Flame speed control using a countercurrent swirl combustor

The Countercurrent Swirl Combustor (CSC) is a modified cyclone design that utilizes fluid dynamic mechanisms as a means of control. Previous investigations have demonstrated the CSC's ability to operate as a low NOx combustion source1. Although low pollutant concentrations are demanded by present and future legislation, a practical combustor must also exhibit variable energy release, or turn down. Inherent in the CSC design is the potential to effectively control flame speeds and thus turn down. The CSC geometry consists of two axial counterflowing but tangentially co-swirling annular reactant ring jets, at different radii, contained within a cylindrical vessel. An exhaust port is located on the axis of the cylinder at one end. The countercurrent shear layer between the two annular jets pumps fluid from the outer ring jet radially inward along the entire axis of the cylinder. A selfstabilized, constant diameter cylindrical flame sheet resides inside of the shear layer, with the low density products confined along the axis by the swirl field. The turbulence levels in the near field of the flame are controlled by manipulating the vorticity in the shear layer through axial shear, tangential shear, and radial ring jet separation. Flame speed suppression is achieved by increasing the global swirl velocity. The present fundamental study has experimentally observed flame speeds ranging from laminar to 3.5 times laminar values using natural gas as a fuel. A dimensionless parameter is proposed that incorporates turbulence generation mechanisms, swirl suppression, and turbulent scale adjustments. Data collapse is observed over a wide range operating conditions and geometries using this parameter.