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

Modern central heating systems use low NOx premixed burners with a large modulation range. This can lead to noise problems that cannot be solved via trial and error, but need accurate modeling. An acoustical analysis as part of the design phase can reduce the time-to-market considerably, but the acoustical response of the flame is an unknown and complex key factor. A large class of burners currently used in boiler equipment are formed by radiant surface burners. These burners are generally made of a porous material on which a (nearly) flat flame can be stabilized. This will lead to heat transfer from the flame to the burner, and consequently a cooler flame and reduced NOx emissions. However, the heat loss towards the burner has a significant influence on the behavior of the flame when the flow velocity is acoustically disturbed. In that case at certain frequencies a feedback loop can occur, and the velocity fluctuations are amplified. In this paper, a detailed model for the heat transfer between the gas and the burner is used to numerically study the influence of several different material properties on the acoustical transfer coefficient. The model is tested with measurements of the transfer function for four different burner deck types with known material properties. It is found that an accurate description of the heat transfer in the porous material is essential for a good modeling of the acoustical transfer function.