A Review of Fundamental Studies Relevant to Flame Lift-off in Diesel Jets

Recent experiments have shown that flame lift-off, fuel-air premixing and soot formation in Diesel jets are interrelated. Hence, understanding and characterizing lift-off is important. Experimentally observed dependence of lift-off on injection pressure, injector orifice diameter, chamber temperature, density and O₂ concentration are discussed. Theories for lift-off in atmospheric gas jet diffusion flames and supporting experimental and numerical work are reviewed. The relevance of these theories to flame stabilization in Diesel jets is explored. In addition, key differences between lift-off in atmospheric gas jets and Diesel sprays are highlighted. Some of the recent computational models employed to predict lift-off in Diesel jets, including recent computations of flame lift-off employing a representative interactive flamelet model with multiple flamelets, are described. It is shown that all the models are able to predict the measured trends in lift-off for varying injection pressures, chamber temperatures and O₂ concentrations. The underlying assumptions in the models and associated uncertainties are critically examined. The role of fundamental direct-numerical-simulation studies in clarifying the mechanisms of lift-off is discussed.