Ignition of ultra-lean premixed hydrogen/air by an impinging hot jet

Abstract

The ignition characteristics of a hot turbulent jet impinging on a flat plate surrounded by an ultra-lean premixed H2/air was studied both experimentally and numerically. The hot turbulent jet was generated by burning a small quantity of stoichiometric H2/air mixture in a separate small volume called the pre-chamber. The higher pressure resulting from pre-chamber combustion pushed the combustion products into the main chamber through a small nozzle (0.75–4.5 mm in diameter) in the form of a hot turbulent jet, which then impinged on a flat plate. Six different plates with varying impinging heights and angles were used. Two important parameters controlling the impinging characteristics of the jet, the ratio of the impinging distance to the nozzle diameter, H/D and the impinging angle, θ were examined. Simultaneous high-speed Schlieren and OH∗ chemiluminescence imaging were applied to visualize the jet penetration/impinging and ignition process inside the main combustion chamber. Results illustrate the existence of two distinct types of ignition mechanisms. If the impinging distance is short and the hot turbulent jet hits the plate with high enough momentum, the temperature increases around the stagnation point and ignition starts from this impinging region. However, if the impinging distance is long, the hot turbulent jet mixes with the ambient unburned H2/air in the main chamber and ignites the mixture at the upstream from the plate. For such type of ignition, the impinging plate has a minimum role on main chamber ignition. Employing the stagnation point ignition, a leaner limit of H2/air in the main chamber was achieved. Numerical modeling of the turbulent hot jet impingement process was carried out to explain the impinging jet ignition mechanism. It was found that H/D ratio was the controlling parameter between the two ignition mechanisms. The limiting H/D ratio was found to be 21.6, below which ignition occurred via jet impingement. Unlike the H/D ratio, the impinging angle did not affect the ignition mechanism; however, it affected the main chamber burn time.