Effect of Flame Onset Timing on Pulsating Combustion Startup and Thermal Cycles

The high efficiency and low emissions of pulsating combustion holds significant potential for applications for energy and power applications. However, the instability during the initiation process and the underlying dynamic evolution mechanisms warrant further detailed investigation. Based on flame visualization techniques and high-frequency pressure signal analysis, this study systematically investigates the influence mechanism of the flame onset timing–defined as the instant when the flame first emerges upstream of the combustion chamber in high-speed imaging—governs the initiation and stable operation of a Helmholtz-type pulsating combustor. Experimental results indicate that successful excitation during the initiation stage depends on the precise synchronization between the flame onset timing and the descending phase of the negative-pressure oscillation. Conversely, when the flame ignites during the positive-pressure phase, the thermoacoustic coupling in the combustion chamber is disrupted, leading to attenuation of pressure amplitude and disorder in the oscillation cycle. During the stable pulsating combustion stage, flame dynamics and pressure signals exhibit strong correlation. The development phase of the blue flame, which indicates complete combustion, closely corresponds to the rising edge of the pressure waveform. Furthermore, PV (P: Pressure, V: volume) thermodynamic cycle analysis reveals the nonlinear interactions among intake, combustion expansion, and exhaust processes.