A PDF method for HCCI combustion modeling : CPU time optimization through a restricted initial distribution

Abstract

Probability Density Function (PDF) is often selected to couple chemistry with turbulence for complex reactive flows since complex reactions can be treated without modeling assumptions. This paper describes an investigation into the use of the particles approximation of this transport equation approach applied to Homogeneous Charge Compression Ignition (HCCI) combustion. The model used here is an IEM (Interaction by Exchange with the Mean) model to describe the micromixing. Therefore, the fluid within the combustion chamber is represented by a number of computational particles. Each particle evolves function of the rate of change due to the chemical reaction term and the mixing term. The chemical reaction term is calculated using a reduced mechanism of n-heptane oxidation with 25 species and 25 reactions developed previously. The parametric study with a variation of the number of particles from 50 up to 10 4 has been investigated for three initial distributions. The numerical experiments have shown that the hat distribution is not appropriate and the normal and lognormal distributions give the same trends. As expected when the number of particles increases for homogenous mixture (i.e. high turbulence intensity), the in-cylinder pressure evolution tends towards the homogeneous curve. For both homogeneous and inhomogeneous (i.e. low turbulence intensity) cases, we have found that 200 particles are sufficient to model correctly the system, with a CPU time of a few minutes when a restriction of initial distribution is adopted.

Optical and Numerical Investigations on Combustion and OH Radical Behavior Inside an Optical Engine Operating in LTC Combustion Mode

“…In previous studies [16,17], the micro-mixing has been described through an IEM (Interaction by Exchange with the Mean) model. The main advantages of IEM model are its simplicity and its short computational time.…”

“…In addition, the above-mentioned studies have shown that the turbulent time scale introduced by the IEM model requires physical description instead of a fixed value parameter. Moreover, to further reduce the computation time, random initial distribution was restricted via a fixed parameter [17]. This latter also has an important effect on the solution.…”

“…OH radical was simulated by the previous semi-detailed n-heptane kinetic mechanism containing 37 species and 61 elementary chemical reactions [18] and compared to experimental results. Note that this reduced mechanism is more accurate to simulate species evolutions compared to the reduced n-heptane mechanism with 25 species and 26 elementary chemical reactions used in the previous studies [16,17,19]. On another hand, the confidence coefficient allowing the restriction of random initial distribution describes the initial heterogeneities amplitude of temperature and of species mass in the combustion.…”

“…The previous studies [16,17] have shown that normal and/or lognormal distributions are suitable to generate random initial mass and temperature for each particle. In addition, the same previous studies have also shown that 200 particles are sufficient to reach good accuracy.…”