Abstract
A systematic study relying on Direct Numerical Simulations (DNS) of premixed hydrogen-air mixtures has been performed to investigate the hotspot ignition characteristics and ignition probability under turbulent conditions. An ignition diagram is first obtained under laminar conditions by a parametric study. The impact of turbulence intensity on ignition delays and ignition probability is then quantified in a statistically-significant manner by repeating a large number of independent DNS realizations. By tracking in a Lagrangian frame the ignition spot, the balance between heat diffusion and heat of chemical reaction is observed as function of time. The evolution of each chemical species and radicals at the ignition spot is checked and the mechanism leading to ignition or misfire are analyzed. It is observed that successful ignition is mostly connected to a sufficient build-up of a HO2 pool, ultimately initiating production of OH. Turbulence always delays ignition, and ignition probability goes to zero at sufficiently high turbulence intensity when keeping temperature and size of the initial hotspot constant.
Highlights
Ignition is an important and complex issue, which has been extensively investigated during many decades
Results are presented in a statistical manner towards characterizing successful ignition events relevant to safety issues
The different critical temperature between S0D and S1D is due to heat diffusion to the cold surroundings in S1D case, which does not exist in S0D and obviously impacts noticeably ignition delay
Summary
Ignition is an important and complex issue, which has been extensively investigated during many decades. In order to obtain results independent from a specific geometry or confinement, only convection and diffusion are retained in the present study, radiative transfer in the gas phase being of minor importance for open-flow stoichiometric hydrogen-air mixtures. A parametric study of auto-ignition scenarios for lean n-heptane/air [20] and hydrogen/air [21] mixtures with thermal stratification at constant volume and high pressure have been later conducted using 2D DNS, concentrating on the influence of imposed initial temperature fluctuations T and of the ratio of turbulence to ignition delay timescale. Further DNS studies considered 3D flames with complex kinetic schemes (see e.g., [22]) but did not investigate the ignition mechanism. Results from homogeneous laminar configurations are first presented (Section 3.1) to validate the employed numerical models with corresponding experimental data, and obtain a laminar ignition diagram. The findings obtained by tracking the hotspot in a Lagrangian frame are presented (Section 3.3), before concluding
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