The near-field instability of variable property jets of air, CO 2, and He, issued into the ambient air, has been investigated experimentally within normal gravity and microgravity fields. The density ratio to the ambient air is unity for air jets, more than unity (1.53) for CO 2 jets, and less than unity (0.14) for He jets, respectively. The ratio of kinematic viscosity to the ambient air is unity for air jets, less than unity for CO2 jets (0.53), and more than unity for He jets (7.75), respectively. The jet velocity is varied from 0.4 to 1.8 m/s and then the jet Reynolds number varies from 60 for Helium jet to 2,000 for CO2 jet, while the Richardson number varies from negative to positive values. The motion of the jet is visualized using a laser tomographic method and recorded by a high-speed digital video camera with 250 frames/s. The result shows that the instability of the jet is intensified when Re > 800 while it is weakened at Re < 800 at the microgravity field, indicating that the viscosity plays an important role in weakening the instability. Under a normal gravity field, the buoyancy also becomes important. In order to quantify the instability criteria, the quantity of the instability is introduced, which consists of the Kelvin-Helmholtz instability, buoyancy effect and viscous effect. When the ratio of the sum of Kelvin-Helmholtz and buoyancy forces to viscous force exceeds a certain value, around 12 in the present study, the jet becomes unstable even when Re < 800. These results reveal that the instability of variable property jets is influenced by the Kelvin-Helmholtz instability, the viscous effect and the buoyancy effect.
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