Oscillatory combustion of heterogeneous flames

Two different steady reacting modes can be experimentally observed in heterogeneous combustion, depending on the nature of the burning material and operating conditions (pressure, temperature, diabaticity). The first is a steady, time-invariant regression rate of the burning surface (interface between the condensed and gas phases); the second is an oscillatory combustion with regression rate of the burning surface cyclically varying in time. For adiabatic systems, oscillatory combustion regimes can be observed for a more or less wide pressure range above the pressure deflagration limit (PDL). Experimental tests conducted on ÄP84/CTPB16 composite solid rocket propellant, at 300 K of ambient temperature, revealed oscillatory combustion in the pressure range 0.07<p<0.3 atm. For p<0.07 atm (the PDL), no self-sustained reacting mode was observed, while, for p>0.3 atm, the combustion was characterized by a time-invariant burning rate. The nature of the reacting mode can also be affected by adding energy to the system: an external energy source (CO2 laser) was employed in order to drive the combustion process of the same AP-based solid propellant. Several tests at different pressure levels and radiant flux intensities were performed. In this experimental configuration, reacting modes were detected even for p<0.07 atm. It was found that low values of pressure and radiant flux intensity lead to pulsating combustion. By increasing pressure and/or radiant flux intensity, the frequency of the oscillations increases.