The ignition characteristics of ultra-lean premixed H_2/air using a hot turbulent jet impinging on a flat plate was studied experimentally and numerically. The hot turbulent jet was generated by burning a small quantity of stoichiometric H_2/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 connected by a small diameter nozzle (1.5-3 mm) in the form of a hot turbulent jet, which then impinged on the flat plate and ignited the ultra-lean premixed H_2/air in the main chamber. Six different plates with varying heights and angles were used. Two important parameters controlling the impinging characteristics of the jet, the impinging distance, and the impinging angle were examined. Simultaneous high-speed Schlieren and OH* chemiluminescence imaging were applied to visualize the jet penetration 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 the ignition starts from this impinging region. However, if the impinging distance is long, the hot turbulent jet mixes with the unburned H_2/air in the main chamber and ignites the mixture at the upstream from the plate. For such type of ignition, the impinging plate has no role in main chamber ignition. Numerical results helped to understand the flow dynamics near the impingement location responsible for the lean limit extension.
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