Intrinsic thermoacoustic modes and their interplay with acoustic modes in a Rijke burner

Abstract

The interplays between acoustic and intrinsic modes in a model of a Rijke burner are revealed and their influence on the prediction of thermoacoustic instabilities is demonstrated. To this end, the system is examined for a range of time delays, temperature ratios and reflection coefficients as adjustable parameters. A linear acoustic network model is used and all modes with frequency below the cut-on frequency for non-planar acoustic waves are considered. The results show that when reflection coefficients are reduced, the presence of a pure ITA mode limits the reduction in the growth rate that usually results from a reduction of the reflection coefficients. In certain conditions, the growth rates can even increase by decreasing reflections. As the time delay of the flame and thus the ITA frequency decreases, the acoustic modes couple to and subsequently decouple from the pure ITA modes. These effects cause the maximum growth rate to alternate between the modes. This investigation draws a broad picture of acoustic and intrinsic modes, which is crucial to accurate prediction and interpretation of thermoacoustic instabilities.

Thermoacoustic interplay between intrinsic thermoacoustic and acoustic modes: non-normality and high sensitivities

“…Practically, we have implemented this using only the growth rates, as the ITA modes in our cases are strongly damped. This definition is different from the one proposed in previous work (Hosseini et al 2018;Hoeijmakers et al 2014;Emmert et al 2015), where a mode is classified as ITA (or acoustic) based on the limit of reducing the flame gain (reflection coefficients) to zero. Our choice is based on the argument that, as the parameters of the system are changed, the behaviour of…”

“…2015; Hosseini et al. 2018), where a mode is classified as ITA (or acoustic) based on the limit of reducing the flame gain (reflection coefficients) to zero. Our choice is based on the argument that, as the parameters of the system are changed, the behaviour of the modes can alternate between the two types (see discussion of figure 3).…”

“…In addition, Emmert et al (2016) showed that a stable system can be driven into instability, if excited at intrinsic frequencies. The work by Hosseini et al (2018) shows that, as the parameters of the system are changed, there is significant interplay between the ITA and the acoustic modes; in particular, they identify a coupling mechanism that causes ITA modes "attach and detach" from their acoustic counterparts.…”

Study an Acoustic-Intrinsic Thermoacoustic Feedback on Swirling Flame Combustors

“…They are commonly characterized as acoustic features of the combustion system driven by unsteady heat release of the flame, which feeds perturbation energy into acoustic cavityoriented resonant modes ( [3,4]). However, recent research, including theoretical modelling ( [5][6][7][8]), numerical simulations ( [9,10]) and experimental verifications ( [11][12][13]), show that thermoacoustic instability can occur even in a fully anechoic environment where acoustic reflections from the inlet or outlet of the combustor are not involved. Due to their independence on chamber acoustic resonant behavior, they are often identified as intrinsic thermoacoustic (ITA) instability.…”

“…Recently, ITA modes were reported to be dominant in a perfectly premixed combustion system [15] and partially premixed turbulent combustion system [16]. In a Rijke burner, Hosseini et al [13] analysed the interactions between the ITA modes and acoustic modes.…”