Effect of slot positions on multi-regime turbulent combustion of CH4-air flame: A numerical study

The presence of inhomogeneities in the premixed and non-premixed fuel-air mixtures leads to the formation of multi-regime combustion. This study numerically investigates the influence of slot-2 positions on the re-circulation characteristics of a multi-regime burner (MRB) operating under premixed and non-premixed <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">CH</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> -air combustion conditions. The axisymmetric, steady-state, compressible turbulent flow field was simulated using the standard <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mi>κ</mml:mi> <mml:mo>−</mml:mo> <mml:mi>ϵ</mml:mi> </mml:mrow> </mml:math> turbulence model coupled with the Eddy Dissipation Concept (EDC) to resolve the reactive flow dynamics. Detailed chemical kinetics incorporating <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mn>36</mml:mn> </mml:mrow> </mml:math> species and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mn>218</mml:mn> </mml:mrow> </mml:math> reactions based on the GRI-3.0 mechanism were implemented. Five slot-2 positions were examined by varying slot-1 wall thickness for equivalence ratios <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>ϕ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>j</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>1</mml:mn> <mml:mo>.</mml:mo> <mml:mn>8</mml:mn> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>.</mml:mo> <mml:mn>6</mml:mn> </mml:mrow> </mml:math> at slot-1 velocity 15 m/s. Numerical results of the mixture fraction, temperature, major species, minor species, and progress variable were validated against prior experimental and numerical datasets. The analysis revealed that varying the slot-2 position significantly alters the radial width of the re-circulation zone and the flame neck thickness. The peak of the temperature ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mo>≈</mml:mo> <mml:mn>2200</mml:mn> </mml:mrow> </mml:math> K) and enhanced minor species concentrations ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>Y</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">CO</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>0</mml:mn> <mml:mo>.</mml:mo> <mml:mn>0465</mml:mn> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>Y</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>0</mml:mn> <mml:mo>.</mml:mo> <mml:mn>0031</mml:mn> </mml:mrow> </mml:math> ) were found in the inner premixed reaction zone. The outer premixed region exhibited a gradual decline in temperature and mixture fraction due to entrainment of co-flowing air, leading to a shift of the outer reaction zone downstream. These findings highlight the strong geometric sensitivity of turbulent flame structure and stabilization in MRB configurations.