Forced and Self-Excited Instabilities From Lean Premixed, Liquid-Fuelled Aeroengine Injectors at High Pressures and Temperatures

Abstract

Measurements of unsteady pressure and chemiluminescence during flow forced operation of aeroengine lean direct injection fuel spray nozzles were made, with a goal to determine the response of the flame, subject to a range of air fuel ratios, fuel flow splits between pilot and mains injectors, and cooling flows. A rotating shutter installed at the downstream choked nozzle provided the excitation for forcing the mass flow rate between 100 to 600 Hz, at normalized intensities of 0.1 to 0.7 relative to the mean velocity at the injector. The experiments were performed at inlet conditions of 800 K and 5.7 bar. Self-excitation created by the coupling between the flame and the combustor cavity was observed, in the form of a broad peak around 275 Hz. Numerical studies indicate that the peak is associated with an entropy spot (a region of non-uniform temperature) travelling from the flame to the choked nozzle, followed by the ensuing expansion wave towards the injector and amplification of the excitation. Investigation of previous studies suggests that similar phenomena may have been present in other studies at high pressure. The main impact of the self-excitation is the significant amplification of the velocity fluctuations from 0.1 of the mean velocity away from the self-excitation frequency to around 0.7 at the peak. The flame response, represented by the ratio of the fractional fluctuations in OH* chemiluminescence to the fractional velocity fluctuations at the injector, can be determined under conditions where the self-excitation heat release contributes only a small portion of the forced heat release, based on the measured background. The flame response shows a significant dependence on both air fuel ratio and fuel splits, with a decreasing gain towards higher frequency. The results show that it is possible to generate high amplitude fluctuations on the flow using this method, and demonstrate the role of entropy spots during normal operation in lean direct injection systems. Finally, the results suggest that there is an interaction between the forcing frequency and the self-excitation, which may behave in a non-linear manner, and which deserves further investigation.

Thermoacoustic Shape Optimization of a Subsonic Nozzle

“…The upstream propagating acoustic wave generated by indirect combustion noise has been suspected to possibly couple with the flame dynamics and close a feedback loop leading to combustion instabilities for years [7,8]. Very recent evidence, supporting the existence of this feedback mechanism in realistic configurations, has been brought to the attention of the research community [9,10]. Recently, Moase et al [11] have extended the work of Marble and Candel [4], taking into account the subsonic ' part of the flow in a choked nozzle.…”

Mixed acoustic–entropy combustion instabilities in gas turbines

“…Zhu et al (2001) and Yao et al (2012) considered an academic combustor and applied the Reynolds Averaged Navier-Stokes (RANS) methods although the accuracy of this approach when dealing with reacting turbulent flows is highly questionable. Unsteady RANS computations have been performed by Hochgreb et al (2013) in an academic combustor. These authors found that the mechanism of convection of entropy spots to the downstream nozzle has a keyrole on the establishment of a self-sustained thermoacoustic oscillation, whose computed frequency match the experiments.…”

Entropy noise: A review of theory, progress and challenges

“…It is therefore unsurprising that several analytical, experimental and numerical studies have identified that entropy noise introduces extra low frequency thermoacoustic combustor modes. 21,29,67,[71][72][73][74] For example, aero-engine combustors with fuel-spray atomisers are particularly susceptible to a low-frequency oscillations in the range 50-120 Hz at idle and sub-idle conditions, commonly called ''rumble.'' In many cases, these are likely to be associated with entropy noise modes.…”