Multiple V-flame burner: investigation on flame interaction

Lean premixed combustion is today one of the more promising techniques aiming to reduceenvironmental impact of combustion processes.In the field of small burners (that is, domestic heating applications) the use of lean premixed methane-air V-flames (inverted Bunsen cone flames) seems highly advantageous, with respect to traditionalones, in order to reduce thermal NOx emissions. Behaviour of a single V-flame generated by means ofa stabilization rod downstream the efflux of the combustible mixture has been already investigated inliterature, under the point of view of stability limits and fluid dynamic features. It has beenreported that flame morphology strictly depends upon operating conditions of the burner (excess air, efflux velocity of the mixture) and can range from conventional Bunsen one (for rich or stoichiometricmixtures) to V morphology (for lean flames) till blow-off limit. This paper deals with the experimental characterization of multiple adjacent V-flames in a burnerprototype, with the possibility of feeding a controlled air stream (separated from the air in thecombustible mixture) parallel to the main mixture flow, in inter-flame position. Secondary air feeding (present also in conventional burners with Bunsen flames) seems to produce deep effects upon thecombustion process, for instance enlarging the operating field of the burner. In fact, it promotes thetransition from an undesired unstable regime to a fully stable one with V-flames, maintaining constantflow rate and composition of the combustible mixture, but lowering flame temperature and reducingpollutants emissions (especially Thermal NOx, for local mixture dilution). Therefore, influence of thesecondary air stream upon the combustion process has been investigated by temperature and fluiddynamic measurements, by means of thermocouples and LDA (laser Doppler Anemometry) respectively.