Large eddy simulation of a turbulent ethylene/air diffusion flame

As combustion generated nano-organic particles (NOC) may pose significant health and environmental problems, there is great scientific interest in studying their formation and evolution in turbulent combustion systems. Traditional approaches to turbulent combustion numerical modeling apply Reynolds averaging techniques (RANS) to predict the behavior of the mean values of the reacting flow properties. In this way, unsteady effects are not taken into account in the formation of nanoparticles. Large Eddy Simulation represents an attractive methodology for studying turbulent reacting flows and this approach is becoming possible as computational resources are increasing. A LES approach involves the direct numerical resolution of the large turbulence scales while the small ones and their interaction with the large-scale flow is modeled. The chemistry model used here is based on the mixture fraction transport so that turbulent combustion is modeled as a simple laminar diffusion reactor in an unsteady straining environment created by turbulent advection. In flamelet models the explicit dependence on velocity is removed, from the species concentration transport equation, by relating scalars to the mixture fraction which, in turn, is related to the velocity field. A reaction progress variable is introduced to take into account the transition from premixed combustion occurring near the flame base to non-premixed combustion occurring downstream in the jet flame. In the present study, LES of a turbulent ethylene jet diffusion flame are performed with the aim of computing the formation of nano-organic particles; the results are then compared with an ad hoc experiment.