Non-premixed ignition by vitiated air in counterflow configurations

Ignition studies have been conducted in counterflow configurations in recent years by using heated air as the ignition source. In the present investigation, an alternative methodology has being advanced for studying ignition, by utilizing vitiated air that is produced from the oxidation of ultra-lean H-2/air mixtures supplied from one burner. Non-premixed ignition is achieved by counter-flowing the hot vitiated air against a fuel-containing jet. The ultra-lean H-2/air mixtures are oxidized on a catalyst positioned at the burner exit, allowing thus for the effective variation of the temperature of the hot gases, which are mainly composed by N-2, excess O-2, small amounts of H2O, and negligible amounts of radical species. Thus, the heat release of the H-2 oxidation serves as the ignition source and eliminates the need of heating the air. This new methodology was tested for non-premixed ignition of H-2 and H-2-enriched CO. H-2 and CO were studied first, given that the kinetics of these fuels, constitute the fundamental "building blocks" of the hydrocarbons oxidation kinetics. For the H-2 studies, the ignition temperatures were measured for global strain rates varying between 100 and 250 s(-1) and mole fractions of H-2 in the (H-2+N-2) stream varying between 10-60%. Similar studies were conducted for non-premixed H-2-enriched CO, with H-2 molar fractions ranging from 0.3-3% in the fuel stream. The fuel stream was not diluted with N-2 in these studies, given the relatively low ignition propensity of CO, and the need to avoid excessively high ignition temperatures as they could impact the performance of the catalyst and its supporting ceramic material. The present experimental results compare favorably with previously reported ones in similar configurations, providing thus confidence in the proposed ignition methodology. Agreements with numerical predictions were partially satisfactory.