Numerical investigation of time-dependent properties and extinction of strained methane and propane-air flamelets

Abstract

Laminar premixed strained flames are numerically modeled for time-dependent strain rates and pressure. The simulations include detailed chemistry in terms of a set of elementary reactions, as well as a multispecies transport model. The motivation to consider time dependence in modeling of strained flames is the consequence of the concept of strained laminar flamelets in unsteady turbulent combustion, such as occurs in internal combustion engines where the turbulent flow field and pressure are subject to large temporal variations. Nonstationary conservation equations for strained flames have been derived including one-parameter (strain rate or tangential pressure gradient) and two-parameter (tangential pressure gradient and flow field divergence) formulations. These equations then have been used to study the following problems: Extinction limits of methane-air flames at different equivalence ratios in the stationary case, to test the model and the input data used (e.g., different levels of methane oxidation chemistry). Influence of temporally periodical change of the strain rate on the flame front behavior. Actually, a lean methane-air flame with sinusoidally varying strain rate is simulated numerically. Influence of simultaneous application of strain and time-dependent pressure decrease, as occurs in the decompression phase in an Otto engine. A propane-air flame (starting at P = 8 bar) is used to elucidate pressure decrease at strain rate values typical in this environment. Temporal behavior of a strained flame front to determine the characteristic extinction time. Lean and rich propane-air flames are considered during quenching under the influence of a constant strain rate. The results are discussed and compared-as far as possible-with experimental and computational results given in the literature.