Influence of the Inter-structural Gap between the Combustion Zone and the Flame Trap on the Properties of the dual Layer Porous Burners in Operation V. Jovicic *, 1,2 , E. Friedmann 1 , C. Regensburger 1 , A. Zbogar-Rasic 1 , A. Delgado 1,2 1 Institute of Fluid Mechanics (LSTM), University of Erlangen-Nuremberg, Germany 2 Erlangen Graduate School in Advanced Optical Technologies (SAOT), Germany Abstract One of the flame stabilization principles for the porous burner technology is based on the sudden change of the pore size, i.e. application of two adjacent porous zones. When the temperatures exceed 1500°C, material damage at their contact surface can occur. Conducted experimental investigation shows the influence of the existence of an axial distance between the two porous zones on the burner properties (pressure drop, flue gas composition, operation range, surface temperatures). The modified burner, i.e. with the gap between the zones, has lower CO but higher NOx emissions and a wider operational range at the equivalent inlet conditions compared to the reference burner. Experiment showed when the axial distance between the zones is small, the operating characteristics of the burner are not significantly changed. * Corresponding author: vojislav.jovicic@fau.de Proceedings of the European Combustion Meeting 2015 Introduction Porous burner combustion is a combustion technology where a fully premixed fuel gas – air mixture burns inside the holes of a porous, inert structure, mainly made of ceramic materials [1 – 4]. Due to the presence of the porous material in the combustion zone, an intensive thermal energy transfer between the gas phase and the porous medium occurs, resulting in a homogenization of the temperature field and lower pollutant emissions (CO, NOx) [1, 2]. This combustion technology offers, as in the literature well documented [2, 3], benefits for both domestic heating devices and different industrial applications, through its high thermal radiation output, reduced space requirement, high power modulation range, short reaction time, etc. The flame stabilization concepts for the porous burners are based on the modified Pécklet number (dual layer stabilization) [5, 6], velocity– [7] and jet stabilization [8], and active cooling [9]. The flame stabilization based on the modified Pécklet number, shown in Figure 1, enables a wide operational range with regard to the thermal power, and is commonly applied in commercially available porous burners. Figure 1. Scheme of the burner layout. In this case, the flame is stabilized due to a sudden change of the pore diameter between the combustion zone (big pores) and the flame trap/preheating zone (small pores). Smaller pore diameter of the flame trap zone leads to flame quenching within it, thus preventing the flame flash-back. This stabilization principle provides the power modulation range of up to 1:10 under normal operating conditions [2, 3]. As the combustion takes place within a porous medium, the porous structure of a combustion zone commonly reaches the temperature between 1000°C and 1400°C [1, 3]. The highest operating temperature is generally limited by the physical properties of the applied (ceramic) materials. As a result, the porous burner technology is dependent on high temperature and oxidation resistant porous materials, which can withstand high temporal and spatial temperature gradients (leading to high thermal stresses), occurring e.g. during the ignition phase [1, 10, 11]. Regarding the combustion zone of a dual layer porous burner, silicium infiltrated silicium-carbide (SiSiC) is the most commonly applied. SiSiC has good thermal shock resistance, high thermal conductivity and good material stability [1, 12]. Its maximal operating temperature is limited to ca. 1450°C due to the separation of the free silica and due to the material that follows oxidation [13]. Apart from SiSiC, materials like Al2O3, ZrO2, C/SiC composite ceramics, etc. [11] can be also used as porous burner components. In comparison to the combustion zone material, the flame trap material should possess very low thermal conductivity in order to prevent flame flash-back. Most commonly, vacuum-formed aluminum oxide is applied for this purpose [1]. Further development of the porous burner technology often demands higher radiation output and higher overall efficiency, which can be achieved only by increase of its maximum operating temperature. For