Large Eddy Simulation of the Diffusion Flame Characteristics of Liquid Fuel Films

This study presents three-dimensional large eddy simulations of fuel film flames to clarify the effects of the quiescent ambient pressure and oxygen content on the flame characteristics of isooctane fuel films in a constant-volume chamber. The ambient pressure, P _amb , and oxygen content, χ _O2 , were set to 2–5 bars and 16–30%, respectively. The results indicate that, regarding the effect of P _amb , the flame characteristics of the fuel film transition from a laminar flame with flickering to a turbulent flame where the flame oscillates irregularly in the horizontal direction as P _amb increases. The flame characteristics are principally influenced by the behaviour of the vortex rings that are periodically formed around the flame. As the buoyancy effect increases with increasing P _amb , the axisymmetry of the vortex rings disappears early in the formation process, leading to rapid breakdown. Consequently, the flame behaviour becomes turbulent. The χ _O2 conditions affect the flame temperature of the fuel film flame. The change in flame temperature with χ _O2 affects not only the buoyancy effect but also the viscous effect acting on the flame. The rate between both effects changes the axisymmetry of the vortex rings formed around the flame, consequently leading to changes in the flame characteristics. This effect is more noticeable when χ _O2 is lower than it is under atmospheric conditions. The relationship between the flame characteristics of the fuel film and the rate of buoyancy and viscosity effects acting on the flame can be expressed by the Grashof number, Gr, considering the P _amb and flame temperature proposed in the present study. The proposed Gr indicates that the Gr of a flame is proportional to the square of P _amb but inversely proportional to the cube of the flame temperature.