Research On Turbulent Combustion Characteristics And No Formation Of Steam Diluted H₂-Air Micro-Mixing Flame

In recent years,natural disasters caused by environmental issues have occurred frequently around the world,and energy structure transformation is gradually accelerating on a global scale.And Large-scale hydrogen（H₂）utilization is the only option to achieve the goal of decarbonization.As the cleanest and most efficient facility in the current fossil fuel-based thermal power generation system,gas turbine combined cycle technology is considered to be the main pathway for future hydrogen electrification.However,the chemical properties of H₂ lead to the risk of flashback and autoignition when using H₂in conventional combustion methods.In addition,the conflict between increasing the flame temperature to improve the cycle efficiency of gas turbines and suppressing NO_x emissions is more prominent when using H₂.Therefore,the development of safe,efficient and near-zero NO_x emission H₂combustion technologies is one of the current popular research topics in gas combustion science,which is of great significance to solve the problem of harmonious development of energy system and environment in the future.On the basis of summarizing the development of existing combustion technology,this thesis proposes the H₂O diluted H₂-Air micro-mixing combustion method.On the one hand,micro-mixing combustion improves the fuel-Air mixing quality and flow field by physically adjusting the combustion organization,which in turn affects flame stability and NO_x generation.On the other hand,H₂O dilution affects the combustion process and NO_x generation by changing reactants’physical properties and even directly participating in radical reactions.When the two are combined,micro-mixing combustion can ensure the mixing quality and combustion stability,while H₂O dilution can reduce the activity of H₂ and further reduce the risk of flashback or autoignition.This greatly improves the safety of H₂ combustion.At the same time,this technology can suppress NO_x generation from both physical and chemical perspectives,and achieve near-zero NO_xemission,which is of great practical significance and value.In order to promote the practical application and development of the H₂O-diluted H₂-Air micro-mixing combustion method,this thesis takes its combustion characteristics and NO_x reduction mechanism as the main research objects.An optical diagnostic micro-mixing combustion platform and high resolution numerical simulation methods were carried out in conjunction to research the H₂O-diluted H₂-Air micro-mixing flame.First,a new type of H₂O-diluted H₂ micro-mixing burner that can achieve high-efficiency mixing is designed.The effects of key structural parameters on fuel-Air mixing qualities,pressure loss and outlet flow field are studied,and a database of optimal parameters is obtained.The results show that the cross-jet mechanism with Air swirl（incidence angleθbetween 45°～60°）and exit expansion section（expansion angleβbetween 40°～60°）can obtain the best mixing capability and flame stability with relative low exit pressure loss under the currently studied structure.On this basis,the burner was built,and its flame stability and ultra-low NO_x emission performance were experimentally verified.Secondly,the flame structural characteristics and the spatio-temporal evolution characteristics of the heat release oscillations were studied based on the high-frequency PLIF data.The analysis found that the structural characteristics of H₂O-diluted H₂-Air micro-mixing flame are greatly influenced condtions.Increasingφor decreasing D leads to more small-scale folds with curvature radius close to 1 mm in the flame,indicating that micro-mixing combustion is a combustion reaction process at higher turbulence intensity compared to conventional scale combustion.The frequency analysis shows that the heat release oscillations of H₂O diluted H₂-Air micro-mixing flames tend to be low frequency oscillations.Asφincreases,the heat release frequency gradually decreases and a second harmonic frequency region appears near the cyclonic shear layer.In addition,the change of D causes the concentration and heat release rate of O and H radicals to change significantly,which in turn changes the intensity and location of the oscillation region.The DMD analysis reveals that the low-frequency oscillation region is mainly distributed in the flame arm region.With the increase ofφ,the oscillation region first develops along the swirling shear layer from the middle downstream to the entire region until it reaches the peak level,and then gradually disappears from the flame head and moves downstream.Finally,the mechanism of NO reduction by H₂O dilution of H₂-Air micro-mixing combustion was explored using DNS.The chemical effect of H₂O dilution on NO generation was found to be non-negligible,accounting for-16.3%to 20.1%of the total NO alteration value due to dilution.Under typical operating conditions（0.1 MPa,φ=0.8,D=25%）,NO generation can be reduced by 2/3 after considering the chemical effect of H₂O dilution.The inhibition impact of H₂O chemical effect on NO is mainly from the direct chemical reaction effect of H₂O.While the effect of turbulence intensity on the thermal dilution effect and chemical effect of H₂O dilution is negligible.However,the increase of turbulence intensity leads to a trend that the peak concentration of NO at the hotspot increases first and then decreases rapidly.Compared with the laminar flame,the peak temperature and NO concentration at Ka=0.4 increased by about 85 K and 120%on average.And when the turbulence intensity Ka was increased to 36,the peak temperature and NO concentration decreased by about 45 K and 40%,respectively.This implies that increasing the turbulence intensity is beneficial to suppress the NO generation at the flame hotspot,which is beneficial to reduce the overall NO emission.In addition,the N₂→NNH path is significantly enhanced in the low-temperature region of the flame,while the opposite is true within the flame hotspot.This is because the H atom consumption paths are enhanced at high turbulence intensities,and the H atom in the flame are rapidly consumed by H→HO₂path and H→OH path during the propagation to the hotspot,and finally the concentration of H atom reaching the hotspot is relatively reduced,thus suppressing the NO generation in the high temperature region of the flame.The concentration of NO in the super-adiabatic temperature region,which was originally considered as the main source of NO formation in the flame,was thus suppressed.