The effect of ammonia addition on the low-temperature autoignition of n-heptane: An experimental and modeling study.

Ammonia (NH 3) is receiving increasing attention as an alternative engine fuel due to its carbon-free nature. However, its fundamental combustion characteristics, such as higher autoignition temperature and lower burning velocity compared to conventional hydrocarbons, limit its direct use in traditional engines. To confront this dilemma, hydrocarbon/NH 3 blending fuels are seen as one of the most appropriate ways to exploit its advantage and offset its weaknesses. Using n-heptane as a representative of hydrocarbons, this study investigated the effect of NH 3 addition on the low-temperature autoignition of n-heptane. The ignition delay times of five n-heptane/NH 3 mixtures with NH 3 fractions of 0%, 20%, and 40% were measured in a rapid compression machine at temperatures of 635–945 K, pressures of 10 and 15 bar, and equivalence ratios of 1.0 and 2.0. Experimental results show that the n-heptane/NH 3 blending fuels exhibit pronounced low-temperature reactivity, and both the total and the first-stage ignition delay times increase with the increase of NH 3 fraction. A blending mechanism of n-heptane/NH 3 was compiled based on the existing n-heptane mechanism and NH 3 mechanism. It is found that the blending mechanism is capable to predict the inhibition effect of NH 3 addition on n-heptane autoignition and qualitatively capture the dependence of ignition delay time on equivalence ratio and oxygen mole fraction. Nevertheless, there are significant discrepancies between the experiments and the simulation. Furthermore, kinetic analyses, including species evolution, rate of production and sensitivity of OH radical were conducted sequentially to reveal the autoignition kinetics of NH 3 /n-heptane blends and the interaction between n-heptane and NH 3. Suggestions are provided for the further development of the blending mechanism. [ABSTRACT FROM AUTHOR]