LES of a Round Impinging Jet: Investigation of the Link Between Nusselt Secondary Peak and Near-Wall Vortical Structures

Abstract

In an attempt to improve our understanding of the fundamental flow problem that is an impinging jet, a wall-resolved Large Eddy Simulation (LES) is produced to investigate large-scale unsteady flow features, mixing processes near the wall and heat transfer. The simulation focuses on a single unconfined round jet normally impinging on a flat plate at a Reynolds number (based on the pipe diameter and bulk velocity) of Re = 23 000 and for a nozzle to plate distance of H = 2D. This configuration is known to lead to a double peak in the Nusselt distribution. Evaluation of the high order statistics, such as Skewness and Kurtosis of the temporal evolution of axial velocity and wall heat flux, provides first-ever insights into the effect of the vortical structures on the mean wall heat transfer. Heat transfer statistics such as probability density functions (PDF) confirm the ability of LES to reproduce the strong intermittent thermal events responsible for the increase of the mean wall heat transfer radial distribution. Axial velocity and temperature temporal distributions are analysed simultaneously to gain further insight into the mixing process near the wall. In particular, the probabilities of the different cold/hot fluid ejection/injection events prove that the strong intermittent thermal events are linked to a change in the mixing behavior induced by the passage of the large-scale vortical structures. These structures are found to preferentially produce a cold fluid flux towards the wall leading to the local heat transfer enhancement usually identified by the secondary peak.