2. Ignition Mechanisms

As described in the literature review section in the previous chapter, there exists a knowledge gap how ignition initiates by a hot turbulent jet. What are the ignition mechanisms from a fundamental point of view? What are the nondimensional parameters governing the ignition mechanism? To explore the fundamental ignition mechanisms by a hot turbulent jet, an experimental setup was built that uses a dual-chamber design (a small pre-chamber resided within the big main chamber). Two fuels, methane and hydrogen, were studied. Simultaneous high-speed schlieren and OH* chemiluminescence imaging were applied to visualize the jet penetration and ignition processes. It was found there exist two ignition mechanisms – flame ignition and jet ignition. A parametric study was conducted to understand the effects of several parameters on the ignition mechanism and probability, including orifice diameter, initial temperature and pressure, fuel/air equivalence ratios in both chambers, and pre-chamber spark position. The mean and fluctuation velocities of the transient hot jet were calculated according to the measured pressure histories in the two chambers. A limiting global Damköhler number was found for each fuel, under which the ignition probability is nearly zero. Lastly, the ignition outcome of all tests (no ignition, flame ignition, and jet ignition) was marked on the classical turbulent combustion regime diagram. These results provide important guidelines for design and optimization of efficient and reliable pre-chambers for natural gas engines.