Enhancing ammonia combustion using reactivity stratification with hydrogen addition

In this paper, we propose a new method to enhance the burning rate of ammonia through reactivity stratification by using the binary fuel of ammonia and hydrogen. It is found that stratification from the high-reactivity fuel, hydrogen, to the low-reactivity fuel, ammonia, can substantially increase the flame speed of the low-reactivity, ammonia/air mixture. Moreover, the enhancement can sustain for a certain period even when the flame is entirely within the original low-reactivity mixture, indicating a memory effect of the flame. Such sustaining effect is found to be caused mainly by ammonia decomposition and the associated hydrogen production initiated by the hydrogen and active radicals. The reactions of NH_x+H as well as reactions involving HNO and N₂H₂ were found to be important for this process. The length scale of the flame speed sustenance is further investigated in terms of the stratification thickness, pressure, flame stretch and stoichiometry. Furthermore, the flammability limits of the low-reactivity fuel are found to be substantially extended by the reactivity stratification. These findings suggest a strategy to facilitate the direct combustion application of ammonia.