Numerical investigation on the strategy of separating burner-flame thermoacoustics

Thermoacoustic instability in combustion systems is contingent upon the thermoacoustic properties of the burner when coupled with a flame. In instances where the burner is acoustically transparent, a straightforward approach involves the separation of the burner from its corresponding flame. Acoustic velocity measurement at the upstream of the burner, in such cases, can be equated with the acoustic velocity at the downstream of the burner due to its transparency. However, when the burner possesses a complex geometry with inherent acoustic properties, this separation strategy becomes pivotal. Near-field acoustics immediately following the burner may influence the effectiveness of this separation. Additionally, burners are typically characterized in the absence of a flame (cold condition), with a common assumption that the acoustic properties of the burner remain unchanged in the presence of the flame (hot condition). This paper employs numerical simulations to scrutinize these assumptions and the separation strategy of the burner from the flame. Utilizing direct numerical simulation and an acoustic network approach, we compare the direct evaluation of the total transfer matrix of the burner with flame against the separate evaluation of the burner and flame, derived from linearized Rankine-Hugoniot relations.