Interpreting diffusion flame structure by simultaneous mixture fraction and temperature measurements using optical and acoustic signals from laser-induced plasmas

The measurement of mixture fraction and temperature is of great importance in non-premixed flame structure studies and combustion optimization. However, obtaining profiles of mixture fraction and temperature simultaneously for a wide range of fuels and compositions has proven to be challenging, especially when suitable spatial and temporal resolutions are desired. In this study, the acoustic and optical signals from laser induced plasmas were simultaneously used for the first time to obtain acoustic-based laser-induced breakdown thermometry (LIBT) combined with the laser induced breakdown spectroscopy (LIBS). The system was first calibrated in an ethylene-air premixed flame. Then the atomic-ratio and temperature distributions along the centerline of an ethylene counterflow diffusion flame were measured. The strong compositional and temperature gradients in diffusion flames represents a potential challenge, but simultaneous measurements were successfully performed within a 1.5 mm wide physical region, where the equivalence ratio ranged from 0.5 to 12 and temperature ranged from 1100 K to 2000 K. The elemental mass fractions and mixture fraction distribution were inferred based on measured atomic ratio distributions. The preferential diffusion of H relative to C was directly observed by the measurement of C/H ratio. Lastly, the physics behind the LIBT technique is discussed and analyzed. This work demonstrates that the combination of LIBT and LIBS holds promise as a simple tool to measure the mixture fraction and temperature in a broader range of combustion conditions; and facilitates a better understanding of flame structure.