The effect of oxygen content on the turbulent flame speed of ammonia/oxygen/nitrogen expanding flames under elevated pressures

Abstract Ammonia is a promising, but weakly reactive, carbon-free fuel and a promising hydrogen carrier for renewable energy. In this study, the turbulent flame speed (ST) of stoichiometric ammonia/oxygen/nitrogen mixtures under oxygen enrichment conditions was investigated at elevated pressures using a fan-stirred constant volume combustion chamber. Turbulent flame speed is found to increase with oxygen content at all pressures and turbulent intensities (u′) studied. In contrast, the turbulent-to-laminar flame speed ratio (ST/SL) was found to decrease with the oxygen content, mainly due to SL increasing faster than ST. The self-similar propagation characteristics of ammonia flame as flame radius develops are similar to other fuels and scale as a one-half power law, but it bends down as oxygen content increases. Several correlations of turbulent flame speed are validated against the present data, and it is found that a fitting relationship including pressures and turbulent intensities is independent of fuel type and performs best among literature proposed correlations. Moreover, the correlations of ammonia get similar results with other fuels when turbulent length scale adopted same definition of laminar flame thickness, in contrast, the ST/SL is larger for ammonia compared to methane at the same Reynolds number. Two new correlations of turbulent flame speed based on Karlovitz number (Ka) and Damkohler number (Da) are presented and narrowed the data scatter. Ammonia/oxygen/nitrogen mixture with ηO2 = 0.4 get similar turbulent flame speed as methane/air. A flame surface wrinkling analysis and stretch sensitivity analysis showed that the sensitivity of the higher stretch rate combined with the wrinkling acceleration was responsible for the observed increase in ST/SL ratio.