Near Field Characteristics of a Rectangular Jet and Its Effect on the Liftoff of Turbulent Methane Flame

Abstract

Understanding the stability of turbulent flames is a key for the design of efficient combustion systems. The present paper reports an experimental study on the effect of the internal geometry of a rectangular orifice on the characteristics/stability of a turbulent methane flame. Three rectangular nozzles with different orifice lengths having an identical exit aspect ratio (AR) of 2 were used. The coairflow strength was also varied to evaluate its effect on the jet flow emerging from the rectangular nozzle. The experimental data revealed that the jet initial conditions affect both the flow characteristics and the liftoff of turbulent diffusion methane flame. That is, increasing the orifice length of the rectangular nozzle resulted in delaying the occurrence of the axisswitching phenomenon, reducing the length of the jet potential core, and accelerating the liftoff transition of the attached flame. The coairflow was found to reduce the velocity strain rate in the shear layer, displace the occurrence of axisswitching farther downstream of the jet, and delay flame detachment. The results revealed also that there is a clear interplay between the flame liftoff and the jet nearfield molecular mixing and flow characteristics. That is, a rectangular jet which spreads faster and generates higher nearfield velocity strain and turbulence intensity causes flame detachment at a lower fuel jet velocity. Based on this, a correlation was found between the flame liftoff velocity, the fuel molecular thermal diffusivity, the stoichiometric laminar flame speed, and the fuel jet strain rate at the nozzle exit. This relationship was shown to successfully predict the liftoff velocity of methane flame as well as other common gaseous hydrocarbons and hydrogen flames.