Study On The Characteristics Of Methane No-premixed Flame In The High-temperature Environment

Burning in the high temperature environment is one of the hot issues in industrial and academic.The knowledge of combustion characteristics of methane non-premixed flame with high temperature air combustion and high temperature O₂/CO₂ combustion can led to higher combustion efficiency and low pollution gas emission,but these researches have been few reported.In this study,the extinction characteristics of CH₄/CO₂ versus O₂/CO₂ and CH₄/N₂versus O₂/N₂ counterflow non-premixed flames were studied numerically.The flame shape,height and liftoff height of CH₄/CO₂ and CH₄/N₂ non-premixed jet flames in co-flow air with different temperatures and CH₄ non-premixed jet flames in co-flow O₂/CO₂ with different temperatures were studied experimentally.The results show that:?1?The extinction strain rates of both flames are increasing with mole fraction of fuel or oxygen and oxidizer temperatures.Comparing with extinction limits of two types of flame shows with different fuel mole fraction show that CH₄/N₂ versus O₂/N₂ counterflow non-premixed flames are more sensitivity to oxidizer temperature.The flame strength of two flames near extinction are close,it means chemical kinetic is very important for stretch extinction.The analyses of integral reaction rates,impact factors and sensitivity coefficients suggest OH radical plays an important role in flame extinction.Higher oxidizer temperature can increase the whole reaction rates to enhance extinction limit.The differences of two types of flames are large in chain initiation reactions of methane,R33?H+O₂?+M?=HO₂?+M??,R38?H+O₂=O+OH?and R99?OH+CO=H+CO₂?.The reaction rate of third body reaction of CH₄/CO₂ versus O₂/CO₂ flame is increasing with the oxidizer temperature up to 1000 K.It decreases OH generation in the flame to inhibit promotion effect of high oxidizer temperature.?2?When flames have not got rid of fuel nozzle,the flame heights of CH₄/CO₂ flames are close to the flame heights of CH₄/N₂ flames and which are the lowest with O₂/CO₂ co-flow at same condition.Critical velocities of liftoff are increasing but liftoff heights are decreasing with higher co-flow temperature.Stabilization theory for lifted non-premixed turbulent flame,which termed as premixed flame model and large-scale flame model were employed to analysis characteristics of liftoff height of the two flames.The result shows that liner relationship between non-dimension liftoff height and non-dimension jet velocity exists in co-flow air flames.The large-scale model cannot get a fitting curve in co-flows O₂/CO₂ with different oxygen mole fraction.A quantitative analysis on the liftoff characteristics based on the premixed flame model and large-scale model was conducted,it is found that the ratio of fuel and air and stoichiometric laminar flame speed have significant effect on liftoff height.