Computational Radiative Heat Transfer

RHT-001

COMPUTATIONAL TECHNIQUES FOR THE EVALUATION OF TURBULENCE-RADIATION INTERACTIONS IN COMBUSTION SYSTEMS

Michael F. Modest, The Pennsylvania State University

Radiation, chemical kinetics and turbulence individually are among the most challenging problems of computational science and engineering. In turbulent combustion, these phenomena are coupled in highly nonlinear ways. In much the same way as convection is aided by turbulence, so is radiation, which in the presence of chemical reactions may increase several fold due to turbulence interactions. Evaluation of such turbulence-radiation interactions (TRI) is extremely difficult and computationally demanding. Any such computational method requires knowledge of turbulent fluctuations, commonly expressed in terms of a probability density function (PDF) for the pertinent flow variables.

The PDF and resulting TRI may be determined using different levels of sophistication, which will be briefly discussed:

(i) Reynolds-averaged solutions (RAS) to the governing equations together with an assumed PDF,

(ii) RAS together with a transported PDF solver,

(iii) large eddy simulation (LES) with a transported FDF (filtered density function), and

(iv) direct numerical simulation (DNS), which requires no separate PDF solution.

Standard radiative transfer equation (RTE) solvers can only resolve the TRI partially, requiring the so-called OTFA (optically-thin fluctuation assumption). Full TRI can only be predicted using a photon Monte Carlo approach, which is essentially a transported PDF for photons. Using photon Monte Carlo in conjunction with the Lagrangian particle field of transported PDF/FDF schemes requires development of a fundamentally new Monte Carlo approach. Similarly, the high-order finite difference or spectral LES and DNS solvers require the use of similar order RTE solution schemes.

RHT-002

SOME SPECTRAL AND GEOMETRICAL ACCELERATION TECHNIQUES FOR THE COMPUTATION OF RADIATIVE TRANSFER IN GASEOUS MEDIA

Anouar Soufiani, École Centrale de Paris

Radiative intensity is a function of seven variables, namely, three space coordinates, two direction coordinates, radiation frequency or wave number, and time. The last dependence can be either due to intrinsic variations (short time phenomena for instance) or to extrinsic variations (time variation of other fields). Complete description or computation of this 7-dimensional problem generally requires enormous CPU times, especially for gases with very complex spectra, and many attempts to reduce this dimensionality have been worked on over more than one century. Recent developments have focused on Monte Carlo techniques for which the increase of computational times with space dimension is much smaller than for other deterministic techniques.

The aim here is to present some computational acceleration techniques for the spectral dependence of radiative intensity and for the treatment of complex geometries. Various gas radiative property models are briefly described, and the possibility of high-resolution spectral computations (line-by-line approach) in conjunction with Monte Carlo methods will be discussed. The associated additional cost in comparison with other approximate spectral models is shown to be very small if some specific techniques are used for wave number tracking. The use of Monte Carlo techniques in complex geometries encounters other specific problems related to the very frequent computation of ray intersections with boundaries. To overcome this geometrical problem, some acceleration techniques are described including multi-grid techniques, and 'octree' recursive techniques, which are commonly used in the field of virtual images. Applications to the propagation of an arc plasma and its extinction in an electrical circuit breaker will be presented.

RHT-003

MODELING OF RADIATION HEAT TRANSFER IN MULTIPHASE FLOWS TYPICAL FOR FUEL-COOLANT INTERACTION

Leonid A. Dombrovsky, Joint Institute for High Temperatures, Moscow

Thermal radiation of core melt droplets falling in water is an important heat transfer mode which strongly affects both thermal and hydrodynamic interaction of the high-temperature melt with water. A considerable part of thermal radiation emitted by the droplets lies in the range of water semi-transparency. As a result, the radiation is not completely absorbed in water and one needs to account for radiation heat transfer between the particles at different temperatures. The scattering of radiation by steam bubbles and melt droplets separated from ambient water by a thin steam layer is also important. The problem is further complicated by semi-transparency of small oxide droplets and temperature differences between the center and surface of the droplets. Nevertheless, the specific radiative properties of the multiphase flow components allow for a simplified approach, which can be implemented in problem-oriented CFD codes. A more sophisticated approach for visible radiation of multiphase media will also be presented. The latter is expected to be important for optical diagnostics of the flow in small-scale experiments including those using various stimulant melts.

RHT-004

COMPUTATIONAL ASPECTS OF NEAR-FIELD RADIATIVE TRANSFER

M. Pinar Mengüç, University of Kentucky

With the recent interest in shrinking dimensions and exploding research in nanosciences and nano-scale engineering, further understanding of radiative transfer at small scales became important. This new knowledge is particularly more crucial for better design and control of the nanostructures and processes. To this end, the physics of radiative exchange in nanoscale configurations needs to be explored in systematic way. The objective of this presentation is to provide an overview of different computational techniques for modeling 'near-field radiative transfer.'

Two different physical problems need to be considered here. The first one involves two objects of any size within close proximity to each other; the typical gap distances considered here are in the order of the characteristic wavelength of emission, as determined roughly from the Wien law. The second one involves nanometer-size objects of about 100 nm or less.

Both of these cases need to be tackled with the wave theories, rather than the classical theories of radiative transfer and the well known radiative transfer equation (RTE). For these cases the Maxwell equations need to be employed, and polarization, interference, tunneling, and scattering of waves need to be fully accounted for in the calculations. Several examples will be provided on these topics, which are currently explored, and future directions and practical applications of thermal radiation at nanoscales will be given.