Based on two groups of gaseous and supercritical CO2 release experiments using an industrial scale CO2 pipeline (258 m long

233 mm i.d.) through a 100 mm diameter orifice

the dynamic pressure and visible cloud in the near field

and the temperature and concentration changes in the diffusion region were analyzed. The results show that the sharp drop in inventory pressure near the orifice produced a highly under-expanded jet and induced a sharp drop in temperature as a result of Joule-Thomson cooling. This effect led to the formation of solid CO2 particles inside the under-expanded jet. The visible cloud remained a metastable state after the rapid expansion

and followed by a slow attenuation stage. The visible white cloud entraining the dry ice particles and condensed water rapidly expanded in the rapid expansion stage and dry ice particles have been sublimated into the ground before and did not form the ice bed. At the moment of the rupture

the temperature in the discharge area of gaseous and supercritical CO2 releases dropped rapidly and then increased slowly

while the CO2 concentrations increased quickly and then decreased. The dangerous distance of the 5% concentration of gaseous and supercritical CO2 releases were at least 30 m and 50 m respectively.