Effect of droplet diameter and global equivalence ratio on n-heptane spray auto-ignition

Three dimensional DNS are performed to investigate the auto-ignition behavior of n-heptane droplets in high-temperature products of premixed H₂/air combustion. A skeletal chemistry which contains 44 species 122 element reactions is adopted. Two-way coupling is considered for the interaction between liquid phase and gas phase by Eulerian-Lagrangian point-source approach. The initial turbulence is homogenous and isotropic. Five cases of diverse initial droplet size and global equivalence ratio are carried out to study their effects on the ignition process. The most reactive mixture fraction η_MR is determined. The statistical characteristics of mixture fraction η and the scalar dissipation rate χ are examined, in terms of the probability density function (PDF) P(η) and conditional averaged dissipation rate 〈χ|η〉 respectively. The temporal evolution of joint PDF of η and χ at the point (η=η_MR, χ = 0) is analyzed. It is found that P(η_MR,χ=0) is a key parameter relating to the happening of ignition.