Active Control of Methane-Air Coaxial Jet Mixing and Combustion with Arrayed Miniature Jet Actuators

Abstract

Large-scale vortical structures and associated mixing in a methane-air coaxial jet are actively controlled by manipulating the initial jet shear layer with miniature jet actuators installed on the inner surface of the annular nozzle. The periodic radial miniature jet injections are realized by using a rapid-response pneumatic servovalve, and sinusoidal/pulsed flowing are employed in the present study. The spatio-temporal primary jet structures are investigated through phase-locked 2C-PIV (2 Component Particle Image Velocimetry) and Stereoscopic-PIV. In the pulsed configuration, it is found that intense vortex rings are produced in phase with the periodic control input regardless of the valve-driven frequency fv examined. When the Strouhal number Stv, which is defined with fv, is larger than unity, the vortex rings are densely-shed and thus methane/air mixing is prompted with low periodic fluctuation. The diameter of the vortices becomes small as Stv is increased, so that the transport range of the inner methane and outer air fluids can be controlled by changing Stv. In addition, the evolution of counter-rotating streamwise vortex pair is also confirmed. These streamwise vortices are formed as a result of the radial injection of the miniature jet, which leads to entrainment of the ambient fluid near the primary jet shear layer. They contribute the mixing enhancement in the downstream, where the vortex rings are broken down. Moreover, it is demonstrated that the coaxial jet flame characteristics such as flame holding are drastically improved by the present jet control scheme.