[en] High pressure two-phase discharge flow from a vessel through a short pipe into an open space at atmospheric pressure was numerically analyzed using one- and two-dimensional non-equilibrium two-fluid models, in order to investigate effects of two-dimensional flow structure on thermal hydraulic phenomena in critical two-phase flows. Two-dimensional calculations revealed that flow contraction at the pipe inlet caused a liquid core in the central region of the pipe cross section, while void fraction in the vicinity of the pipe inner wall was large. This uneven phase distribution, which cannot be treated with one-dimensional calculations, seemed to result in larger discharge flow rate predictions because of phase slip promotion in two-dimensional calculations than that in one-dimensional calculations. Barrel-shaped shock waves, which are commonly observed in critical gas flow, did not appear in the calculated two-phase jets because large volumetric expansion due to flashing prevented the discharged two-phase mixture from over-depressurization. (author)