The buoyancy effect on micro hydrogen jet flames in still air was numerially studied.The results show that when the jet velocity is relatively large(V≥0.2 m/s),the flame height,width and temperature decrease,whereas ...The buoyancy effect on micro hydrogen jet flames in still air was numerially studied.The results show that when the jet velocity is relatively large(V≥0.2 m/s),the flame height,width and temperature decrease,whereas the peak OH mass fraction increases significantly under normal gravity(g=9.8 m/s^2).For a very low jet velocity(e.g.,V=0.1 m/s),both the peak OH mass fraction and flame temperature under g=9.8 m/s^2 are lower than the counterparts under g=0 m/s^2.Analysis reveals that when V≥0.2 m/s,fuel/air mixing will be promoted and combustion will be intensified due to radial flow caused by the buoyancy effect.However,the flame temperature will be slightly decreased owing to the large amount of entrainment of cold air into the reaction zone.For V=0.1 m/s,since the heat release rate is very low,the entrainment of cold air and fuel leakage from the rim of tube exit lead to a significant drop of flame temperature.Meanwhile,the heat loss rate from fuel to inner tube wall is larger under g=9.8 m/s^2 compared to that under g=0 m/s^2.Therefore,the buoyancy effect is overall negative at very low jet velocities.展开更多
Lateral-confined coaxial jet diffusion flame is common in micro thrusters,and the specific impulse is mainly obtained through thermodynamic calculations with an assumption of fuel combustion with an equivalence ratio,...Lateral-confined coaxial jet diffusion flame is common in micro thrusters,and the specific impulse is mainly obtained through thermodynamic calculations with an assumption of fuel combustion with an equivalence ratio,regardless of the stability of the combustion process.However,the flame behavior plays an important impact on the performance of a micro thruster through the varied combustion efficiency.The stability of confined coaxial jet diffusion flames with air coflow was studied by experiments and numerical simulation.Methane,hydrogen,and propane were used as fuels.Flame attachment,liftoff,blowout(extinction limits of lifted flame),and blowoff(extinction limits of attached flame) behaviors with the effect of confinement ratios and fuel properties were focused on.Among the range of the jet flow velocity in this research,the hydrogen flame is always attached to the jet exit,the flame tip goes from closed to open as the jet velocity increases,while the flame transitions from attachment to liftoff in the case of CH_(4) and C_(3)H_(8) .Further,in a narrow confined space,the attached flame for both CH4 and C_(3)H_(8) undergoes liftoff followed by blowout.However,in a space with a high confinement ratio,the CH4 flame transitions directly from attachment to blowoff.The critical modified Craya-Curtet number,which is used to predict the onset of the recirculation,is determined through simulation and experiment,and the number is about 1.77.This work provides valuable data on flame stability inside a confined space and gives insights into the design of a thruster.展开更多
基金Project(51576084)supported by the National Natural Science Foundation of China。
文摘The buoyancy effect on micro hydrogen jet flames in still air was numerially studied.The results show that when the jet velocity is relatively large(V≥0.2 m/s),the flame height,width and temperature decrease,whereas the peak OH mass fraction increases significantly under normal gravity(g=9.8 m/s^2).For a very low jet velocity(e.g.,V=0.1 m/s),both the peak OH mass fraction and flame temperature under g=9.8 m/s^2 are lower than the counterparts under g=0 m/s^2.Analysis reveals that when V≥0.2 m/s,fuel/air mixing will be promoted and combustion will be intensified due to radial flow caused by the buoyancy effect.However,the flame temperature will be slightly decreased owing to the large amount of entrainment of cold air into the reaction zone.For V=0.1 m/s,since the heat release rate is very low,the entrainment of cold air and fuel leakage from the rim of tube exit lead to a significant drop of flame temperature.Meanwhile,the heat loss rate from fuel to inner tube wall is larger under g=9.8 m/s^2 compared to that under g=0 m/s^2.Therefore,the buoyancy effect is overall negative at very low jet velocities.
基金supported by the Space Application System of China Manned Space Program,and the National Key Research and Development Program of China under grant number 2022YFF0504500.
文摘Lateral-confined coaxial jet diffusion flame is common in micro thrusters,and the specific impulse is mainly obtained through thermodynamic calculations with an assumption of fuel combustion with an equivalence ratio,regardless of the stability of the combustion process.However,the flame behavior plays an important impact on the performance of a micro thruster through the varied combustion efficiency.The stability of confined coaxial jet diffusion flames with air coflow was studied by experiments and numerical simulation.Methane,hydrogen,and propane were used as fuels.Flame attachment,liftoff,blowout(extinction limits of lifted flame),and blowoff(extinction limits of attached flame) behaviors with the effect of confinement ratios and fuel properties were focused on.Among the range of the jet flow velocity in this research,the hydrogen flame is always attached to the jet exit,the flame tip goes from closed to open as the jet velocity increases,while the flame transitions from attachment to liftoff in the case of CH_(4) and C_(3)H_(8) .Further,in a narrow confined space,the attached flame for both CH4 and C_(3)H_(8) undergoes liftoff followed by blowout.However,in a space with a high confinement ratio,the CH4 flame transitions directly from attachment to blowoff.The critical modified Craya-Curtet number,which is used to predict the onset of the recirculation,is determined through simulation and experiment,and the number is about 1.77.This work provides valuable data on flame stability inside a confined space and gives insights into the design of a thruster.
