Abstract
The present work investigates the modeling of turbulent heat transfer in flows where radiative and convective heat transfer are coupled. In high temperature radiatively participating flows, radiation is the most relevant heat transfer mechanism and, due to its non-locality, it causes counter intuitive interactions with the turbulent temperature field. These so-called Turbulence-Radiation Interactions (TRI) largely affect the temperature field, modifying substantially the turbulent heat transfer. Therefore, in the context of modeling (RANS/LES), these interactions require a closure model. This work provides the inclusion of TRI in the modeling of the turbulent heat transfer by adopting a unique approach which consists in approximating the fluctuations of the radiative field with temperature fluctuations only. Based on this approximation, coefficients of proportionality are employed in order to close the unknown terms in the relevant model equations. A closed form of all radiation-temperature-velocity correlation is explicitly derived depending on the chosen turbulent heat transfer model. This model is applied to a standard two-equation turbulent heat transfer closure and used to reproduce results obtained with high-fidelity DNS simulations. While a standard approach (i.e., neglecting TRI) is not able to correctly predict the DNS data, the new model’s results shows exceptional agreement with the high-fidelity data. This clearly proves the validity (and the necessity) of the proposed model in non-reactive, radiative turbulent flows.
Highlights
Many engineering applications work with high temperature fluids which are able to absorb and emit thermal radiation, such as H2O, CO2 or CH4
This model is applied to a standard twoequation turbulent heat transfer closure and used to reproduce results obtained with high-fidelity direct numerical simulation (DNS) simulations
The dashed lines show the results obtained with a simple constant turbulent Prandtl number, while the solid lines are the calculations using the two equation turbulent heat flux model
Summary
Many engineering applications work with high temperature fluids which are able to absorb and emit thermal radiation, such as H2O, CO2 or CH4. Our previous work (Silvestri et al, 2018) showed that TRI has a non linear dependency on optical thickness τ, which is caused by the contrasting roles of radiative emission and absorption Building on these results, we were recently able to parameterize TRI for a wide range of optical thicknesses in gray and non-gray partici pating turbulent flows (Silvestri et al, 2019). To previous works, we highlighted the de pendency of radiative dissipation on the ratio κg/ωc, where κg is a TRIequivalent absorption coefficient and ωc is a characteristic wave number which accounts for anisotropic turbulent structures While all this knowledge has been gathered regarding the TRI mechanism and the coupling of convective and radiative heat transfer, standard models have not yet been adapted to these recent findings. This “TRI” closure model is applied to a standard two equation model and tested against various DNS cases to demonstrate both its necessity and validity
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