Pressure–heat release measurements during start-up conditions in a pulse combustor

An experimental study focusing on the temporal evolution of the global OH heat release (q′) and dynamic pressure (p′) from ignition to limit cycle conditions in an aerovalved pulse combustor has been carried out. The motivation of the work was to investigate how the thermo-acoustic relationships evolve, as very little is understood regarding how pressure and heat release couplings develop prior to establishing limit cycle conditions. The start-up experiments demonstrated that the total start-up sequences occurred within 100 ms and can be subdivided into three regimes: (i) ignition and decay; (ii) instability growth; and (iii) onset of limit cycle operation. The main results showed that upon ignition the high amplitude impulse pressure wave corresponded to the natural frequency of the pulse combustor at ambient gas temperature and was verified by an acoustic model. The pressure field over the growth period exhibited two main trends, either steady amplitude growth or a short delay interval followed by steady amplitude growth to limit cycle conditions. Overall, no reproducibility in frequency or phase during the growth period was observed pointing to the influence of strong non-linear interactions. When operating under limit cycle conditions, the heat release and pressure oscillations were in phase, possessed high levels of coherence, and exhibited narrow band frequency response at the operating frequency and several harmonics.