applied sciences Article  
Large Eddy Simulation of the Effect of Hydrogen Ratio on the Flame Stabilization and Blow-Off Dynamics of a Lean CH4/H2/Air Bluff-Body Flame  
[PERSON], [PERSON],*(c), [PERSON], [PERSON] and [PERSON]  
Footnote  
Lean premixed combustion (LPC) is viewed as one of the most promising combustion techniques in the context of gas turbines due to its capability to significantly reduce \(\rm NO_{x}\) emissions [7; 8; 9]. However, the implementation of this technique is associated with the risk of flame instability and blow-off [10; 11]. Several studies have suggested that blending hydrogen with carbon oxides (i.e., methane or DME, etc.) as the fuel can reduce the emission of carbon oxides, exergy losses, and the likelihood of blow-off [12; 13; 14; 15]. This is because hydrogen addition enhances the combustion intensity of the \(\rm CH_{4}/air\) flame. However, it may increase the risk of flashback, especially in high-hydrogen-ratio conditions. Although enriching methane with hydrogen may lead to an increase in \(\rm NO_{x}\) emissions due to the higher burnt gas temperature, this issue can be addressed by LPC [16]. Meanwhile, fueling gas turbines with hydrogen is also an efficient way to consume hydrogen produced from wind or solar energy [17]. The bluff body and swirl geometries are widely used to anchor and stabilize the flame in a GT combustion chamber by establishing a recirculation zone (RZ) of hot gas near the burner outlet, igniting the fresh gas from the burner [18; 19]. Turbulent premixed flames stabilized by bluff bodies have received considerable attention [20]. [PERSON] et al. reviewed the recent progress made in flame stabilization methods for micro-combustors [21]. They investigated single-flame stabilization technologies including flame holders, and pointed out that a bluff body is a classic flame-anchoring method that has been extensively employed in various industrial combustion devices. [PERSON] et al. studied a short lean flame stabilized by bluff bodies using a fast-response apparatus in pure \(\rm CH_{4}/air\) conditions [22]. They described the mechanism of the blow-off process and mentioned that the structure of the flame close to extinction supported the underlying assumptions behind well-stirred reactor concepts of blow-off. [PERSON] et al. examined the recirculation zone of a bluff body-stabilized \(\rm CH_{4}/air\) flame numerically and experimentally [23]. They analyzed the flame's characteristics under different conditions and demonstrated that the position of the outer recirculation zone was affected by the size of the bluff body and the swirl strength. A simplified mechanism with only 16 species was used in their study. A high-precision LES with a more detailed chemical mechanism is needed for the further study of flame stabilization mechanisms. In addition, these studies focused on the stabilization mechanism of bluff-body \(\rm CH_{4}/air\) flames, but hydrogen addition can also largely influence flame characteristics and stabilization mechanisms. [PERSON] et al. researched the effect of hydrogen addition in premixed \(\rm CH_{4}/air\) flames experimentally using a maximum hydrogen ratio of 9% [24]. They found that low swirl intensity assisted in reducing the cold-flow recirculation into the reaction zone and elevated the stability limit at lower adiabatic temperature conditions. Furthermore, the addition of hydrogen shifted the reaction zone to the upstream locations of the flame. [PERSON] et al. numerically investigated the role of mixture composition and temperature in defining the steady-flow structure in premixed \(\rm CH_{4}/H_{2}/air\) flames stabilized by bluff bodies [25]. They showcased the flow structures and flame stability maps for fuels with different hydrogen ratios and inlet conditions using the extinction strain rate as the chemical time scale. Their simulation was performed for a 2D domain, and the results yielded rather qualitative implications.