Unravelling convective heat transfer in the Rotated Arc Mixer

Thermal homogenization is essentially a transient problem and convective heat transfer by (chaotic) advection is known to accelerate this process. Convective heat transfer traditionally is examined in terms of heat-transfer coefficients at domain walls and characterised by Nusselt relations. However, though of proven worth, such Nusselt relations offer limited insight into the underlying thermal transport phenomena. This study seeks to address this by considering convective heat transfer from an alternative perspective. To this end, the temperature field is decomposed into a conductive and convective contribution, where the latter incorporates the impact of fluid motion. This representation enables explicit isolation and visualisation of the energy redistribution and energy fluxes induced by the flow and thus facilitates a more direct description and analysis of convective heat transfer compared to conventional methods. The alternative method is demonstrated for the Rotated Arc Mixer, a prototypical industrial mixer/heat-exchanger. This exposes the internal transport phenomena by which the flow enhances heat transfer. Moreover, this reveals that, though having a net beneficial effect on thermal transport, the flow typically also causes episodes of reduced instead of enhanced heat transfer.