Experimental and numerical study of combustion characteristics of ammonia–hydrogen–air swirling flame with/without secondary air injection

Ammonia (NH3) is currently considered to be a potential carbon-free alternative fuel, and its large-scale use as such would certainly decrease greenhouse gas emissions and meet increasingly stringent emission requirements. Although the low flame propagation speed and high NO production of NH3 hinder its direct application as a renewable fuel, co-combustion of NH3–H2 is an effective way to overcome these challenges. In this study, the combustion characteristics of NH3–H2 swirling flames under different equivalence ratios and H2 blending ratios conditions are both numerically and experimentally investigated. Numerically, the One-Dimensional (1D) laminar flame computation presents a comparison base and the Three-Dimensional (3D) numerical simulation yields detailed flame property distributions. Experimentally, the high-speed camera takes instantaneous swirl flame images and the gas analyzer measures the NO emission at the exit plane of the flame chamber. Qualitative and quantitative analysis is performed on the flame structure and NO emission for a series of NH3–H2 swirl flames. The variation trends of the NO emission calculated using different techniques agree very well. The quantitative results show that the NO emissions are much higher at lean equivalence ratios than those at rich equivalence ratios, and such difference is closely related to the combustion flame structure. Moreover, it is shown that the utilization of secondary air injection can achieve a significant reduction in NO emissions at the exit of the combustion chamber at equivalence ratios less than or equal to 0.9.