The Acoustic Response of Hydrogen/Ammonia Flames

Abstract

Ammonia is a carbon-free fuel which could be used in gas turbines. Ammonia’s potential as a fuel can be improved when mixed with hydrogen. However, this could cause an increased propensity to thermoacoustic oscillations in the combustor. The acoustic response of hydrogen/ammonia flames was evaluated by calculating the flame transfer functions (FTFs). A local level set approach, combined with an incompressible velocity perturbation model, was used to calculate the flame front response to different frequencies of acoustic oscillations. The unsteady heat release rate was calculated from the flame front surface area, obtained by solving the G-equation, thereby calculating the gain and phase of the FTF. The unstretched flame speed, was obtained from experimental values and two chemical kinetic models: CRECK-NH3 and GRI-Mech3.0. The models’ accuracy was assessed by comparing the modelled to experimental values from literature. CRECK-NH3 was fitter for ammonia/hydrogen modelling, as it was always within 12% of experimental values, compared to GRI-Mech3.0 which always differed by over 35%. The FTFs suggested an increase in hydrogen enrichment led to an increase in the flame acoustic response, as the gain drop off occurred at a higher frequency with higher hydrogen content. The flame acts as a low-pass filter to acoustic waves and the bandwidth of this filter (the frequency at which the gain drops off) increases with hydrogen content. This was due to higher flame speeds with higher hydrogen content. These FTFs were also compared to those of hydrogen/methane flames and the non-linear response was briefly analysed.