, Russia
The paper presents a discussion of the use of both transport approximation for scattering phase function and diffusion-based models for radiative transfer in absorbing and anisotropically scattering media like many disperse systems in nature and engineering. The main attention is paid to heat transfer problems and traditional methods of identification of spectral radiative properties of dispersed materials when the details of angular distribution of the radiation intensity are not so important. The latter makes reasonable use of the above-mentioned approximations. In more complex applied problems, the diffusion approximation appears to be a good approach as the first step of a combined two-step solution. Some example problems solved recently by the author and his colleagues are used to illustrate the approach considered in the paper.
NUMERICAL STUDY OF UNSTEADY AIRFLOW PHENOMENA IN A VENTILATED ROOM
317-333
Kana
Horikiri
Faculty of Science, Engineering and Computing, Kingston University, London SW15 3DW, United Kingdom
Yufeng
Yao
Jun
Yao
School of Engineering, Isaac Newton Building,
University of Lincoln, Brayford Pool, Lincoln LN6 7TS, UK
Numerical simulation of airflow in an indoor environment has been carried out for forced, natural, and mixed convection modes, respectively, by using the computational fluid dynamics (CFD) approach of solving the Reynolds-averaged Navier−Stokes equations. Three empty model rooms in two-dimensional configuration were studied first; focusing on the effects of grid refinement, mesh topology, and turbulence model. It was found that structured mesh results were in better agreement with available experimental measurements for all three convection scenarios, while the renormalized group (RNG) к − ε turbulence model produced better results for both forced and mixed convections and the shear stress transport (SST) turbulence model for the natural convection prediction. Further studies of air velocity and temperature distributions in a three-dimensional cubic model room with and without an obstacle have shown reasonably good agreement with available test data at the measuring points. CFD results exhibited some unsteady flow phenomena that have not yet been observed or reported in previous experimental studies for the same problem. After analyzing the time history of velocity and temperature data using fast Fourier transformation (FFT), it was found that both air velocity and temperature field oscillated at low frequencies up to 0.4 Hz and the most significant velocity oscillations occurred at a vertical height of an ankle level (0.1 m) from the floor, where temperature oscillation was insignificant. The reasons for this flow unsteadiness were possibly a higher Grashof number, estimated at 0.5 × 106 based inflow conditions, and thus strong buoyancy driven effects caused the oscillations in the flow field. The appearance of an obstacle in the room induced flow separation at its sharp edges and this would further enhance the oscillations due to the unsteady nature of detached shear-layer flow.
NUMERICAL STUDY OF COUPLED MOLECULAR GAS RADIATION AND NATURAL CONVECTION IN A DIFFERENTIALLY HEATED CUBICAL CAVITY
335-350
Laurent
Soucasse
CNRS, UPR 288, Laboratoire EM2C, Chatenay-Malabry, France; Ecole Centrale Paris
Philippe
Riviere
Laboratoire EM2C, CNRS, CentraleSupelec, Universite Paris Saclay, 3 rue Joliot Curie, 91192 Gif-sur-Yvette Cedex, France
Shihe
Xin
Univ Lyon, CNRS, INSA-Lyon, Universite Claude Bernard Lyon 1, CETHIL UMR5008, F-69621,
Villeurbanne, France
Patrick
Le Quere
LESTE UA CNRS 1098 Universite de Poitiers,40 av du Recteur Pineau,86022 Poitiers Cedex- France
Anouar
Soufiani
Laboratoire EM2C, CNRS, CentraleSupelec, Universite Paris Saclay, 3 rue Joliot Curie, 91192 Gif-sur-Yvette Cedex, France
The coupling between natural convection and gas and wall radiation is studied numerically in a differentially heated cubical cavity filled with an air/CO2/H2O mixture. In order to solve coupled flow, heat transfer, and radiation equations, we develop a 3D radiative transfer model based on the deterministic ray tracing method, coupled with a pseudo-spectral Chebyshev method for natural convection under Boussinesq approximation. An absorption distribution function (ADF) model is used to describe gas radiative properties. Coupled simulations are performed at Ra = 105, 106, and 3 × 107, considering wall and/or gas radiation. Steady solutions were obtained except at the highest Rayleigh number in the case of radiating walls. Results show a strong influence of radiative transfer on temperature and velocity fields. The global homogenization of the temperature field induced by radiation leads to a decrease of the thermal stratification parameter. Two different mechanisms leading to this behavior, involving either wall/wall or gas radiative exchanges, are identified. In addition, we observe a thickening of the vertical boundary layers and an increase of the global circulation in the cavity. The influence of the Rayleigh number and 3D effects are also discussed.
ADVECTION AND DIFFUSION SIMULATIONS USING LAGRANGIAN BLOCKS
351-363
Vincent H.
Chu
Department of Civil Engineering and Applied Mechanics, McGill University, 817 Sherbrooke St. West, Montreal, Quebec H3A0C3, Canada
Wihel
Altai
Department of Civil Engineering and Applied Mechanics, McGill University, 817 Sherbrooke St. West, Montreal, Quebec H3A0C3, Canada
The Lagrangian block advection and diffusion is developed as an alternative numerical procedure to the solution of the transport equation. The blocks are the computational elements, which are defined by the zero, first, and second moments of the blocks. The centers of mass of the blocks move with the advection velocity. The second moment of the blocks increases at a rate proportional to the diffusivity. The accuracy of the simulations by the Lagrangian block method is assessed by comparing the block simulation with an exact solution of the advection-and-diffusion equation. Unlike most numerical methods, the error associated with the Lagrangian block method is small and is not cumulative even when a very coarse block size is employed for the computation. False numerical diffusion error is totally avoidable when Lagrangian blocks are used to do the computation.
COMBINED TWO-FLUX APPROXIMATION AND MONTE CARLO MODEL FOR IDENTIFICATION OF RADIATIVE PROPERTIES OF HIGHLY SCATTERING DISPERSED MATERIALS
365-378
Leonid A.
Dombrovsky
Joint Institute for High Temperatures, 17A Krasnokazarmennaya Str., Moscow,
111116, Russia; Tyumen State University, 6 Volodarsky Str., Tyumen, 625003, Russia
Krithiga
Ganesan
Department of Mechanical Engineering, University of Minnesota, Minneapolis, USA
Wojciech
Lipinski
Research School of Engineering, The Australian National University, Canberra ACT 2601, Australia
An identification procedure is developed for obtaining spectral radiative properties of highly scattering dispersed materials such as porous ceramics. Traditional techniques based on measurements of the directional-hemispherical reflectance and transmittance are of limited use because of difficulties in fabricating sufficiently thin and mechanically stable samples to obtain reliable values of directional-hemispherical transmittance. However, one can use the directional-hemispherical reflectance measurements for optically thick samples to obtain the transport scattering albedo. A one-dimensional analytical solution employs the modified two-flux approximation for the identification of transport scattering albedo. An additional transmittance measurement is required to identify the transport extinction coefficient. Binormal narrow cone transmittance is measured for this purpose. Because the one-dimensional analytical solution is not applicable to model the binormal narrow cone transmittance, the Monte Carlo ray-tracing technique is used to identify the transport extinction coefficient. The identification procedure is applied to obtain near-infrared radiative properties of porous ceria ceramics used in solar thermochemical reactors. The identified transport scattering coefficient is shown to be in good agreement with theoretical estimates based on the Mie theory for polydisperse pores and grains. This verifies the applicability of a model based on independent scattering and Mie theory for theoretical predictions of radiative properties of two types of ceria ceramics with porosity of 0.08 and 0.72, and for extrapolating the properties of both ceramics in a limited near-infrared range to the range of significant absorption.