A unified approach for the identification of fluctuating phenomena in combustors based on Doak’s Momentum Potential Theory

Kurzfassung

The present work investigates the definition of a novel general framework for the description of fluctuating phenomena in reacting continua, with a particular emphasis on identification of the acoustic emissions. The approach presented here relies on Doak's "Momentum Potential Theory of Energy Flux carried by Momentum Fluctuations'' (MPT). This study presents, in fact, the results of the adaptation of Doak's MPT to the dynamics of multi-species and reacting flows. Several applications of the method to large-eddy simulation data of complex reacting and non-reacting test cases are used to demonstrate the main theses of the investigation, which can be summarized as follows: (i) the derived approach offers a reliable strategy to analyze the thermoacoustic behavior of reacting continua based on the single assumption of having a time-stationary fluctuating flow; (ii) vortical, thermal, and acoustic disturbances can be effectively identified and separated from each other, using the proposed approach; (iii) the acoustics of the system can be reliably described by means of a ``Generalized Acoustic Variable'', which can be identified as the acoustic part of the fluctuating total stagnation enthalpy (TFE); (iv) the acoustic production can be related to clearly separated vortical, acoustic, and thermal sources, which balance the radiation of the mean fluxes of the TFE; (v) the decomposition of the fluctuations obtained with the MPT are capable of highlighting several (in some cases already well-known but also new) insights characterizing the multiphysics coupling in stable and unstable combustor systems. The demonstration of such results is collected in five papers, which report: (1) the theoretical extension of Doak's original MPT formulation to multi-species and reacting flows; (2) the main concepts of the MPT application to combustion noise, explained by means of the analysis of the thermoacoustic behavior of a non-reacting lab-scale combustor, the so-called FACTOR configuration; (3) the application of the extended model for multi-species and reacting continua, in case of the so-called VOLVO Test Rig in stable operations; (4) the analysis of the thermoacoustic behavior of a stable, partially premixed flame, in case of the PRECCINSTA burner configuration; (5) the multiphysics coupling arising between the fluctuations in the same burner, this time in self-excited conditions. The validity of the MPT as a reliable tool for identifying fluctuations and determining their coupling in combustors has been confirmed by these studies. In particular, the MPT-based approach was found to be capable of describing the main characteristics and the main mechanisms of generation for the acoustic disturbances. In this regard, it was demonstrated that the overall acoustic production in (reacting, fully- or partially-premixed, lab-scale) combustors is, as expected, principally related to the behavior of the thermal fluctuations. However, the other kinds of disturbance, namely vortical and mixture disturbances, remain of paramount importance for the coupling between acoustic, thermal, and turbulent dynamics, which has also been highlighted based on the presented approach. In summary, this MPT-based framework provides a description of the acoustics (along with the other modes of disturbance) in reacting continua that is at the same time clear-defined, general, and comprehensive. Such a framework can provide an important base for improving the understanding of fluctuating phenomena in combustors and for enabling the development of future low-order thermoacoustic models that are characterized by higher complexity, consistency, and reliability. By means of such arguments, the MPT is proposed as a consistent, unified framework for the description of thermoacoustic problems.