Unsteady Flamelet Response in the Near Field of High-Reynolds-Number Jets

Abstract

In this work, we perform numerical studies of the unsteady response of laminar diffusion flamelets relevant to the near field (x/d < 30) of high-Reynolds-number gaseous-fuel jets that are injected into high-pressure and high-temperature chambers. A large-eddy simulation database of a 70,000-Reynolds-number variable-density round jet is employed to compute turbulent time histories of the scalar dissipation rate in the near field. With this information, studies of diffusion flamelets subjected to scalar dissipation rate fluctuations are performed in which the unsteady flamelet equations are solved with the assumption of unity species Lewis number. The commonly employed diesel fuel surrogate n-heptane is chosen as the fuel, and its oxidation chemistry is modeled by a kinetic mechanism incorporating 159 species among 1540 reaction steps. Results show that in the simulated near field of the 70,000-Reynolds-number jet, transient temporary flame-weakening events followed by flame recovery are probable. Although the flame temperature, major species, and pollutants such as unburned hydrocarbons show a relatively fast response and good agreement with steady flamelet predictions, the pollutant, nitric oxide, responds with a significant phase lag, rendering steady flamelets inadequate. The analysis is extrapolated to higher-Reynolds-number jets with higher-intensity scalar dissipation rate fluctuations, in which transient flame-extinction/reignition events are observed. The applicability of steady flamelets to predict temperature and species responses during extinction/ reignition is assessed, and the implications of extinction/reignition events for jet near-field phenomena, such as flame liftoff, are explored.