Response of a spray of n-heptane or dodecane at an acoustic pressure antinode in reactive conditions
Résumé
Lean liquid-fueled combustion is a promising technology to reduce the environmental impact of the aeronautical industry. However, lean flames in a confined environment like an annular combustor are prone to develop thermo-acoustic instabilities, potentially leading to a decrease of combustion efficiency or even safety issues. To better understand the driving mechanisms, a collection of three swirling spray flames is aligned in a cavity, simulating an unfolded sector of an annular combustor which exhibits self-excited azimuthal instabilities of frequency fr about 730 Hz. Experiments are performed with n-heptane or dodecane, and the central flame is placed at a pressure antinode of a forced transverse acoustic field at a frequency fr. The gain of the Flame Describing Function depends strongly on the fuel, while its phase is quite unchanged. Phase-averaged spray quantities (count, velocity and size), obtained with Phase Doppler Particle Analyzer, are modulated at fr with an amplitude that depends on the fuel. Droplet number waves are generated alternatively in the inner region and the outer region of the spray. This pattern induces equivalence ratio fluctuations, the amplitude of which depends on the fuel. The increased presence of small droplets with n-heptane compared to dodecane suggests that they dominate the flame response.
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