Experimental Study Of The Characteristics Of Blended Hydrogen-methane Non-premixed Jet Flames

As an efficient and clean energy carrier,hydrogen has a broad development prospect in improving energy supply security and achieving zero carbon target.Transportation problem has been one of the biggest bottlenecks restricting the industrialization of hydrogen energy at present.It is a new effective way to solve this problem to blend hydrogen into natural gas pipeline for transportation.However,the leakage of gas pipeline is easy to cause non-premixed jet fire accidents.Therefore,it is of great practical significance for the safety design of hydrogen transport system and the emergency prevention and control of jet fire accidents to thoroughly study the characteristics of blended hydrogen-methane non-premixed jet flames.The morphological characteristics,radiation characteristics,lift-off height and lift-off limit of H2/CH4 jet flames were investigated in this paper.The main conclusions are as follows:The characteristics and mechanisms of H2/CH4 flames were explored(H2/CH4 flame with H2 volume fraction fv=20%was taken as the research subject).The simplified model of olive flame morphology was optimized,and a prediction correlation for H2/CH4 turbulent flame volume with fv=20%was obtained.The results show that the flame volume is directly proportional to the cube of flame height.The flame width evolution of 20%H2/CH4 and CH4 with the change of heat release rate(HRR,Q)can be divided into two stages:when Q<7.2 kW,there is a small difference between the two flame widths,and both increase with HRR.When Q≥7.2 kW,the flame widths are tend to remain unchanged,but 20%H2/CH4 flame width(0.1 m)is greater than CH4 flame width(0.09 m).In addition,a dimensionless prediction model for the radiation fraction of 20%H2/CH4 flame was developed.The radiation fraction can be predicted by dimensionless flame height,dimensionless fuel jet velocity and dimensionless HRR.The radiant heat flux and radiant fraction of 20%H2/CH4 flame measured at different heights increase with increasing HRR.However,when the flame changes from attached to lifted regime,the radiation fraction decreases significantly.This study examined the effect of H2 volume fraction(fv)on the height,width and lift-off height of H2/CH4 flames.Comparisons of different previous correlations of flame height with current experiments were conducted to obtain flame height semiempirical model for H2/CH4 flames with different fv.When the flame is in subsonic regime or choked and supersonic regime,the normalized flame height is positively correlated with the 0.4 power of dimensionless HRR,but the specific forms of the models are different in the two regimes.When nozzle diameter dj=3 mm,the flame height decreases with the increase of fv.However,under the condition of dj=5 mm,when fv increases from 0 to 50%,the flame height does not change significantly.When fv>50%,the flame height would decrease with fv again.The phenomenon is related to two competitive effects that influence flame height increase caused by blending H2,namely,inhibiting effect and promoting effect.fv has little effect on the ratio of flame width to height of H2/CH4 flame,which first increases with dimensionless HRR and then becomes nearly constant(～0.145).By reasonable assumption of maximum laminar burning velocity of H2/CH4 mixture with fv≤50%,a dimensionless prediction model of jet flame lift-off height applicable to pure fuel was extended to H2/CH4 mixture with fv≤50%.Lift-off height can be predicted by fuel jet velocity and hydrogen volume fraction.In addition,when dj is constant,the H2/CH4 flame lift-off height under the same HRR decreases with fv,while the lift-off height under the same fv increases with HRR.In addition,this paper revealed the influence of nozzle diameter(dj)on the flame characteristics of H2/CH4 flames.For laminar H2/CH4 flame,the flame height decreases with dj.For turbulent H2/CH4 flame,the flame height increases with dj.The variation trend of flame width with dj is basically consistent under different volume flow rates.The flame width increases with dj when 2 ≤dj≤5 mm,and remains unchanged or slightly decreases with dj when 6≤ dj≤10 mm.The evolution of flame width can be divided into three regimes:small eddy control,large eddy control and joint control regimes.They correspond to three trends of flame width evolution respectively,i.e.,increase,decrease and remaining unchanged with dj.Moreover,the critical jet velocity of H2/CH4 flame liftoff decreases linearly with dj,and the critical volume flow increases linearly with dj.