A numerical investigation in buoyancy effects on micro jet diffusion flame

Abstract

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/s2). 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/s2 are lower than the counterparts under g=0 m/s2. 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/s2 compared to that under g=0 m/s2. Therefore, the buoyancy effect is overall negative at very low jet velocities.