Ignition of a parallel hydrogen injection into a supersonic hot air stream and ignition of a cold premixed combustible hydrogen/air stream heated by a hot air jet were investigated by a high-resolution TVD LU-SGS numerical algorithm. The present two-dimensional simulation included detailed chemistry in terms of elementary reactions, as well as a multispecies molecular transport model and the Baldwin Lomax algebraic turbulent model. In the case of a cold hydrogen injection into a hot air stream, numerical simulation showed that, with a decrease of the width of fuel jet, ignition point moves upstream quickly. The present results also showed that ignition distance was linearly dependent on the logarithmic function of the width of the fuel jet. Furthermore, the phenomenon could be still observed even for a very low concentration of hydrogen in the fuel jet. In the case of a premixed hydrogen/air stream ignited by a hot air jet, with a decrease of the width of air jet, ignition point moved very slightly downstream. However, there existed an ignition limit for the width of air jet, that is, if the width of the air jet was less than this limit, ignition became impossible. Furthermore, this ignition limit depended strongly on the initial temperatures of air stream and premixed hydrogen/air stream.
Flame spread phenomena in a suspended fuel droplet array were experimentally investigated for n-hexadecane in a high-pressure ambience. Seven droplets of the same size were arranged at equal horizontal spacings. Flame spread rates were measured based on OH emission histories detected by a high-speed video camera with an image intensifier for droplet diameters of 0.50, 0.75, and 1.0 mm at ambient pressures from 0.1 to 2.0 MPa. Results show that, as droplet spacing becomes smaller, flame spread rate increases and attains a maximum value at a specific spacing. A further decrease in droplet spacing causes the spread rate to decrease due to the large latent heat of vaporization. Experiments were also conducted in a microgravity field to determine if these characteristics of flame spread are affected by natural convection.
Experimental and numerical investigations were performed for the Iaminar burning velocity and the flame structure of laminar premixed CH4/O2/CO2 flames. Measurements of the laminar burning velocity were conducted by using a flame cone angle method for a circular nozzle burner. Numerical simulation was performed using one-dimensional plane flame code including radiation heat loss with an optically thin model. It was shown that the laminar burning velocity decrease with CO2 addition even though the adiabatic flame temperature is the same as that for CH4/Air flames. The radiation heat loss is significant for the CH4/O2/CO2 flames, and the flame temperature and laminar burning velocity decreases when the radiation heat loss is considered. Effects of thermal properties, radiation, and chemical reaction on the determination of the laminar burning velocity of CH4/O2/CO2 flames were discussed.
Experimental investigation on flame spread along suspended droplet arrays have been conducted with various droplet spacings and ambient air velocities. Especially, an opposed air stream is introduced to simulate fundamental flame spread behaviors in spray combustion. High-speed chemiluminescence imaging technique of OH radicals has been adopted to measure flame spread rates and to observe various flame spread behaviors. The fuel used is n-Decane and the air velocity varies from 0 to 17cm/s. The pattern of flame spread is grouped into two: a continuous mode and an intermittent one. It is found that there exists droplet spcings, above which flame spread does not occur. The increase of ambient air velocity causes the limit droplet spacing of flame spread to become small due to the increase of apparent flame stretch. As the ambient air velocity decreases, flame spread rate increases and then decreases after taking a maximum flame spread rate. This suggests that there exists a moderate air flowing to give a maximum flame spread rate due to enhanced chemical reaction by the increase of oxidizer concentration.
