Subadiabatic and superadiabatic performance of a two-section porous burner

Flames may be stabilized in porous media at velocities either above or below the laminar flame speed. These two regimes are often called superadiabatic and subadiabatic, respectively. In this paper, several burners are investigated experimentally at both superadiabatic and subadiabatic conditions. The upper and lower velocity limits of stable combustion are reported. For equivalence ratios of 0.70 and below, both sub- and superadiabatic performance was seen. For both sub- and superadiabatic performance, the flame was stabilized at or near the interface between the upstream and downstream sections of porous media. The lower velocity limit for superadiabatic performance was extinction of the flame. The upper limit for superadiabatic performance was blowoff, which occurred for all burners at a flow velocity several times the adiabatic laminar flame speed, a phenomenon which is attributed to heat recirculation within the porous media. For equivalence ratios above 0.70, only subadiabatic performance was observed. The lower velocity limit occurred when the flame reached extinction. The upper velocity limit occurred when heat recirculation enhanced the laminar flame speed to a point at which the flame front propagated through the upstream section of porous media and flashed back upstream of the burner. Both the upper and lower velocity limits minimized at or near an equivalence ratio of 1.1. For equivalence ratios between 1.3 and 1.7 and flow velocities between 4 and 9 cm/ s, oscillations of the flame front within the downstream section of porous media were observed.