Dynamics and Chemistry of Laminar and Turbulent Expanding Flames

Abstract

The present work consists of a series of experimental studies on expanding laminar and turbulent flames. The first goal is to use the laminar stable expanding flame to study laminar flame speed and flame chemistry. The second goal is to study the dynamics and stability of premixed flames with/without the presence of turbulence. A unique dual-chamber, high-pressure, fan-stirred, preheated combustion vessel was developed as the main experimental apparatus. The stretch extrapolation of laminar flame speeds from experimental raw data is a state-of-the-art practice. Its associated uncertainty was first quantified using numerical simulations of spherical expanding flames. Then laminar flame speeds of cyclo-alkanes, butanol isomers, toluene, <italic>o</italic>-xylene and mixtures of H2 with C1-C4 hydrocarbons were acquired for a wide range of pressures (1-20 atm). The molecular structure effects on the oxidation chemistry of cyclic alkanes and butanol isomers were studied. On the dynamics of premixed flames, the self-acceleration of expanding flames due to Darrieus-Landau and diffusional-thermal instabilities was quantified through experimentation on hydrogen flames. Results show that most cellular flames exhibit self-similar acceleration, which suggests that the wrinkled flame surface can be described by a fractal. Additionally, high pressure and high turbulent Reynolds number turbulent flames propagating in near isotropic turbulence were studied. Results show that the turbulent flame speeds can be scaled by a Reynolds number defined based on the properties of the corresponding laminar flame and the flow. Finally, spark ignition in turbulent flows was studied. Results showed that turbulence can enhance ignition for mixtures with sufficiently large Lewis number, which is contrary to the general belief that turbulence always renders ignition more difficult.