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Large Eddy Simulation of heat transfer in ribbed ducts

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VKI PHDT 2009-09, Márton Lohász, Budapest University of Technology and Economics, 2009, ISBN 978-2-87516-008-9

Large Eddy Simulation of  heat transfer in ribbed  ducts

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Large Eddy Simulation of  heat transfer in ribbed  ducts

Márton Lohász

VKI PHDT 2009-09, Budapest University  of Technology  and  Economics, 2009, ISBN 978-2-87516-008-9

Turbulent flow over ribbed surfaces is of great interest in the understanding of separated turbulent flows and in turbomachinery applications. Under this second point of view the goal is to optimize the internal ribbing, which enhances the internal cooling of the turbine blades. This need justifies the experimental investigation, which the Turbomachinery and Propulsion Department of the Von Karman Institute for Fluid Dynamics has been performing over the last decade.

The flow under consideration is strongly 3 dimensional (3D), so that numerical investigation can be considered necessary for a complete understanding of the overall phenomenology. However, direct numerical simulation, which would give a complete physical description of turbulent flow, is still unaffordable for this class of flows, characterized by high Reynolds number (104-105). Therefore, some part of the turbulent field has to be modelled, leading to the proposal to apply to this problem the Large Eddy Simulation (LES) approach. In this method a space filter is applied to separate the large (energy carrying structures) and the small (redistributing and dissipating structures). The former are explicitly computed by the numerical solution while the latter are modeled by an ad hoc model.

LES was applied to the simulation of the flow in a high blockage stationary duct with transversal ribs, using the commercial program Fluent 6. The results were validated against PIV measurements carried out in the past by Luca Casarsa. Effect of the SGS model was investigated and found not to have strong influence compared to the numerical diffusion introduced by a monotone upwinding discretization of the advection term. Simulation of unsteady periodic heat transfer was implemented in Fluent. The uniform heat flux heat transfer problem was investigated,which was already measured and published at VKI in Murat Çakan’s thesis. The comparison of the liquid crystal measurement to our simulation showed qualitative agreement. The same computation procedure was applied for the case of a 45° inclined rib configuration without validation. The LES results were already postprocessed in terms of averaged flow field topology and coherent structure motion. A conditional averaging procedure was also implemented for better analysis of the coherent structures and their impact on flow and heat transfer.

An emphasize will be put in the thesis on the comparison of the two geometry configuration considering the flow topology, the heat transfer characteristics and vortex dynamics.
In Figure 1 an example of this comparison is provided: the heat transfer enhancement factor is shown using contour plots and the bifurcation lines (separation and reattachment lines). In the majority of the duct wall the heat transfer enhancement (increase in heat transfer coefficient) is at the place where reattachment can be found, the separation lines on the other side are well correlated with the decrease in the heat transfer coefficient.

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