Effect of methane addition on ammonia jet flames into a high-temperature co-flow

Ammonia, as an alternative fuel for achieving carbon neutrality, exhibits extensive application prospects. However, narrow combustion limit and worse combustion stability are two main challenges for ammonia applications. Integrating methane into ammonia enhances the combustion performance compared with pure ammonia fuel, offering a more sustainable and efficient solution. The aim of this work is to explore the methane combustion enhancement effect on autoignition characteristics of ammonia jet combustion into high temperature co-flow. An analysis of the ammonia-methane flame has been performed to identify the best methane molar fraction: the effect of methane addition on the flame length, area and normalized lifted height at different co-flow conditions has been evaluated. For the optimized ammonia-methane blend (molar fraction equals to 8%), flame core development characteristics in different co-flow temperatures have been investigated, proving that flame core formation is promoted by an elevated co-flow temperature, due to the increased heat exchange. Methane addition significantly reduces the autoignition boundary temperature, compared with pure ammonia. Furthermore, combustion efficiency improves due to the methane presence, reducing ignition delay time and increasing jet flame stability. As the co-flow temperature increases, flame length and area enlarge, while lifted height decreases, hindering the methane enhancing effect. An increment in the co-flow temperature also produces a more stable jet flame. As far as the effect of injection pressure on flame stability is concerned, an increase of such a pressure beyond a threshold leads to jet flame instability and intermittent autoignition phenomena occur.