Low-Order Thermoacoustic Analysis of Real Engines

This article illustrates the capability of the recently introduced low-order acoustic-network modeling (LOM) approach (Laurent et al., Combust. Flame, vol. 206, 2019) based on the ’generalized modal-expansions’ and the ’state-space’ framework to study thermoacoustic combustion instabilities in complex realistic configurations under the assumption of zero-Mach mean flow conditions. The acoustics modeling is essentially an improved and generalized version of the classical modal expansions (Galerkin series) technique. Here, one can use as the basis functions either an over- complete set of acoustic eigenmodes (called an Over-Complete (OC) Frame) or a simple orthogonal (OB) basis as has been the norm so far. The former, where deemed necessary, offers enhanced convergence, correct representation of acoustic variables at the interfaces of the subdomains in the network fixing issues such as Gibbs oscillations, and at the same time, presents the opportunity for interconnecting subdomains with 1D/2D/3D acoustics and even modeling advanced features such as complex boundary impedances and multi-perforated liners (Laurent et al., J Comp. Phy., vol. 428, 2021). The potential of the tool is illustrated by performing a linear stability analysis of a real SAFRAN aeronautical engine combustor with twenty 3D quasi-compact flames while keeping all the geometrical complexities intact. The results are similar to those obtained by a 3D finite element based Helmholtz solver but with CPU-time significantly lower (by 3 orders of magnitude). These observations suggest the feasibility of exploiting the tool for extensive parametric studies directly on industrially relevant configurations.