Experimental Study on the Effects of Reynolds Number Ratio on Flame Stability, Macrostructure, and Emissions in Dual Annular Burner for CH₄ and CH₄/H₂ Combustion

Flame stability is crucial for many industrial and aviation applications. Flow parameters, notably Reynolds number and velocity, significantly affect combustor power and flame characteristics. Thus, the influence of the Reynolds number ratio (RR = Re_out/Re_in) from 1 to 2.5 was investigated for CH₄ and CH₄/H₂ fuel in a dual annular jet burner. Twenty percent of the fuel volume for CH₄/H₂ combustion was hydrogen. For each RR, the global equivalence ratio varied from stoichiometric to lean blowoff (φ_g = 0.48), while power ranged from 2.9 to 17.8 kW. The static stability analysis indicates that increasing the RR decreases the lean blowoff limit by approximately 2%. Reynolds number ratios also change the flame macrostructure transitioning from M-shaped flame to V-shaped flame for methane combustion at stoichiometric conditions. At RR = 1, three shear layers are anchored at the central bluff body and inner annulus in the M-shaped flame. However, at higher RR, only the center shear layer is anchored on the bluff body forming V-shaped flame. For CH₄/H₂ combustion at stoichiometric conditions, the flame shape forms the M shape consistently with the increased flame front for RR of 1–2.5. Additionally, emission analysis showed that higher RR produced more thermal NO_x at higher combustor power, while CO emission was found to be highest at RR = 1.5 for methane combustion. For hydrogen-blended methane combustion, the emission of CO and NO_x were found to be decreased compared to methane combustion.