Intrinsic thermoacoustic feedback loop in turbulent spray flames

Abstract

This paper investigates low-frequency thermoacoustic instabilities of a turbulent spray flame, a phenomenon known as ‘rumble’. Based on experimental data, a network model analysis is performed, which suggests an intrinsic thermoacoustic (ITA) feedback loop as the root cause of instability. At first, the ITA nature of the observed instability is confirmed by a parametric analysis, which reveals the sensitivity of the instability frequency to the time delay of the flame. Then, we investigate pure acoustic modes and pure ITA modes, which couple to the full system modes such that the origin and the trajectory of each mode is trackable. Finally, we show that the unstable mode frequency scales with the inlet bulk velocity: a feature that clearly separates the observed instability from classical cavity modes. The network model results are corroborated by phasor plots, in which all relevant phase information is compiled. It reveals the phase of the Flame Transfer Function (FTF) contributing to the instability and provides an estimate of the ITA frequency, which agrees well with the dominant peak in the experimental pressure spectrum. Additionally, the obtained flame phase is used to infer the instability frequency by the experimentally measured droplet burning time τB, which reproduces similar trends as experimental and network model results. This theoretical study confirms that (1) ITA feedback loops are important for spray flames, (2) that the ITA instability of the experiment scrutinized is controlled by the droplet dynamics and (3) ITA modes appear also in acoustically closed systems.