Effects of overall equivalence ratio of premixed spray on flame structure of spray jet with a low-volatility fuel

The flame structure of rich-premixed-spray jets in quiescent atmosphere was experimentally studied for different overall equivalence ratios of the fuel spray/air mixture. N-decane was used as a low-volatility fuel to elucidate the local heterogeneity effect. The overall equivalence ratio of the premixed-spray jet was varied with the fuel flow rate while the air flow rate in the premixed-spray jet was kept constant. As the overall equivalence ratio was decreased from a quite large value, the combustion mode transited from the external flame mode to the continuous internal flame mode and then to the disintegrative internal flame mode. In the disintegrative internal flame mode, the internal flame oscillated largely and a part of the internal flame was sometimes disintegrated into bubble-like internal flames, which disappeared inside the external flame. The oscillation and disintegration of the internal flame was possibly caused by non-uniformity in local flame-spread rate due to non-uniform droplet spatial distribution. Characteristics of the internal flame were discussed based on the analogy to the spreading flame in the premixed spray.