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12th Spring School on Transition and Turbulence
EVALUATION OF THE ACCURACY OF THERMAL RADIATION MODELS IN THE ABSENCE AND PRESENCE OF TURBULENT FLUCTUATIONS
Bruno de Luca Zimmer Gomes , RS
Co-Authors: Bruno de Luca Zimmer Gomes, Guilherme Fraga, Francis França
Presenter: Bruno de Luca Zimmer Gomes
Radiation is often the main heat transfer mechanism for problems that involve high temperatures such as reactive flows, where the presence of species such as H2O and CO2, produced in the combustion process, further enhance the radiative transfer problem, for they participate in the radiation exchange. The treatment of the spectral variations of the radiative properties of these species is a particular challenge when modeling radiative heat transfer, due to the highly complex dependence of their absorption spectrum on the wavenumber. In this framework, the present paper analyzes the accuracy of spectral models in the presence or absence of turbulent fluctuations. Fluctuations of the medium properties are known to affect the radiation field, in a phenomenon that is named turbulence-radiation interaction (TRI), which originates from the highly non-linear coupling between fluctuations of the radiation intensity and fluctuations of temperature ()) and medium composition (-8 ). In particular for reacting flows, neglecting these fluctuations when solving the radiation field can lead to large errors, with time-averaged radiative quantities (e.g., the radiation intensity or radiative heat flux) differing significantly from their values evaluated from the mean ) and -8 fields. In this paper, TRI is accounted for through the optically thin fluctuation approximation alongside an empirical correction for the mean radiative emission. The accuracy of different formulations of the weighted-sum-of-gray-gases model (WSGGM), a widely-used global spectral model for gas radiation, is then assessed through comparisons with a reference solution given by line-by-line (LBL) integration of the radiative transfer equation. The test cases consists of a line-of-sight calculations based on scalar fields obtained experimentally for the Sandia flame D, a turbulent non-premixed jet flame of methane and air. The mean total radiation intensity along the line-of-sight is computed considering and neglecting turbulent fluctuations of temperature for both the WSGGM and the LBL method. Results show that the fluctuations substantially increase for both the WSGGM and LBL solutions. The average error associated to the WSGGM is of approximately 5% for the formulations currently studies, when TRI is considered; when turbulent fluctuations are neglected, this error does not change significantly. This seems to indicate that TRI do not alters the accuracy of the WSGGM. However, this may not be the case for all WSGGM formulations and if the intensity of fluctuations are increased; these points will be addressed in the final version of the paper.
thermal radiation, temperature fluctuations, Spectral models, Sandia Flame D, Turbulence-radiation interaction