# Numerical Simulation of Natural Gas/Hydrogen Combustion in a Novel Laboratory Combestor
## 1 Introduction  
The world is facing an unprecedented and fast transformation in energy supply sources. In the 1970s, fossil fuels accounted for more than 80% of global primary energy consumption [1]. Nowadays, the share of renewable energy sources is rising sharply, and projections indicate that fossil fuels will play a significantly reduced role in the near future [2]. This shift in the global energy paradigm is primarily driven by concerns over climate change, which is strongly linked to greenhouse gas emissions. A substantial portion of these emissions originates from the combustion of fossil fuels, making their reduction a key strategy in mitigating global warming [3]. Moreover, renewable energy sources help reduce dependence on external energy supplies, thereby enhancing resilience to pandemics, environmental disasters, and geopolitical disruptions. However, the shift towards alternative energy sources requires gradual implementation and cannot be accomplished in a short timeframe [4]. Meanwhile, cleaner fuels are emerging as promising alternatives to traditional fossil fuels. Among them, hydrogen stands out as a viable option [5]. However, several challenges must be addressed before hydrogen can be widely adopted. Although hydrogen is the mostabundant element in the universe, it does not exist in a free state on Earth and is instead bound to other elements in compounds such as water and methane. Extracting hydrogen requires energy-intensive processes such as water electrolysis or methane reforming, which should ideally be powered by renewable energy sources to minimize carbon emissions [6]. Another major challenge is the lack of infrastructure for hydrogen transport and distribution. While existing natural gas pipelines can accommodate hydrogen when blended in low concentrations, large-scale hydrogen transport remains a technical and economic hurdle [7]. Additionally, hydrogen storage presents significant challenges due to its low density, high diffusivity, material compatibility issues arising from hydrogen embrittlement, high permeation and leakage potential, and the associated risks of explosion. Safe and efficient storage solutions are under development but require further advancements for widespread implementation [7]. In addition to the problems associated with production, transport, and storage, the use of hydrogen as a fuel is also challenging. In fact, hydrogen has a high laminar flame speed, making it prone to flashback in premixed combustion systems, and a low ignition energy and extended flammability range, that may cause premature or unintended ignition. Accordingly, it cannot be readily burnt [8]. An interesting alternative that has been widely investigated, particularly in the last 2 decades, is the blending of hydrogen with natural gas or methane [9]. The present work reports a computational investigation of a novel swirl combustor burning mixtures of natural gas (NG) and hydrogen, with the molar fraction of hydrogen in the fuel ranging from 0% to 100%. Among the methods used to stabilize turbulent flames, swirl is one of the most widely used ones, and several works addressing the combustion of \(\mathrm{CH_{4}/H_{2}}\) or \(\mathrm{NG/H_{2}}\) mixtures in swirl burners have been reported, as summarized below. Unconfined swirling flames were experimentally studied by [PERSON] et al. [10], who focused on the impact of hydrogen enrichment and swirl strength in methane-air premixed flames in a swirl-stabilized laboratory-scale burner, at constant thermal power. They found that hydrogen accelerates chemical reactions, increases the temperature, and reduces the recirculation flow in the reaction zone. The NO concentration in the reaction zone decreased with the increase in the hydrogen content at low swirl intensity, while an opposite trend was found at high swirl intensity. The influence of the hydrogen content and swirl strength was further investigated by [PERSON] et al. [11] for an unconfined bluff-body stabilized swirling burner, usually referred to as a Sydney swirl burner.