Pulsed impinging jets: Momentum and heat-transfer

Abstract

With increasing efficiencies of internal combustion engines, the accurate prediction of the heat transfer across walls, such as from temporally varying impinging jets, is of significant importance. To avoid the excessive cost of fully resolved simulations, this prediction often relies upon the modelling of the resulting non-equilibrium boundary layer using wall functions. Given the unsteady nature of the problem, it is important to understand the development of these boundary layers under non-equilibrium conditions and assess whether existing wall functions are suitable for modelling such flows. Thus, in this paper, the momentum and thermal boundary layers developing from a single pulsed impinging jet are investigated through the use of highly-resolved large eddy simulations of a representative canonical set-up. The simulations are conducted for three jet Reynolds numbers of 20,000, 50,000 and 100,000, based on the peak velocity of the temporal pulse. The jets are simulated at temperature ratios of 0.25 and 0.8, where the ratio is defined as the temperature of the wall to the centreline temperature of the jet. The resulting boundary layers produced by the jets’ impingement are examined in terms of the mean flow, the wall shear stress, wall heat-flux, and the wall-scaled profiles of velocity and temperature. Where possible, the results are used to test select wall functions, and it is observed that the momentum boundary layer is over-predicted with the classic log-law of the wall while the thermal boundary layer can be predicted to a certain degree with certain wall functions, particularly in cases where the Reynolds analogy is valid for the data used to compute the temperature profile.