Impact of pressure fluctuations on the dynamics of laminar premixed flames

• Published in: Proceedings of the Combustion Institute 2019, Vol.37 (2), p.1895-1902
• Main Authors: Beardsell, Guillaume, Blanquart, Guillaume
• Format: Article
• Language: English
• Subjects: Detailed chemistry; Direct numerical simulations; Laminar flames; Thermo-acoustic instabilities
• ISSN: 1540-7489; 1873-2704
• DOI: 10.1016/j.proci.2018.07.125

Thermo-acoustic instabilities are problematic in the design of continuous-combustion propulsion systems such as gas turbine engines, rocket motors, jet engine afterburners, and ramjets. Conceptually, the coupling between acoustics and flame dynamics can be divided into two categories: flame area fluctuations and changes in the local flame speed. The latter can be caused by the thermodynamic fluctuations that accompany an acoustic wave. This coupling is the focus of the present work. In this paper, we are concerned with the dynamics of laminar premixed flames involving large hydrocarbon species. Through high-fidelity numerical simulations, we investigate the flame response for a wide range of fuels and acoustic frequencies. The combustion of hydrogen and methane is considered for verification purposes and as baseline cases for comparison with two large hydrocarbon fuels, n-heptane and n-dodecane. We extract the phase and gain of the unsteady heat release response, which are directly related to the Rayleigh criterion and thus the stability of the system. For all fuels, we observe a local peak in the heat release gain. At high frequencies, we find that the fluctuations of the different species mass fractions decrease with the inverse of the acoustic frequency, leading to chemistry being “frozen” in the high-frequency limit. This allows us to predict the flame behavior directly from the steady-state solution.