Large-Eddy Simulation of local heat transfer in plate and pin fin heat exchangers confined in a pipe flow

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Large-Eddy Simulation of heat transfer in plate and pin fin heat exchangers is carried out under constant surface temperature assumption while resolving the surface heat flux by a dense mesh of 28 M cells. The heat exchangers are mounted inside a pipe in a highly constrained space. The heat transfer takes place by forced convection at ReD=20000. The numerical simulations are validated by comparison of velocity profiles against experimental flow measurements on 3D-printed aluminium heat exchangers. Both the experiments and simulations indicate higher rate of heat transfer on the outer fins. The numerical findings indicate that, for the plate heat exchanger, the average Nusselt number of the outermost fins is approximately 50% larger than at the inner fins (Nu=7.54). For the pin fin heat exchanger, we observe a 20% increase in the Nusselt number between the inner and the outer fins. A strongly localized vortex shedding region is observed for the pin fin heat exchanger increasing the local heat transfer by a factor of two. In contrast, for the planar fins, the flow appears steady and laminar in the interior. In addition to strong flow diversion observed for both geometries, we show that for the pin fin array, almost 50% of the flow that enters the heat exchanger leaks to the clearances through the side and tip gaps. For the planar fins, the respective leakage is only 30%. Numerical evidence is also provided on the relevance of variety of flow phenomena including recirculation zones, vortex shedding, flow laminarization as well as flow separation and reattachment, all affecting the local heat transfer.