A DNS study of ignition characteristics of a lean iso-octane/air mixture under HCCI and SACI conditions

Proceedings of the Combustion Institute, Volume 34, Issue 2, 2013, Pages 2985–2993
A DNS study of ignition characteristics of a lean iso-octane/air mixture under HCCI and SACI conditions
Chun Sang Yoo (a), Zhaoyu Luo (b), Tianfeng Lu (b), Hongjip Kim (c), Jacqueline H. Chen (d)
a School of Mechanical and Advanced Materials Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea
b Department of Mechanical Engineering, University of Connecticut, Storrs, CT 06269, USA
c Department of Mechanical Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea
d Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94551-0969, USA
Abstract
Reports results of a study of the effect of thermal stratification, spark-ignition, and turbulence on the ignition of a lean homogeneous iso-octane/air mixture at constant volume and high pressure using direct numerical simulations (DNS) with a new 99-species reduced kinetic mechanism developed for very lean mixtures from a detailed mechanism.
Researchers conducted two-dimensional DNS in a fixed volume with two-dimensional isotropic velocity spectrums, temperature fluctuations, and an ignition source superimposed on the initial scalar fields. They elucidated the influence of variations in the initial temperature field imposed by changing the variance of temperature, the ignition-timing by changing the time at which the ignition source is superimposed, and the turbulence intensity and length scale on ignition of a lean iso-octane/air mixture. The mean heat release rate increases more slowly and ignition delay decreases with increasing thermal stratification under homogeneous charge compression-ignition (HCCI) conditions since the present mean temperature lies far outside of the negative temperature coefficient (NTC) regime. The spark-ignition induces relatively short ignition delay under spark-assisted compression ignition (SACI) conditions while mildly spreading out the mean heat release rate. Study results indicate that thermal stratification provides smooth operation of HCCI engines and moreover, spark-ignition can precisely control the ignition timing for SACI combustion.
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