# Numerical Simulation of Natural Gas/Hydrogen Combustion in a Novel Laboratory Combestor
## 1 Introduction
Their numerical simulations reveal that hydrogen addition reduces the flame length, but this effect is attenuated with the increase of the swirl number. The NO reaction pathways in an unconfined swirl burner at constant thermal power were computationally investigated by [PERSON] et al. [12]. They found that, for the studied conditions, prompt NO contributes the most to NO formation for hydrogen molar fractions up to 60%, even though its relative contribution decreases with the increase of the hydrogen content. Although thermal NO becomes more important with hydrogen addition, it only accounts for about 25% of the total NO emissions at 60% H\({}_{2}\) (vol.) in the fuel due to the relatively low flame temperatures caused by co-flow air and dilution. The importance of the NNH (diazenyl radical) pathway increases significantly, contributing to around 20% of the total NO emissions with hydrogen enrichment of 60%. The N\({}_{2}\)O pathway for NO production is the least important. Most studies on swirling flames have been conducted under confined conditions for premixed, non-premixed of partially premixed flames. The influence of the swirl intensity on the premixed combustion of natural gas/hydrogen or methane/hydrogen mixtures was explored by [PERSON] et al. (2013) and [PERSON] et al. (2014). The former study found that CO and NO emissions gradually decreased with the increase of the swirl intensity, while combustion performance improved when the swirl angle was set to 45\({}^{\circ}\). The latter one reported that increasing swirl intensity enlarges the inner recirculation zone, shortens the flame, and raises the temperature. The flame structure of CH\({}_{4}\)/H\({}_{2}\) flames has been extensively investigated. [PERSON] and [PERSON] (2015) studied non-premixed flames with hydrogen contents ranging from 0% to 100% in a co-flow swirl burner, maintaining a constant volumetric fuel flow rate. Their results showed that hydrogen addition shortened the flame, bringing it closer to the burner head. Building on their previous work ([PERSON] et al., 2013), [PERSON] et al. (2016) found that under confined conditions, adding hydrogen to methane enhanced the flame stability, shifted the reaction zone upstream, and increased the peak flame temperature. The increase in flame temperature with hydrogen addition was corroborated by other studies ([PERSON] et al., 2014; [PERSON] et al., 2015; [PERSON] et al., 2015), while the reduction in flame size was confirmed by [PERSON] et al. (2016) and [PERSON] et al. (2017). In the latter studied, [PERSON] et al. observed that the flame structure remained nearly unchanged for hydrogen contents up to 5%. However, at concentrations above 11%, the flame shrank and expanded towards the wall. Most studies on emissions from swirling CH\({}_{4}\)/H\({}_{2}\) flames have reported a decrease in CO emissions with increasing hydrogen molar fraction in the fuel, e.g., ([PERSON] et al., 2014; [PERSON] et al., 2015; [PERSON] et al., 2015; [PERSON] et al., 2015), [PERSON] et al. (2016). This reduction is primarily due to the lower carbon content in the fuel and the enhanced formation of OH radicals with higher hydrogen content, which promote CO oxidation. However, the extent of CO reduction varies between studies. Some, like [PERSON] et al. (2017), observed a small decrease, while others, such as [PERSON] et al. (2016), reported a significant reduction. Moreover, some studies have reported opposite trends. [PERSON] and [PERSON] (2016) experimentally investigated hydrogen enrichment in swirling and non-swirling inverse diffusion flames. At constant energy per unit mass input, CO emissions decreased with increasing hydrogen content. However, when hydrogen enrichment was conducted at a constant fuel volumetric flow rate, CO emissions increased. This latter trend aligns with the findings of [PERSON] and [PERSON] (2015), who observed a monotonic increase in CO emissions for hydrogen molar fractions up to 0.8 in a co-flow swirl burner while maintaining a constant volumetric fuel flow rate.