Flow pattern and its transition of nitrogen gas–water two-phase flow in U-tubes and inverted U-tubes

The flow pattern and its transition of the mixture of nitrogen gas and water are observed in U-tubes and inverted U-tubes using high speed CCD camera. In U-tube, it is observed that the dispersed bubble before U-bend converges to be Taylor bubble after U-bend; the Taylor bubble to the slug bubble and the smaller slug bubble to the larger one. And, the slug bubble merges before U-bend and separates in U-bend and finally develops slug bubble again after U-bend. In addition, the film falling annular flow before U-bend and shear in U-bend and at last form Taylor bubble or slug bubble after U-bend. Besides, a different freezing plug in U-tubes is observed. In inverted U-tube, the converging transition observed is from Taylor bubble to slug bubble, Taylor bubble to film falling annular, and slug bubble to film falling annular. At last, the flow pattern map for U-tubes and inverted U-tubes is proposed, respectively.