Dynamic Behavior of Premixed Flames Propagating in Non-Uniform Velocity Fields —Assessment of Intrinsic Instability in Turbulent Combustion—

Abstract

The dynamic behavior of premixed flames propagating in non-uniform velocity fields was investigated to assess the significance of intrinsic instability in turbulent combustion. Two-dimensional unsteady calculations of reactive flows based on the compressible Navier-Stokes equations including a one-step irreversible chemical reaction were performed. A sinusoidal disturbance was superimposed on the velocity field of the unburned gas, and its wavelength was set equal to the linearly most unstable wavelength, i.e. the critical wavelength, at the Lewis number Le=1.0. To investigate the effects of intrinsic instability on the dynamic behavior of premixed flames, we treated two basic types of intrinsic instability, i.e. hydrodynamic instability and diffusive-thermal instability. The dynamic behavior of cellular flames generated both by intrinsic instability and by velocity disturbances was numerically simulated. When Le=1.0, at the initial evolution, we observed a cellular flame whose cell size was equal to the wavelength of velocity disturbances. After that, cells combined together, and one large cell appeared. When Le=0.5, on the other hand, several cells smaller than the wavelength of velocity disturbances were found, and the combination and division of cells were observed. This is because that the size of cells caused only by intrinsic instability is smaller than the wavelength of velocity disturbances. Thus, the dynamic behavior of premixed flames is drastically affected not only by velocity disturbances but also by intrinsic instability. The burning velocity of cellular flames propagating in non-uniform velocity fields was larger than that of planar flames, since cellular flames had larger surface area. The burning velocity became larger as the intensity of velocity disturbances became higher, and the dependence of the burning velocity on the intensity was strongly affected by the Lewis number. The relative significance of intrinsic instability on the evolution of turbulent premixed flames was identified by comparing the growth rate and production rate of flame fronts due to intrinsic instability and turbulence. In the region where the growth rate due to intrinsic instability was larger than that due to turbulence, the dynamic behavior was strongly affected by intrinsic instability. This region includes the combustion conditions of many industrial combustors, and then intrinsic instability plays a significant role in the characteristics of turbulent combustion.