Experimental measurement of turbulent burning velocity of premixed biogas flame

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Ayache, Ahmad
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Biogas is a renewable source of energy produced by anaerobic digestion of organic material, and composed mainly of methane (CH4) and carbon dioxide (〖CO〗_2). Despite its lower heating value, biogas can replace fossil fuels for applications such as stationary internal combustion engine, stationary gas turbine and industrial burners. However, comprehensive experimental data of the turbulent burning velocity of biogas (St), which is a fundamental property for optimizing combustion applications and validating numerical codes, is still lacking. Therefore, the present study is dedicated to experimentally determine the biogas turbulent burning velocity under normal temperature and pressure conditions. In this study, three different biogas surrogate compositions are investigated. Turbulent premixed biogas flame is centrally ignited in a 29 L fan-stirred spherical combustion chamber of nearly homogeneous and isotropic turbulence with an intensity, u’, varying between 0.5 and 1.5 m/s. Outwardly propagating biogas flame is tracked and imaged using Schlieren imaging technique via a high-speed camera, and St values are obtained by measuring the flame front area using an in-house developed Matlab code. To verify the experimental methodology, turbulent burning velocity of methane-air mixture is determined and compared with published data. Biogas results show that by increasing turbulence and reducing CH4%, biogas-air flammability limit reduces from 0.6<∅<1.2 to 0.7<∅<1.1. Additionally, the curve fits of the turbulent burning velocity versus the equivalence ratio show two different trends. At fixed u’, when more CH4 is added into the mixture, the peak of the turbulent burning velocity (50 cm/s) shifts away from the nearly-lean equivalence ratio towards the stoichiometric one. On the contrary, when u’ is increased from 0.5 to 1.5 for the same biogas compositions, the peak of the turbulent burning velocity shifts away from the stoichiometric equivalence ratio towards the leaner one.
Turbulent burning velocity, Biogas, Spherical combustion chamber, Turbulent flame