Abstract
In this paper, a wall-adapted anisotropic heat flux model for large eddy simulations of complex engineering applications is proposed. First, the accuracy and physical consistency of the novel heat flux model are testified for turbulent heated channel flows with different fluid properties by comparing with conventional isotropic models. Then, the performance of the model is evaluated in case of more complex heat and fluid flow situations that are in particular relevant for internal combustion engines and engine exhaust systems. For this purpose large eddy simulations of a strongly heated pipe flow, a turbulent inclined jet impinging on a heated solid surface and a backward-facing step flow with heated walls were carried out. It turned out that the proposed heat flux model has the following advantages over existing model formulations: (1) it accounts for variable fluid properties and anisotropic effects in the unresolved temperature scales, (2) no ad-hoc treatments or dynamic procedure are required to obtain the correct near-wall behavior, (3) the formulation is consistent with the second law of thermodynamics, and (4) the model has a similar prediction accuracy and computational effort than conventional isotropic models. In particular, it is shown that the proposed heat flux model is the only model under consideration that is able to predict the direction of subgrid-scale heat fluxes correctly, also under realistic heat and fluid flow conditions in complex engineering applications.
Highlights
Many energy systems, such as internal combustion engines or exhaust after treatment devices, are confined by solid walls
The main features of the novel model are that (1) it accounts for variable fluid properties and anisotropic effects in the unresolved temperature scales, (2) no ad-hoc treatments or dynamic procedure are required to obtain the correct near-wall behavior, and (3) the formulation is consistent with the second law of thermodynamics
In order to account for fluid flows with variable molecular Prandtl numbers, the same behavior is assumed for th√e characteristic subgridscale time scale in the present model formulation leading to c ∼ Pr ksgs
Summary
Many energy systems, such as internal combustion engines or exhaust after treatment devices, are confined by solid walls. As implied by the discussion above, a better representation of the subgrid-scale heat flux vector for complex heat and fluid flow situations can be obtained by accounting for the anisotropic behavior of small temperature scales and introducing a tensor subgrid-scale thermal diffusivity Some of these models were derived in analogy to the general gradient diffusion hypothesis (Daly and Harlow 1970) as it is often applied in the RANS context. From this brief literature review it appears that only a few advanced anisotropic heat flux models exist in the literature and they are almost never used in LES practice Most of these models are relatively complex, do not account for the effects of local fluid properties and do not provide the correct asymptotic near-wall behavior. Some concluding remarks are provided at the end (Sect. 5)
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