The influence of burner material properties on the acoustical transfer function of radiant surface burners

Modern central heating systems use low NO$_x$ premixed burners with a largemodulation range. This can lead to noise problems which cannot be solved viatrial and error, but need accurate modelling. An acoustical analysis as part ofthe design phase can reduce the time-to-market considerably, but the acousticalresponse of the flame is an unknown and complex key-factor.A large class of burners currently used in boiler equipment is formed byradiant surface burners. These burners are generally made of a porous materialon which a (nearly) flat flame can be stabilized. This willlead to heat transfer from the flame to the burner, and consequently a coolerflame and reduced NO$_x$ emissions. However, the heat loss towards the burnerhas a significant influence on the behavior of the flame when the flow velocityis acoustically disturbed. In that case at certain frequencies a feedback loopcan occur and the velocity fluctuations are amplified.In this paper, a detailed model for the heat transfer between the gas and theburner is used to numerically study the influence of several different material properties on the acoustical transfer coefficient. The model is tested withmeasurements of the transfer function for four different burner deck types withknown material properties. It is found that an accurate description of the heattransfer in the porous material is essential for a good modeling of the acousticaltransfer function.