Combustion and Flame, Vol.225, 272-290, 2021
A 2-D DNS study of the effects of nozzle geometry, ignition kernel placement and initial turbulence on prechamber ignition
A parametric direct numerical simulation study was conducted to investigate the effects of the initial flow field (quiescent or turbulent), nozzle inlet sharpness and width, main chamber composition (lean and stoichiometric), and ignition kernel placement in a two-dimensional prechamber (PC) ignition system. The strongly coupled operating and geometric parameters determine the time at which the flame exits the prechamber, the transient structure and penetration of the initially cold and subsequently hot reactive jet and their impingement on the lower main chamber (MC) wall, affecting the combustion mode and the fuel consumption rate. The temperature of the flame reaching and crossing the nozzle is affected by the flame exit time and is significantly lower than the adiabatic flame temperature of the planar flame, although no quenching is observed. Interaction with the flow field (strong small scale vortices for narrow and sharp entry nozzles, large vortices for wide nozzles) generated close to the exit increases the surface area of the flame and its interaction with the MC mixture. Jet penetration and impingement on the lower MC wall is determined by combustion in the PC and the flow field it generates in the main chamber. Impingement results in large scale vortical structures, which further contribute to the flame area increase and accelerate the consumption of the MC charge at later times. For the conditions studied, budget analysis shows that the main combustion mode is premixed deflagration with locally enhanced or reduced reactivity. Local flame-flame interactions which are more pronounced close to the nozzle exit and the lower MC wall can increase the propagation speed up to six times compared to the planar flame. The evolution of the probability density functions of different quantities is used to characterize the strongly transient process. (C) 2021 The Authors. Published by Elsevier Inc.
Keywords:Turbulent jet ignition;Prechamber ignition;Natural gas engines;Methane combustion;Direct numerical simulation