Combustion characteristics of lean, turbulent, premixed flames with a very lean co-flow

The lean stability and NOX emissions of a combustor employing a lean, partially premixed fuel injection strategy were examined experimentally. A co-annular combustor configuration was run at atmospheric pressure with a simulated natural gas fuel. The core flow consisted of fuel premixed with air; the co-flow was fuel premixed with air, sufficiently lean to be below the flammability limit, or air only. The flame was stabilized on axisymmetric, bluff body flameholders mounted on the centerline of the combustor from either above or below. The bottom-mounted flameholder resulted in a lower lean stability limit than for the top-mounted case. An enhancement of lean stability was also achieved by increasing the fuel content of the co-flow stream. This stability change is accounted for by defining an effective equivalence ratio which takes into account the presence of co-flow fuel and the core/co-flow mixing. Up to a factor of four lower levels of total NOX were measured for the case with co-flow fuel compared with the case of core flow fuel only. This decrease in total NOX was accompanied by a drop in peak temperature. The total exhaust NOX concentration was comparable for all configurations when run close to their corresponding lean limits.