Combustion and Lift-Off Characteristics of n-Heptane Sprays Using Direct Numerical Simulations

Fundamental simulations using DNS type procedures were used to investigate the ignition, combustion characteristics and the lift-off trends of a spatially evolving turbulent liquid fuel jet. In particular, the spatially evolving n-Heptane spray injected in a two-dimensional rectangular domain with an engine like environment was investigated. The computational results were compared to the experimental observations from an optical engine as reported in the literature. It was found that an initial fuel rich combustion downstream of the spray tip is followed by diffusion combustion. Investigations were also made to understand the effects of injection velocity, ambient temperature and the droplet radius on the lift-off length. For each of these parameters three different values in a given range were chosen. For both injection velocity and droplet radius, an increase resulted in a near linear increase in the lift-off length. Changes (increase) in the ambient temperature resulted in a decrease in the lift-off length with the profile following a power law.

Simulations were performed using an enhanced version of the parallel code for turbulent reacting flows, called S3D, which was developed at Sandia National Laboratories at Livermore. The code comprises a DNS quality Eulerian method to solve the carrier gas flow field, while the Lagrangian method is used to track the liquid fuel droplets. Two-way coupling between the liquid and the gas phases were established via the mass, momentum and energy equations. A four step mechanism consisting of 8 species was used to describe the chemical reactions.