Characterization of Valved-Pulsejet and Single-Element Lean Direct Injection Combustion Systems

Abstract

The pulsejet study investigates twelve configurations of the self-aspirated valved pulsejet, focusing on its operational mechanism. It begins by characterizing the engine's acoustic field with varied boundary conditions, revealing an extended effective acoustic length beyond the combustion chamber and tailpipe sections when the valve is open. Radial and lateral velocity fluctuations are hypothesized as causes for vortex structure generation. Analysis of reacting pulsejets using gasoline and ethanol fuels shows that the operating frequencies are unaffected by fuel type, with geometric arrangement as the primary frequency factor. Fluctuations in dynamic pressure, microphone readings, and thrust characterize engine stability, impacted by low-frequency modes and higher harmonics. Tailpipe length emerges as the key geometric factor for performance enhancement with the medium size being the optimal option. Pressure field analysis demonstrates shock wave generation during ignition, serving as an excitation mechanism. Notably, different configurations with varied operating frequencies can yield equivalent thrust, indicating that thrust production is not solely dependent on pressure rise during combustion, despite similar pressure rise observed across most cases. The second study investigates the impact of the equivalence ratio, inlet air temperature, confinement ratio, and exit boundary on the flame dynamics of a single-element, low-emission nozzle used in a multipoint lean direct injection (MLDI) combustion system. High-speed OH* chemiluminescence, combined with sound pressure measurements, is used to analyze the flame structure and its correlation to sound intensity. Three distinct flame types are identified: the V-flame, M-flame, and lifted-distributed flame. The V-flame, occurring at higher equivalence ratios, is associated with axial fluctuation modes and is coupled (in-phase) with the acoustic field, leading to higher sound intensity. Notably, the matching of frequency values between the flame's coherent mode (830 Hz) and sound pressure (822.6 Hz) during the V-flame condition indicates flame-acoustic interaction. The M-flame, observed at lower equivalence ratios, is associated with radial fluctuation modes and is decoupled (out-of-phase) from the acoustic field, resulting in lower sound intensity. The sound intensity is linearly correlated with the equivalence ratio. As the equivalence ratio approaches lean blowout (LBO), the flame loses its coherent structure and transitions into a random turbulence mode, suggesting that the noise measured outside the combustion chamber is dominated by turbulence rather than flame-acoustic coupling mechanisms. Increasing the inlet temperature and adding the exit plate both shift the flame anchoring point upstream. This occurs because a higher inlet temperature decreases air density, which increases axial velocity and reduces the size of the inner recirculation zones, leading to a V-flame anchored near the nozzle exit. Similarly, adding the exit plate raises the pressure gradient in the reverse flow region, pushing the flame anchoring point upstream. Each parameter promotes a different flame dynamic mode. The axial fluctuation mode, associated with flame vortex roll-up, is magnified with increasing inlet temperature, particularly in more confined flames, such as those with a 5.6 confinement ratio. In contrast, the newly revealed axial-radial fluctuation mode (where both vortex roll-up and flame angle fluctuations occur simultaneously) is amplified after adding the exit plate. As the confinement ratio increases, the size of the inner recirculation zone grows linearly, producing larger flames with lower OH* intensity, as the OH* species become more distributed. Larger confinement ratios, such as 6.9 and 9.6, promote the radial fluctuation mode associated with flame angle fluctuations, due to the increased inner recirculation vortices push the jet radially. Significant changes in sound pressure level (SPL), around 15 decibels (dB), are observed in microphone measurements as the flame structure and associated dynamic modes transition between the identified flame types. Finally, the LBO limit, important for reducing thermal NOx formation, is influenced by these parameters.