Active control of coaxial jet flames under different fuel compositions through manipulation of mixing process with miniature jet actuators

抄録

The gaseous fuel-air coaxial jet flow and combustion under different fuel compositions are actively controlled through periodic excitation of the initial jet shear layer with arrayed miniature jet actuators equipped on the inner surface of the annular nozzle. In the present study, methane (CH₄) is diluted by nitrogen (N₂) or carbon dioxide (CO₂) with various dilution rates to mimic biogas with different ratios of combustible and non-combustible components. The spatio-temporal development of the controlled jets are examined with phase-locked two-component particle image velocimetry (PIV). Firstly, it is found that the large-scale vortical structures and the associated mixing in the cold coaxial jets are flexibly controlled by changing the injection frequency f_v of the miniature jets even for the different fuel dilution rates, which affect the momentum flux ratio of the coaxial jet. Based on the cold jet experiments, the present control scheme is applied to bluff-body held coaxial jet flames. The flame holding and emission characteristics for the controlled flames are evaluated under different dilution rates. It is demonstrated that the blow-off limits of the controlled flames are significantly extended to higher dilution rate as increase in f_v. Moreover, carbon monoxide (CO) emission for different fuel dilution rates can be improved by manipulating the mixing upstream of the flame base. At low dilution rate, CO emission is drastically reduced through the enhanced mixing by the intense vortices, which realize highly-oxygenated combustion. On the other hand, at high dilution rate, low CO emission can be achieved through the local mixing enhancement near the inner shear layer by the weak vortices, which lead to relatively high temperature combustion.