Direct Numerical Simulation of Turbulent Non-Premixed Combustion

In many modern combustion systems such as diesel engines and gas turbines, fuel is injected into an environment of hot gases, and a flame may be stabilized through the recirculation of hot air and combustion products. Under such conditions, this leads to a turbulent lifted flame, and the hot environment admits the possibility of autoignition as a mechanism contributing to the stabilization of the flame base. The fuel efficiency and emissions characteristics of combustion devices depend upon the distance the flame is stabilized from the burner. Researchers at Sandia National Laboratories and Oak Ridge National Laboratory, Drs. Jacqueline Chen, Chun Sang Yoo, and Ramanan Sankaran, have recently performed the world�92;s largest direct numerical simulation of a lifted autoignitive turbulent jet flame with detailed hydrogen/air chemistry. This simple laboratory configuration enables the study of flame stabilization mechanisms in an autoignitive, heated coflow. The roles of autoignition, flame propagation, and large-eddy structure in the near field of the jet were elucidated. In particular, the results show that autoignition is the key mechanism responsible for flame stabilization, and HO2 radical is important in initiating the autoignition ahead of the flame base. This film clip is an animation of HO2 and OH in the lifted jet flame. HO2 is a marker of pre-ignition chemistry, and OH is a marker of the high-temperature flame chemistry.

The calculation was performed with 1 billion grids and detailed hydrogen/air kinetics on the ORNL Leadership Computing Facility�92;s Cray XT-3-based Jaguar, requiring 2.5 million cpu-hours, and ran on 9000 processors. The Reynolds number was 11,000. There were 14 billion degrees of freedom in the simulation. The run represents the world's largest direct simulation of an autoignitive turbulent flame. Multi-variate volume rendering in time of the terabytes of simulated data by Drs. Kwan-Liu Ma and Hongfeng Yu (University of California at Davis) was required to understand the complex spatial and temporal relationship between various species in this multi-scale simulation.

Funding for jet flame simulation and combustion science was provided by the BES Chemical Sciences Division; computer time was provided by the LCF at ORNL under INCITE; and visualization was performed with funding from the ASCR SciDAC Program.