Study on the self-ignition mechanism and elimination method of valveless U-bend pulse detonation combustor

The valveless U-bend pulse detonation combustor (PDC) offers pressure gain and compact design advantages for gas turbine applications. However, when operating at high frequencies, with fuel and high-temperature air supplied passively and intermittently in accordance with the periodic pressure fluctuations of detonation wave, the PDC tends to self-ignite after prolonged operation. To understand the characteristics of self-ignition, the study conducted experimental measurements of pressure, flame ion signals, and gas temperature in the PDC. Numerical methods are employed to simulate the process of self-ignition to delve into its underlying mechanisms and to suggest approaches to eliminate self-ignition. The research results indicate that self-ignition can lead to unstable working frequency and the inability to form sufficiently strong detonation waves. Within the studied operating conditions, self-ignition is observed only when the air is heated at frequencies of 25 and 30 Hz. Self-ignition is primarily observed in the ignition section, largely due to high-temperature gas trapped in the recirculation vortex, which prematurely ignites the fresh air-fuel mixture for the subsequent cycle. An active intermittent fuel supply method is proposed to eliminate self-ignition. When fuel supply refills at the t = 8 ms of the detonation cycle in numerical simulation, the low-temperature air that enters the detonation early fully isolates the contact between the fuel and high-temperature gas, thereby eliminating self-ignition effectively. Subsequently, the experimental results further demonstrated that self-ignition could be effectively eliminated when maintaining 82% refill proportion of one entire detonation cycle via active intermittent fuel supply method.