Modelling of pulsating inverted conical flames: a numerical instability analysis

Abstract

The study of combustion-thermoacoustic instabilities is a topic of interest in the development of engines. However, the modeling of these systems involves a high computational burden. This paper focuses on a simpler class of systems that still features such instabilities: inverted conical flames anchored on a central bluff-body. Here these flames are modeled by solving species mass momentum and energy transport equations, coupled with a skeletal methane/air chemical kinetic mechanism. The aim is to characterize the dynamic behavior of inverted conical flames, both due to their natural dynamics and to external incoming velocity fluctuations. The main contribution is the detailed model of the flames, including the smallest scales. The analysis of the impact of the mesh adaption on the flame response shows a trade-off between model accuracy and computational burden that can be adjusted by changing the temperature gradient threshold. The flame response analysis in terms of the temperature and OH mass fraction gives a detailed characterization of the flame front behavior in its different scales, both in time and space. The analysis of the flame front dynamic response employing FFT shows that these have a natural frequency of 35 Hz, and this frequency interacts with the flame response due to incoming velocity excitations. More specifically, when forcing the flame with low frequencies (f ≤ 125 Hz) the flame responds only to the forcing and some harmonics, whereas when forcing between 125 < f ≤ 172 Hz the flame response comprises both the natural and forced behavior. Forcing beyond 200 Hz shows the natural flame response only. KEYWORDSlaminar premixed flame; thermo-acoustic instability; flame transfer function; flame frequency response; computational fluid dynamics CONTACT L. da Costa Ramos.

Reduced order model of diffusion flames based on multi-scale data from detailed CFD: the impact of preprocessing

“…The DRM19 skeletal kinetic model is composed of 19 (plus N 2 ) species and 84 reactions [36]. Such model has already been used to model laminar flames in different configurations and one can refer to [23,37,38] for such applications.…”

“…To overcome the burden linked to modeling such complex combustion systems, a gradient mesh adaptation based on the temperature is applied in this work, where the refinement and coarsening thresholds are 10 K/m and 300 K/m, respectively. A trial and error procedure, based on the temperature sensitivity test in mesh adaptation [38] has been performed to determine the best threshold values for the case of the laminar diffusion flame, with a fuel inlet velocity of 17.5 cm/s. For the sake of brevity, these tests are not included in this work, one may consult [37] for a detailed explanation of the mesh adaption impact on the modeling of a lean premixed flame.…”

Parametric Study of Empirical Constants Used in Soot Formation Models Based on Interpolative Closure Methods of Moments

“…scheme and PISO algorithm were used for pressure interpolation and velocity-pressure coupling, respectively. The mesh utilized here corresponds to a one that was used previously, which was refined through mesh adaptions based on global scaled gradients of specific gaseous chemical species, i.e., C2H6, C3H3 and OH (Da Costa et al, 2022;Ruiz et al, 2021). For these mesh adaptions, normalized threshold values in the range between 0.1 and 0.01 were chosen according to the results obtained progressively.…”