Numerical Study Of Hydrogen Jet And Combustion Characteristics At Methane/Air Premixed Atmosphere

’Zero carbon’hydrogen is one of the important choices for energy transformation under the’double carbon’target.High-pressure direct injection of hydrogen is not only a way of efficient combustion and utilization of hydrogen,but also the plume flame formed by direct injection can be used as an ignition source for low reactive gas.Therefore,it is of great significance to study the hydrogen high pressure direct injection airflow and its ignition characteristics.Nozzle shape is the most important factor affecting gas spray characteristics.In this thesis,based on the Fluent three-dimensional simulation platform,the hydrogen jet characteristics of cylindrical and conical nozzles and the combustion characteristics of methane/air mixture ignited by hydrogen jet were studied by constructing a high-pressure hydrogen jet model.The main work and conclusions of this thesis are summarized as follows:1.Based on the Fluent three-dimensional simulation platform,the hydrogen direct injection and jet combustion models were constructed:the RNG k-εmodel was selected as the high-pressure gas jet turbulence model for model verification,and the eddy dissipation conceptual model（EDC）was used for combustion and turbulence coupling.The jet combustion characteristics of high pressure cylindrical nozzle hydrogen jet at 300K,400K and 500K when the ambient gas equivalence ratio φ=0.6,0.7 and 0.8.2.The high-pressure hydrogen jet characteristics of conical nozzles commonly used in gas were studied:The penetration distance,mass flow rate,near-field core column and far-field vortex ball diameter of gas fuel under different injection pressures and ambient back pressures were analyzed.It is found that the jet penetration increases with the increase of injection pressure and decreases with the increase of ambient back pressure.In the whole stage of gas fuel injection,the mass flow rate curve can be divided into three stages:stage 1（Initial injection）is the establishment of pressure in the nozzle,stage 2（Stable injection）is a stable gas injection process,and stage 3（Tail injection）is the tail injection section with obvious decompression process.The mass flow rate ms strongly depends on the jet pressure and has a weak correlation with the ambient pressure.The under-expanded gas jet structure is divided into three parts:near-field core（Core）,transient region and far-field vortex ball（Vortex ball）.The core length increases rapidly in the range of about 0.20 ms,and then remains at a stable value.The stable core length is defined as L_C,which is linearly related to the Nozzle pressure ratio（NPR）.The diameter of the gas fuel vortex ball decreases with the increase of the ambient pressure,and increases with the increase of the injection pressure.3.The characteristics of high pressure hydrogen jet under cylindrical nozzle are compared and studied:It is found that the jet penetration increases with the increase of injection pressure,and decreases with the increase of ambient back pressure.The mass flow rate of the nozzle is linearly related to the injection pressure,but the correlation with the ambient pressure is weak.The steady-state core length L_Cis linearly related to NPR.The diameter of vortex ball decreases with the increase of ambient pressure and increases with the increase of injection pressure.Under the same injection conditions,the penetration distance and the diameter of the far-field vortex ball of the gas fuel of the cylindrical nozzle are larger than those of the conical nozzle.The core length of the high-pressure cylindrical nozzle gas fuel and the mass flow rate ms degree value in the stable stage are larger than the core length value of the high-pressure conical nozzle under the same conditions.4.The effects of different premixed gas parameters on the combustion characteristics of jet ignition were studied.:The temperature nephogram,cylinder pressure curve,mass fraction nephogram and curve of OH active free radical and CH₂O free radical were compared and analyzed.It is found that when the temperature is the same,the larger the premixed gas equivalence ratio,the faster the flame development and diffusion.The cylinder pressure increases with the increase of the premixed gas equivalence ratio in the cylinder.The larger the premixed gas equivalence ratio is,the more OH active radicals are generated.The CH₂O radicals of the hydrogen-jet flame exist at the boundary junction of the high-temperature flame and the unburned mixture and the outer contour of the high-temperature flame.As the ambient gas equivalence ratio increases,the peak mass fraction of CH₂O radicals decreases,which is negatively correlated with the equivalence ratio.When the equivalence ratio is the same,the flame diffusion rate,the OH active radical diffusion rate and the total mass of OH active radicals increase with the increase of ambient temperature.The formation rate of CH₂O radicals is related to the diffusion rate of flame.The formation rate and maximum mass of CH₂O radicals increase with the increase of ambient gas temperature.The cylinder pressure decreases with the increase of the ambient temperature of the premixed gas in the cylinder.