Due to the low laminar flame speed and heat release rate and high ignition energy, it is difficult to stabilize the flame of ammonia. The combustion of pure ammonia often requires a large-scale combustion chamber to stabilize the combustion, and the stable combustion range of the flame is relatively narrow, which limits the industrial application of ammonia fuel. With the development trend of miniaturization of gas turbines, the size of the combustion chamber will be further reduced and the surface-to-volume ratio will increase, which will lead to serious heat loss problems. At this time, the combustion process of pure ammonia is also more prone to quenching, and the stable combustion range will be further reduced. In order to achieve a wide range of stable combustion of ammonia fuel, the incorporation of hydrogen into ammonia fuel is an efficient method to enhance flame stabilization, and there is no carbon emission in the combustion process. Limited by the storage and transportation cost and safety of hydrogen, the coupling of ammonia fuel pre-cracking device in gas turbine system can effectively solve the above problems. In practical use, an ammonia cracking device can be installed before the combustion chamber, and the ammonia can be partially cracked into hydrogen by using the waste heat to reduce the activation energy through the catalyst, thus avoiding the cost increase caused by the direct use of hydrogen. Mitsubishi Electric has conducted research in this area. In addition, the gas turbine to burn ammonia-hydrogen fuel has been put into use in some projects abroad, but it is blank in China, and most of the research is still in the theoretical stage. In the future, micro gas turbines, as an important part of distributed energy, have excellent applicability for power peak shaving and small-scale power supply in residential areas, industrial areas, and remote areas. Clean carbon-free fuel ammonia and hydrogen are receiving more and more attention in the current society. In order to avoid the problem that the wall temperature of the flame tube is too high and burned out when the gas turbine is working, secondary air is usually used to cool the wall surface. However, this behavior may affect the flame dynamics characteristics of fuel combustion, thus affecting the stability of the flame and the generation of pollutants. Therefore, in this paper, the effects of different wall cooling degrees on the combustion characteristics and emission characteristics of ammonia-hydrogen premixed swirl flame in micro gas turbine combustor are studied by numerical simulation. The wall cooling degree is realized by controlling the wall mixing heat transfer coefficient δ. The realizable k-ε model and the eddy dissipation concept model were used to predict the swirl flame in the turbulence and chemical reaction models, respectively. The grid was divided into structured grids using ICEM CFD, and the main combustion zone, namely the middle and upper reaches, was encrypted, and the grid independence verification was carried out. It was found that the number of 690,000 cells was enough to achieve more accurate simulation results and save computing resources. In addition, the calculation model of this paper is verified by the experimental data of ammonia-hydrogen swirl combustion in the literature, and it is found that the model can accurately simulate the ammonia-hydrogen premixed swirl combustion. The results show that the enhancement of heat loss changes the coupling effect between flame and wall, which can significantly reduce the wall temperature and is negatively correlated. That is, the stronger the wall cooling degree is, the lower the wall temperature is, which achieves the purpose of cooling the flame tube wall. The heat loss will also lead to the quenching of the flame in the outer recirculation zone ( ORZ ), causing the flame topology to change from M-shaped to V-shaped. Two different flame structures can lead to different axial positions of the wall temperature peak point. Under different wall cooling conditions, the wall temperature peak point of M-shaped flame moves downstream with the increase of equivalence ratio, the wall temperature peak point of V-shaped flame moves upstream, and the flames of different shapes show the opposite movement trend. Interestingly, the shape of the wall temperature variation curve with the axial direction of the flames of different shapes is the same as that of the flame shape, which is due to the fact that the M-shaped flame has an additional corner recirculation reaction zone. The study also found that the M-shaped flame on the lean side has a lower NOx emission capacity, and the higher the wall cooling degree, the lower the NOx emission. Therefore, in order to reduce NOx emissions, the flame shape should be maintained in the M shape and the wall cooling degree can be appropriately enhanced. In order to achieve the best working condition of the burner ( refers to the combustion efficiency is greater than 99 % and the NOx emission is low ), this paper also calculates the combustion efficiency from lean combustion to rich combustion under different wall cooling conditions. In the explored working conditions, considering the combustion efficiency and NOx emission, in the case of ensuring a lower δ, the flame has an M-shaped, and the equivalence ratio Φ = 0.9 has the best combustion efficiency and lower NOx emission, which is considered to be the best working condition. However, on the rich-burn side and lean-burn side with equivalence ratio Φ < 0.8, the combustion efficiency is low, and a large amount of unburned ammonia and hydrogen from ammonia pyrolysis are discharged. This problem may need to be solved by the rich-lean two-stage combustion strategy.

ammonia-hydrogen combustion,swirl flame,wall cooling,emission