Numerical analysis of the influence of two-phase flow mass and heat transfers on n-heptane autoignition
Résumé
This paper investigates the influence of liquid fuel presence on the au- toignition of n-heptane/air mixtures over a wide range of conditions en- countered in internal combustion engines. To this end, evaporating droplet physics and skeletal chemistry mechanisms are simultaneously solved con- sidering a homogeneous constant-pressure reactor. A skeletal mechanism is introduced to account for specific kinetics behavior in the Negative Tem- perature Coefficient (NTC) region. The impact of mass and heat source terms during evaporation is emphasized by comparing a two-phase flow sce- nario with a purely gaseous case. The competition between fuel vapor avail- ability and the evaporation-induced gas temperature decrease is specific to two-phase flow autoignition. On the one hand, droplet evaporation delay restricts the gaseous local fuel/air equivalence ratio and consequently the kinetics runaway. On the other hand, temperature reduction due to evapora- tion may either reduce or enhance chemical reactivity depending on the local thermodynamic conditions lying either inside or outside the NTC region. By simultaneously accounting for evaporation source terms and skeletal chem- istry, we can reproduce the already experimentally observed transformation of the NTC region into a Zero Temperature Coefficient (ZTC) region depend- ing on thermodynamic conditions and droplet size. The ZTC phenomenon appears when combustion heat-release starts before complete droplet evap- oration. Since the ZTC behavior can be captured using the point source approach, in which droplets are considered only as zero-dimensional source terms of mass and energy, the present results pave the way for future ex- ploration of NTC chemistry in sprays with a direct numerical simulation of discrete particles considering detailed chemistry and turbulent flows.