Flame propagation through converging-diverging (C-D) microchannels

Heavy-duty natural gas engines utilize prechamber ignition system to achieve ultra-lean combustion. A small amount of near-stoichiometric fuel/air mixture is burned in the prechamber, and the flame passes through a small orifice and then ignites the main chamber mixture in the form of a hot jet. The purpose of this study was to understand the physics of a flame passing through a small orifice (1 - 4 mm) - whether it would survive or extinguish. An experiment has been developed to investigate the characteristics of CH4/air premixed flame passing through a small channel. Both straight and converging-diverging microchannels were considered. For the latter, a cylindrical quartz tube with the inlet to throat ratio of 2:1 was used as a model channel. The influences of the equivalence ratio, inlet, and throat diameters were studied parametrically. Dynamic behavior of flame propagation inside the channels was studied using CH∗ chemiluminescence, as well as direct imaging. Flame dynamics such as flame shape, propagation speed, cyclic oscillatory motions, and local extinction were determined. The stable flame propagation criteria were proposed.