Numerical study on dynamics of local flame elements in turbulent jet premixed flames

Abstract

Direct numerical simulations (DNSs) of turbulent jet premixed flames have been conducted to investigate dynamics of local flame elements. Behaviors of turbulent jet flames are discussed by considering characteristics of local flame elements such as flame displacement speed, local turbulent burning velocity, curvature and tangential strain rate. The peak of probability density function (pdf) of the local turbulent burning velocity locates at laminar burning velocity, whereas the maximum turbulent burning velocity reaches to about 5 times laminar one and the minimum burning velocity is negative. The result indicates that dynamics of turbulent flame cannot be explained by the flamelet concept. The concept of flame stretch is evaluated by growth rate of the flame area, which is a function of flame displacement speed, flame curvature and tangential strain rate. In the concept of flame stretch, the time-averaged growth rate must be zero because flame front should stay in a certain area statistically. However, in DNS, the mean growth rate is not zero, which suggests that flame does not exist as a statistically steady state. The present results indicate a possibility of the existence of flame elements which cannot be explained by the flamelet concept and the concept of flame stretch.