Experimental and numerical study of NOx formation in a domestic H2/air coaxial burner at low Reynolds number

Abstract

Thermal NOx formation in H2/air jet flames from a coaxial burner is studied experimentally and numerically. The aim is to study possible NOx reduction strategies for domestic gas boiler burners. Following a flame splitting method strategy, a single burner is studied at different inlet powers (from 0.2 to 1.0 kW). The effect of three different fuel-air ratios (or equivalence ratio φ) is considered by varying the coaxial air stream, with fuel-air ratios corresponding to values of φ<1, relevant for domestic boiler applications (here φ=0.77, φ=0.83 and φ=0.91). NOx concentrations increase with increasing inlet power between 0.2 and 0.6 kW and numerical results are in good correspondence with available experimental data. The opposite trend is observed above 0.6 kW and no numerical results are obtained, indicating a transition from laminar to turbulent flames. On the other hand, in contrast to the observations made in turbulent non-premixed flames, reducing the equivalence ratio implies higher NOx concentrations in the low Reynolds number flames considered. The numerical results in the laminar regime are used to highlight and quantify three competing main factors concerning NOx production in order to interpret the experimental observations: the volume of the region where NOx is produced, and within this region, the competition between residence time and NOx reaction rate. Based on this analysis, different design strategies for low NOx hydrogen diffusion burners are finally discussed.