Numerical investigation on NOx emissions and thermal performance of hydrogen/ammonia fueled micro-combustors with periodic wall structures

Ammonia, as a carbon-neutral fuel, holds promise in the combustion field but faces challenges stemming from its inherently low combustion rate. For three proposed micro-combustors featuring periodic wall structures: square-wave(C-S), trapezoidal-wave(C-T) and sinusoidal-wave(C-W), the effects of wall periodicity, input power, equivalence ratio and blended ratio on the flow characteristics, NOx emissions, thermal performance and energy conversion efficiency were investigated. The results indicate that wall periodicity plays an important role in reducing NOx emissions and improving energy conversion performance. As the inner wall periodicity decreases, the micro-combustor demonstrates better NOx reduction and higher exergy efficiency. Increasing input power can mitigate NOx emissions and elevate mean outer wall temperature, albeit at the expense of energy conversion efficiency. At an equivalence ratio of 0.9, the micro-combustor results in the highest NOx emissions and radiation efficiency. As the blended ratio increases, NOx emissions initially rise, peaking at a blended ratio of 20 %, before decreasing. Finally, the combustor C-T exhibits the best emission performance and exergy efficiency. At a wall periodicity of 4, input power of 129.92 W, equivalence ratio of 1.0, and blended ratio of 30 %, combustor C-T exhibits a 12.3 % reduction in NOx emissions and an 11.8 % increase in exergy efficiency compared to the reference combustor(C-R). These findings offer insights into optimizing micro combustor design for enhanced exergy efficiency and reduced emissions.