On the characteristics of laminar lifted flames in a nonpremixed jet

Characteristics of laminar lifted flames stabilized in a nonpremixed jet issuing from a small nozzle into ambient air have been studied experimentally. Results show that near the flame base the lifted flame has a tribrachial structure composed of a lean premixed flame, a rich premixed flame, and a diffusion flame extending downstream from the same point. A simple theoretical formula for the liftoff height of nonpremixed jet flames based on a boundary layer type of jet theory predicts that the liftoff height increases with increasing flow rate for Schmidt numbers in the range Sc > 1 or Sc < 0.5, while it decreases for 0.5 < Sc < 1, which is physically unrealistic. This is consistent with the experimental observation that propane and n-butane jets (Sc > 1) have lifted flames while methane and ethane (0.5 < Sc < 1) exhibit blowout directly from the attached flames. A correlation for the explicit dependence of the liftoff height on the flow rate and the nozzle diameter was derived from the experiments for propane and butane and shown to agree well with the theory. This finding emphasizes the importance of the tribrachial flame, approximated as a stoichiometric premixed flame, in determining the characteristics of lifted laminar nonpremixed jet flames.