Effects of hydrogen power ratio on combustion characteristics of hydrogen-methane fuel in arrayed premixed micro-tubes

This study investigates the effects of Hydrogen Power Ratio (HPR) on flame characteristics, emissions, and thermoacoustic instability in a static-mixer-equipped array of premixed micro-tube combustors. Experiments were performed at two air flow rates (23.7 and 35.5 Nm³/h) under constant thermal power, with HPR varying from 0 % to 100 %. Key observations reveal that flame length contracts significantly with increasing HPR, stabilizing at HPR above 80 % as well-defined pale blue conical structures. In terms of emissions, NOx concentration peaks within the 20–30 % HPR range before decreasing, while CO levels show a consistent declining trend. Notably, elevated HPR markedly promotes combustion instability, with excited oscillation frequencies between 700 and 800 Hz exhibiting distinct first-order and second-order modes. This instability mechanism is primarily attributed to the increase in laminar flame velocity, which reduces the heat release delay time and consequently diminishes the phase lag between periodic heat release and pressure fluctuations. Under these conditions, the Rayleigh criterion for oscillation amplification is effectively satisfied. The findings provide critical insights for the design and operation of compact hydrogen-enriched combustion systems, highlighting the inherent trade-off between emission reduction and combustion stability.