Multiphysics coupling in the thermoacoustically unstable PRECCINSTA burner described using Doak's Momentum Potential Theory

This study uses a novel approach, based on Doak's so-called “Momentum Potential Theory (MPT)”, to describe the unstable behaviour of a lab-scale model combustor. The main advantages of using this approach are that of enabling an unambiguous decomposition of vortical, acoustic, and entropic fluctuations, and providing a reliable description of the system's acoustics despite the highly-turbulent, non-isentropic environment. These properties of the MPT-based approach are employed for the first time to characterise a self-excited unstable thermoacoustic feedback-loop, occurring in the PRECCINSTA burner, whose flow dynamics are simulated by large-eddy simulation. Within this framework, two types of decomposition are employed to separate vortical, acoustic, and entropic fluctuations: one in terms of momentum fluctuations, which is able to highlight typical hydrodynamic features of the flow and one in terms of total fluctuating enthalpy, which delivers a cleaner representation of the system state. Utilising such concepts, the following main results are demonstrated: (i) the proposed decomposition of the fluctuations is able to retrieve the unstable behaviour of the combustor, characterised in the considered case by a frequency close to and its successive harmonics; (ii) since entropic and acoustic fluctuations are almost in phase at the injection only, the coupling in this region can be considered as one of the main driving mechanisms of the thermoacoustic feedback-loop; (iii) the acoustics is dominated by a bulk oscillation inside the combustion chamber, which may result from the interplay between an ITA mode and an Helmholtz mode of the chamber; (iv) typical dynamical features of the flow, like flame roll-up or destruction / construction of central vortex core and central recirculation bubble can be identified from vortical and entropic fluctuations. All of these findings contribute to demonstrate the suitability of the MPT – especially regarding the decomposition in terms of total fluctuating enthalpy – to obtain a comprehensive description of thermoacoustic oscillations in unstable combustors. • First analysis of a thermoacoustic instability by means of the MPT approach. • New insights about the multi-physics coupling in the unstable PRECCINSTA. • MPT decompositions represent the system state cleanly and highlight main flow features.