# Numerical Simulation of Natural Gas/Hydrogen Combustion in a Novel Laboratory Combestor
## 1 Introduction
Most studies have reported an increase in NO\({}_{x}\) emissions with higher hydrogen content in the fuel (e.g., ([PERSON] et al., 2014; [PERSON] et al., 2015; [PERSON] et al., 2015; [PERSON] et al., 2015; [PERSON] et al., 2015)). This is primarily due to the rise in adiabatic flame temperature, which promotes thermal NO formation, typically dominant at temperatures above 1800 K. Additionally, higher hydrogen content increases the concentration of O, H, and OH radicals, further enhancing NO formation. However, the magnitude of the NO\({}_{x}\) increase varies widely among studies, being small in some (e.g., ([PERSON] et al., 2016)) and significant in others (e.g., ([PERSON] et al., 2015)). [PERSON] et al. (2016) pointed out that increasing the swirl intensity or the excess air could help mitigate NO\({}_{x}\) emissions. Interestingly, not all studies found a direct correlation between hydrogen content and NO\({}_{x}\) emissions. [PERSON] et al. (2017) observed no significant effect, while [PERSON] and [PERSON] (2016) reported lower NO\({}_{x}\) emissions with hydrogen enrichment at a constant fuel volumetric flow rate. These discrepancies highlight that the sensitivity of both CO and NO\({}_{x}\) emissions to fuel composition depends on burner design, operating conditions, and the specific constraints of each study, such as maintaining constant the Reynolds number, the thermal input, the volumetric fuel flow rate, etc. The stability of CH\({}_{4}\)/H\({}_{2}\) turbulent swirling flames has also been addressed in the literature. [PERSON] et al. [26] experimentally investigated the transition between stable and unstable combustion in a swirl-stabilized premixed combustor. They observed that combustion remained stable at low equivalence ratios but developed harmonic oscillations above a critical equivalence ratio, regardless of the fuel composition. [PERSON] et al. [27] conducted a numerical investigation of a swirl vortex-tube combustor. Their results show that the local flow velocity near the reaction zone decreases, while the tangential velocity remains high due to the strong vortex flow. This effect inhibits blow-out and enhances stability. [PERSON] et al. [28] performed a numerical study on flashback in a bluff-body swirl burner, demonstrating that the preferential diffusion of H\({}_{2}\) significantly increases flashback susceptibility. [PERSON] et al. [29] experimentally investigated flame stabilization mechanisms and emissions in a dual swirl low-NO\({}_{x}\) burner. Hydrogen was introduced into the combustor through a central tube with an internal swirler, while a methane-air mixture was injected via an annular swirler. The swirl imparted to the hydrogen stream improved flame stabilization, resulting in low NO\({}_{x}\) emissions. Additionally, a small recess in the central injector significantly extended the operability range of the burner. NO\({}_{x}\) emissions were further reduced at higher thermal power for a fixed equivalence ratio. [PERSON] et al. [30] conducted large eddy simulations of these flames. Their results indicated that while hydrogen injection had a minimal impact on the overall flame structure, it led to a more intense reaction. [PERSON] et al. [31] investigated the influence of a pilot flame on swirling turbulent premixed CH\({}_{4}\)/H\({}_{2}\) flames in a laboratory gas turbine combustor. Their experiments demonstrated that the pilot flame extended the stable flame regime and that the lean bluff-off equivalence ratio decreased with increasing the hydrogen content in the fuel. A few recent works studied swirling flames in the moderate or intense low-oxygen dilution (MILD) combustion regime. [PERSON] et al. [32] conducted a numerical study of non-premixed CH\({}_{4}\)/H\({}_{2}\) swirling flames in a hot co-flow emulating MILD combustion. Their results showed that swirl extended the reaction zone radially while reducing its length compared to the purely MILD combustion regime. In addition, swirl decreased CO production but enhanced NO\({}_{x}\) emissions.