Turbulent flame speed measurement of NH3/H2/air and CH4/air flames and a numerical case study of NO emission in a constant volume combustion chamber (C.V.C.C.)

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• This work reported new turbulent flame speed measurement and numerical case study of NO emission of the NH 3 /H 2 /air and CH 4 /air flames. • Flame morphologies show that the turbulent flame wrinkling characteristics are mainly dominated by the turbulent intensity ( u ′/ S L ), turbulent length scale ( l T / l F ) and differential-diffusion term ( Le ). • Turbulent flame speed is found to increase as hydrogen content and turbulent intensity increase, but the pressure facilitates more on the increase of S T of thermal-diffusion unstable mixtures. • Scaling law of S T / S L ∼ Re 0.5 / Le ∼ ( u '/ S L ) 0.5 ( l T / l F ) 0.5 Le -1 with equal weighting factors of u' / S L and l T / l F follows the Damköhler's second hypothesis at highly turbulent regimes. • NO emissions are independent of turbulent intensity due to the long residence time and residence time has contrary effects on NO emissions of NH 3 /H 2 /air and CH 4 /air flames due to the contrary role of the thermal-NO x pathway. Ammonia combustion is a meaningful method to retrieve stored amounts of excess variable renewable energy. The practical combustors like engines and gas turbines fired by the common hydrocarbon fuels need to be operated by the carbon-free like ammonia or ammonia/hydrogen blends shortly. Thus, in this study, the turbulent flame speed ( S T ) and NO emission potentials of the NH 3 /H 2 blend (NH 3 /H 2 = 40/60, 50/50, and 60/40 vol%) are investigated in comparison with the methane/air flames in a constant volume combustion vessel. The effects of Lewis number ( Le ), turbulent intensity ( u ′=0.78–2.34 m/s), and pressures (1, 5 bar) on normalized S T / S L are considered and validated against literature proposed correlations. Flame morphologies show that the turbulent flame wrinkling characteristics are mainly dominated by the turbulent intensity ( u ′/ S L ), turbulent length scale ( l T / l F ) at different pressures, and differential-diffusion term ( Le ). The differential-diffusion and turbulent stretch, jointly determine the S T / S L of NH 3 /H 2 /air and CH 4 /air mixtures. The scaling parameter Re T,flame / Le 2 can describe the self-similar propagation characteristics of S T / S L of spherical flame and consider the differential-diffusion effect. The further Damköhler number, Da modification on the general turbulent flame speed correlation proposed as: S T / S L ∼ ADa B ·( Re T,flame / Le 2 ) 0.5 unifies different hydrogen content cases by considering the turbulent stretch effect. When literature experimental data are also correlated together, the scaling law becomes: S T / S L ∼ Re 0.5 / Le ∼ ( u' / S L ) 0.5 ( l T / l F ) 0.5 Le -1 with equal weighting factors of u' / S L and l T / l F terms, this follows the Damköhler's second hypothesis at highly turbulent regimes. The impact of turbulent intensity and residence time on NO emissions in the flue gas of NH 3 /H 2 /air and CH 4 /air flames are independent of turbulent intensity due to the long residence time. Simulated results show that residence time has contrary effects on NO emissions of NH 3 /H 2 /air and CH 4 /air flames due to the contrary role of the thermal-NO x pathway. Practical utilization of NH 3 /H 2 /air in gas turbine combustors could increase the flame residence time by increasing the back-flow zone in the post-flame zone and swirl number to suppress the NO x emissions.