This study focuses on the thermal and emission performance of small-size premixed combustors by investigating the combustion and flow characteristics of various carbon-free and classical hydrocarbon fuels, including ammonia, hydrogen, and methane. More attention is given to the influence of combustor structure and inlet parameters on thermodynamic and emission performance. These novel structural designs demonstrated obvious improvements in combustor performance, offering valuable insights into the optimization of small-scale combustion systems.

One of the designs include a reverse flow single-channel inlet and double-channel outlet (SIDO) combustor aining to enhance its thermal performances. Increasing the inlet pressure (Pin) improves thermal performance and exergy efficiency while reducing nitrogen oxide emissions. Increasing the inlet velocity (Vin) can enhance the temperature uniformity of the combustor wall. Increasing the equivalence ratio (Φ) leads to a reduction of nitrogen oxide emissions, and the micro-combustor has better overall performance, when Φ = 1.0. Increasing the blending ration of hydrogen give rise to a decayed advection but enhanced diffusion, and the pressure loss (Ploss) can be reduced.

Another design is applying porous medium (PM) in the small-size combustors. In comparison with the system without PM, the application of PM is found to lead to a significant improvement on thermal performances. It is found that there is a substantial 37.5% reduction in the standard deviation of the outer wall temperature (ST,W) at Vin = 2.0 m/s. The optimal thermal performance is achieved as Φ=0.9. A higher porosity (σ) gives rise to a lower entropy production within the PM. The lowest entropy production resulting from heat conduction is shown to be achieved when σ = 0.8. By implementing PM, the exergy efficiency (ηexergy) is found to be increased by 23.9% at Vin = 2.0 m/s. In general, this present investigation shed physical insights on the entropy production and thermodynamic exergy performances of ammonia/methane-fueled micro-combustion systems with and without PM.

For comparison, we proposed and studied a double-channel inlet and double-channel outlet (DIDO) combustor, which is shown to be capable of generating a vortex at the outlet, thereby reducing NOx emissions. Specifically, at a ammonia volumetric flow rate of 900 mL/min, the NO concentration at the outlet can be curtailed by 29.23%. The DIDO combustor yields a substantial enhancement in thermal performance, achieving a 51% reduction in ST,W when ammonia volumetric flow rate is set at 500 mL/min which significantly enhances the uniformity of wall temperature. The peak of thermal performance and maximum radiation efficiency (ηradiation) is reached at Φ = 0.9.

Finally, we proposed and investigated a reverse-flow Tesla channel applied in a counter-flow combustor. It is found that such structured combustor has a remarkable improvement of 72.6% to the combustor wall temperature at hydrogen volume flow rate of 100 mL/min. The diodicity (Di) of the Tesla valve is found to be increased with higher hydrogen volume flow rate, and a lower Φ contributes to a higher Di. Besides, Di decreases when Φ goes up, stabilizing at Φ = 0.9. The reverse-flow Tesla valve exhibits a more uniform pressure distribution and entropy production than the forward-flow Tesla valve. At Φ = 0.9, the hydrogen-to-air ratio maximized heat release, producing the highest entropy. Tesla-valve structured combustors demonstrate near complete combustion before Φ reaching 0.9, the combustion efficiency (ηcombustion) gradually decreasing after Φ getting to 1.0. Additionally, the effect of blending ammonia with various ratio of hydrogen was studied. To achieve a stable ammonia-hydrogen combustion within the reverse and forward flow Tesla valves, ammonia ratio can reach 20% for the reverse flow Tesla valve, whereas the ratio for a stable combustion in the forward flow configuration is 10%. The increased flow resistance inside the reverse flow structure promotes more complete fuel-depleted combustion, thereby increasing the wall temperature. In contrast, the forward flow structure, due to its lower flow resistance, extends the flame area of the mixed fuel, thereby improving wall temperature uniformity. The Double-layer Tesla Valve structure improves wall temperature uniformity by over 55% across varying flow rates. Both double-layer and single-layer structures demonstrate a significant enhancement in the combustor's thermal performance and overall performances characterized with Nusselt and Peclet numbers.