Opposed jet burner and Tsuji burner for representative laminar flamelet with differential molecular diffusion for non-premixed combustion modeling

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Modeling differential molecular diffusion in turbulent non-premixed combustion remains challenging for flamelet models. Laminar flamelet is a key building block of flamelet models for turbulent combustion. One significant challenge that has not been adequately addressed is the representativity of laminar flamelet for the characteristics of differential molecular diffusion in turbulent combustion. Laminar flamelet is typically generated based on two conceptual burner configurations, the opposed jet burner and the Tsuji burner. The two burners are commonly viewed to be equivalent for the description of laminar flamelet structures. In this work, a major difference between them is revealed for the first time when they are used to represent differential molecular diffusion. The traditional opposed jet burner yields almost fixed equal diffusion locations defined as the points with equal values of mixture fractions based on different elements in the mixture fraction space. The Tsuji burner, with a slight extension, can produce a continuous variation of the equal diffusion locations in the mixture fraction space. This variation of the equal diffusion locations is shown to be an important feature of non-premixed combustion, as demonstrated in a laminar jet mixing layer, a turbulent jet mixing layer, and a turbulent jet non-premixed flame. Theoretical analysis is conducted based on an idealized opposed jet mixing layer to identify the major cause of the difference between the two burners. It is found that the inlet diffusion caused the major difference in differential molecular diffusion in the two burners. The curvature difference, another main difference between the two burners, however, is not found to cause any major difference in differential molecular diffusion in the two burners. Based on the finding, a modified opposed jet burner with disabled inlet diffusion is introduced and is shown to be able to reproduce the feature of differential molecular diffusion observed in the Tsuji burner. Both burners, after properly generalized with suitable boundary conditions and additional parameters (like the velocity ratio of fuel and oxidizer), are expected to be suitable and equivalent choices for the generation of representative laminar flamelets that can be used eventually in flamelet modeling of differential molecular diffusion in turbulent non-premixed combustion.Novelty and significance statement: (1) A significant difference between the opposed jet burner and the Tsuji burner is revealed for generating representative laminar flamelet with differential molecular diffusion (under specific conditions); (2) The flamelet generated from the Tsuji burner is shown to be representative of differential molecular diffusion found in practically relevant mixing and combustion problems; (3) The treatment of inlet diffusion is identified to be the major cause of the difference observed between the two burners; (4) Modifications are introduced to the opposed jet burner to reconcile the two different burners for producing representative flamelet that can be used in the modeling of differential molecular diffusion in turbulent non-premixed combustion.