Characterization of flame dynamics in a single-element lean direct injection combustion system

This study explores how the equivalence ratio, inlet air temperature, confinement ratio, and exit boundary influence flame dynamics in a single-element, low-emission nozzle used in a lean direct injection combustion system. Using high-speed OH* chemiluminescence and sound pressure measurements, the research identifies three flame types—V-flame, M−flame, and lifted-distributed flame—with distinct behaviors. At higher equivalence ratios, the V-flame type shows axial fluctuation modes, in-phase coupling with the acoustic field, and higher sound intensity. This is highlighted by the near match in frequency between flame mode (830 Hz) and sound pressure (822.6 Hz), suggesting flame-acoustic interaction. In contrast, the M−flame, found at lower equivalence ratios, exhibits radial fluctuation modes, out-of-phase acoustic decoupling, and lower sound intensity. Sound intensity shows a linear correlation with equivalence ratio; as lean blowout (LBO) nears, the flame structure becomes incoherent, and turbulence dominates external noise. Increasing the inlet temperature or adding an exit plate shifts the flame anchoring point upstream. Higher inlet temperatures lower air density, raising axial velocity and shrinking inner recirculation zones, promoting a V-flame near the nozzle exit. Similarly, adding an exit plate increases the pressure gradient in reverse flow regions, pushing the flame upstream. Inlet temperature amplifies axial fluctuation modes, while the exit plate enhances an axial-radial fluctuation mode. Larger confinement ratios, such as 6.9 and 9.6, enlarge the inner recirculation zone and favor radial fluctuation modes, with sound pressure levels shifting by up to 15 dB during flame structure transitions. These parameters influence the LBO limit, critical for NOx emissions.