# Experimental Study of Methane Combustion Efficiency in a High-Enthalpy Oxygen-Containing Flow
## 1 Introduction  
The methane (CH\({}_{4}\)) and natural gas are among the most promising fuels for power plants in the energy and transport industries. This is a consequence of the high-energy capacity and manufacturability of CH\({}_{4}\), its relatively low cost, the availability of a large amount of raw stock and the possibility of minimizing environmental pollution in comparison with other hydrocarbon fuels [1, 2, 3]. In the methane combustion products, a high ratio of the water vapor mass fraction to the carbon dioxide mass fraction gH\({}_{2}\)O/gCO\({}_{2}\) is realized. This reduces the formation of carbon-black on the elements of the power plants relative to the use of other hydrocarbons as fuel. However, ensuring the efficient methane combustion is a complex scientific and technical problem, which is determined by its low oxidation rate and the short concentration range of CH\({}_{4}\) stable combustion with an air [4, 5, 6, 7, 8, 9, 10]. To search for methods of organizing efficient CH\({}_{4}\) combustion, fundamental, and applied studies are being carried out on the chemical and physical intensification of diffusion and kinetic processes during the interaction of methane with oxygen-containing flows. As a method of methane combustion chemical intensification, the addition of more active components, such as hydrogen or ethylene, can be considered. The studies [11, 12, 13, 14, 15, 16, 17, 18, 19, 20] show that the addition of hydrogen allows us to shorten the length of the reaction zone and increase the combustion intensity at the flame foundation due to the combustion rate increase. It is noted that an increase in the hydrogen proportion leads to an increase in the OH radical concentration and to a decrease in the CO and NOx radicals in the combustion products. The addition of -40% hydrogen (by volume) leads to increase the coefficient of methane combustion completeness approximately by 20% [12, 13, 14, 15]. According to the studies [16, 17], the flow turbulization which accompanying the hydrogen addition, increases the average flame propagation velocity in a multicomponent medium. The effects of the diffusion-thermal instability occurrence, as well as an increase in the mixture reactivity due to hydrogen addition to the turbulent flow, were also confirmed by numerical simulation [18, 19, 20]. The similar effects can be obtained due to ethylene addition to the methane [21, 22, 23], however, mixtures of methane with ethylene have a lower explosion capability. A widespread method of methane combustion physical intensification, especially at high rates, is the eddy zones organization [24; 25; 26]. The eddy zones are realized in special niches, behind ledges or bluff bodies and make it possible to increase the local residence time of the mixture, which ensures combustion stabilization. The studies [27; 28; 29] show that nonstationary eddy zones increase gas-dynamic losses and can cause the development of vibrational combustion. Considering this, the methane combustion stabilization methods, based on new physical principles, are being actively developed in recent times. Such methods include an acoustic [30; 31; 32; 33; 34] and electrical effects on flames [35; 36; 37; 38; 39]. Definitely, these methods of intensification are promising, however, they require detailed research and testing. It should be noted that one of the most technologically advanced methods for fuel combustion intensification is preliminary heating of the oxidizer [40], which can be carried out in special heat exchangers. Given the low reactivity, heating (enthalpy increasing) of the oxidizer can significantly improve the methane combustion efficiency. It is worth noting that nowadays the enthalpy of the oxidizer is often limited to -700 kJ/kg in practice, which is determined by the maximum allowable heating temperature in known industrial heat exchangers. A large number of experiments on methane combustion at elevated enthalpies are conducted in smaller laboratory facilities [41; 42].