Abstract:To develop a micro combustion system for carbon-free fuels, the micro pure-hydrogen swirl combustor was designed and tested. This combustor is able to operate efficiently across a wide range of equivalence ratios and input power levels, and to seamlessly switch between hydrocarbon gas fuels and pure hydrogen. The combustor features the non-premixed combustion mode and a low-flow-resistance design, incorporating integrated swirl blades and a preheated annular flow channel. The swirl number is 0.76, and the flow resistance is less than 86 Pa. The aerodynamic characteristics of the micro swirl combustor were explored through numerical simulations. The results demonstrate that well-distributed high-velocity zones and recirculation regions are formed at the combustor outlet. The central recirculation zone exhibits an axisymmetric distribution, effectively entraining downstream hot flue gases and thereby achieving stable swirl combustion. Experimental investigations were carried out to study the effects of the equivalence ratio (0.3 – 0.6) and input power (495 – 990 W) on the temperature distribution and pollutant emissions of pure-hydrogen combustion. The results show that the developed micro swirl combustor operates efficiently and stably within the tested equivalence ratio range. When the input power is 990 W, the average temperatures measured at seven monitoring points at equivalence ratios of 0.3, 0.4, 0.5, and 0.6 are 1 182, 1 277, 1 376, and 1 256 K, respectively. The highest flue gas temperature is achieved at an equivalence ratio of 0.5, with a maximum measured temperature of 1 579 K. The input power rate has a significant impact on the flue gas temperature, with higher heat release rates resulting in increased temperatures. At an equivalence ratio of 0.5 and a heat release rate of 0.62 MW/m2, the NO emission concentration reaches 140.6 mg/m3. Reducing the equivalence ratio while maintaining the same heat release rate significantly lowers NO emissions. For instance, at an equivalence ratio of 0.3 and an input power of 990 W, the NO concentration drops to 60.3 mg/m3, representing a 56.2% reduction, while ensuring complete hydrogen combustion. The flame of pure hydrogen appears transparent and is difficult to observe with the naked eye under illuminated environments. In the absence of ambient light, it exhibits weak luminescence primarily caused by combustion intermediates, manifesting as an orange-red color. Surrounding this orange-red region, a faint bluish-purple hue can be observed. The flame height of pure hydrogen increases continuously with increasing input power. When the input power reaches 495 W, the hydrogen flame becomes anchored at the exit of the micro swirl combustor. At an elevated input power of 990 W, the flame height exceeds 3 cm while maintaining stable combustion dynamics. Close-