Onset of flame-intrinsic thermoacoustic instabilities in partially premixed turbulent combustors

Abstract

We investigate the onset of thermoacoustic instabilities in a turbulent combustor terminated with an area contraction. Flow speed is varied in a swirl-stabilized, partially premixed combustor and the system is observed to undergo a dynamical transition from combustion noise to instability via intermittency. We find that the frequency of thermoacoustic oscillations does not lock-on to any of the acoustic modes. Instead, we observe that the dominant mode in the dynamics of combustion noise, intermittency and thermoacoustic instability is a function of the flow speed. We also find that the observed mode is insensitive to the changes in acoustic field of the combustor, but it varies as a function of upstream flow time scale. This new kind of thermoacoustic instability was independently discovered in the recent theoretical analysis of premixed flames. They are known as intrinsic thermoacoustic modes. In this paper, we report the experimental observation and the route to flame intrinsic thermoacoustic instabilities in partially premixed flame combustors. A simplified low-order network model analysis is performed to examine the driving mechanism. Frequencies predicted by the network model analysis match well with the experimentally observed dominant frequencies. Intrinsic flame-acoustic coupling between the unsteady heat release rate and equivalence ratio fluctuations occurring at the location of fuel injection is found to play a key role. Further, we observe intrinsic thermoacoustic modes to occur only when the acoustic reflection coefficients at the exit are low. This result indicates that thermoacoustic systems with increased acoustic losses at the boundaries have to consider the possibility of flame intrinsic thermoacoustic oscillations.

Parametric Study of Intrinsic Thermoacoustic Feedback-driven Instability in a Partially Premixed Combustor

“…In a real gas turbine engine, the combustor is followed by turbine blades, which lead to a reduction in the exit area of the combustor. In our earlier investigation (Murugesan et al 2018), it was observed that a reduction in the exit area was accompanied by a drop in the acoustic reflection coefficient; as a consequence, the conventional acoustic-driven instability is damped. However, the ITA mode becomes dominant as we reduce the exit area.…”

“…It also stated that with the reflection coefficient tending to zero the ITA mode of instability exist. Murugesan et al (Murugesan et al 2018) performed experimental study on partially premixed flame. In experimental study injection location, area ratio and length of combustor were varied.…”

“…From the above survey we observed that a few experimental investigations (Hoeijmakers et al 2016) (Albayrak et al 2017) (Murugesan et al 2018) have been performed to show the occurrence of ITA mode driven instability in a range of operating conditions. The present work is mainly an experimental work on academic scale test rig.…”

Seeds of phase transition to thermoacoustic instability

“…To investigate the transition from S C to S TI via S IN , we perform experiments on a laboratory scale turbulent combustor 39,[55][56][57][58] and acquire spatiotemporal data. Our study analyses the transition between the dynamical states by continuously varying a system parameter, the air mass flow rate (ṁ air ) from low to high values 47,[59][60][61][62] .…”

Study an Acoustic-Intrinsic Thermoacoustic Feedback on Swirling Flame Combustors

“…ITA feedback have also been found in the practical combustor configurations with acoustic losses [7] or partially reflecting boundaries [14]. 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.…”