Fuel Unsaturation Effects on NOx and PAH Formation in Spray Flames.

The effect of fuel unsaturation on NOx and PAH formation in spray flames is investigated at diesel engine 6 conditions. The directed relation graph methodology is used to develop a reduced mechanism starting from the 7 detailed CRECK mechanism2. The reduced mechanism and spray models are validated against the shock tube 8 ignition data and high-fidelity, non-reacting and reacting spray data from the Engine Combustion Network [26]. 9 3-D simulations are performed using the CONVERGE software to examine the structure and emission 10 characteristics of n-heptane and 1-heptene spray flames in a constant-volume combustion vessel. Results indicate 11 that the combustion under diesel engine conditions is characterized by a double-flame structure with a rich 12 premixed reaction zone (RPZ) near the flame stabilization region and a non-premixed reaction zone (NPZ) further 13 downstream. Most of NOx is formed via thermal NO route in the NPZ, while PAH species are mainly formed in 14 the RPZ. A small amount of NO is also formed via prompt route in the RPZ, and via N2O intermediate route in 15 the region outside NPZ, and via NNH intermediate route in the region between RPZ and NPZ. The presence of a 16 double bond leads to higher flame temperature and thus higher NO in 1-heptene flame than that in n-heptane 17 flame. It also leads to the increased formation of PAH species, implying increased soot emission in 1-heptene 18 flame than that in n-heptane flame. Reaction path analysis indicate that the increased formation of PAH species 19 can be attributed to the significantly higher amounts of 1,3-butadiene and allene formed due to  scission 20 reactions resulting from the presence of double bond in 1-heptene.