Effect of delay time on the combustion instability in a single-element combustor

Abstract

There are fewer studies on combustion instability (CI) characteristics of non-premixed methane-oxygen flames compared to those of premixed flames. Therefore, this study examined the effect of the total delay time (τtotal), which was calculated using the flame structure and speed, on the CI characteristics of a non-premixed methane-oxygen flame using a coaxial injector. The experiments were repeated by varying the O/F ratio, which is a key parameter of τtotal, and combustor length. Increasing τtotal shifted the CI mode from a high harmonic to the fundamental longitudinal mode. Further, increasing the combustor length shifted the CI mode from the fundamental longitudinal to the third harmonic mode resulted in the pressure perturbation occurring only in particular frequency ranges. The CI strength also changed with varying the combustor length, despite a constant τtotal. In conclusion, this study showed that the time-lag analysis based on τtotal and combustor length might be important for determining the CI characteristics of a non-premixed methane-oxygen flame formed by a single coaxial injector, and thus, τtotal could be the main parameter for determining CI characteristics. Further, the relationship between the flame transfer function and CI frequency was investigated. This study is expected to advance our collective understanding of CI mechanisms and provide a means of avoiding CI by applying time-lag analysis to a non-premixed flame.