The acoustic response of burner-stabilized premixed flat flames

The behavior of acoustically driven flat flames has been analyzed experimentally. In this study, pressure transducers and laser Doppler velocimetry are used to characterize the acoustical waves upstream and downstream of a flat flame stabilized on a flame holder. Two different flame holders have been used, a perforated brass plate and a ceramic foam, exhibiting very different surface temperatures. From these experiments, the acoustical transfer function can be derived. This transfer function shows a resonance-like behavior, of which the shape and peak frequency is governed mainly by the surface temperature of the burner and the velocity of the unburned mixture. The brass burner exhibits a resonance frequency around 140 Hz, where the resonance of the ceramic burner seems to have shifted to much higher frequencies and is much more damped. All results can be understood very well with an analytical model in terms of Zeldovich number, standoff distance, and heat conductivity Apart from the analytical model for the brass flame holder, numerical simulations with detailed chemistry have also been performed. Again, the correspondence is good. The most interesting application is the acoustic behavior of central heating systems, in which these burners are frequently used. For the purpose of modeling the acoustical behavior of complete boiler systems, the analytical model can be used with minor adjustments to the Zeldovich number and heat conductivity, yielding a fairly accurate semiempirical model describing the transfer function.