Shock Ignition of N-Heptane With Supplemental Hydrogen

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MacLean, JD
Hydrogen , Shock Tube , n-heptane , Ignition Delay
The objective of the study was to investigate the effect of adding molecular hydrogen to a mixture of n-heptane (surrogate for diesel fuel) and air on ignition delay time at prototypic post-compression diesel engine conditions. Ignition delay time data was measured using the reflected shock wave technique at a pressure of 20 bar (representative of part load engine conditions), for three fuel equivalence ratios (ϕ = 0.833, 1, 1.25) over a temperature range of 700 K to 1150 K. The ignition delay time data, obtained for all heptane-hydrogen fuel combinations and fuel-air equivalence ratios, is characterized by a negative temperature coefficient region between 800 K and 1000 K. This is shown to be consistent with known intermediate temperature large hydrocarbon molecular kinetics. For prototypic diesel post-compression temperatures of less than 1000 K the addition of hydrogen to n-heptane-air, for all three equivalence ratios, resulted in an increase in the measured ignition delay time. This increase was the result of hydrogen acting as a diluent due to the relatively slow dissociation compared to heptane. Therefore, although hydrogen addition to the diesel fuel is expected to reduce soot, it will have a negative impact on the cetane number. However, the impact is expected to be very small as 20% hydrogen addition on average only increases the ignition delay time by 8%. For temperatures above 1000 K, the fast decomposition of hydrogen provides free radicals that speed up the n-heptane abstraction process that result in a reduction in ignition delay time. The ignition delay time prediction using a constant volume model with the Lund reaction mechanism [1] was satisfactory. There was a negative 50 K temperature shift in the computed ignition delay time, compared to the measured data that can be attributed to the zero velocity approximation of the constant volume model. For all three equivalence ratios the constant volume model predicted the increase in ignition delay time with hydrogen addition for temperatures below 1000 K, and the decrease for temperatures above 1000 K.
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