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Compact Heat Exchangers and Enhancement Technology for the Process Industries - 2003

ISBN Print: 978-1-56700-195-2



This study provides a qualitatively visual observation of the two-phase flow patterns for air-water mixtures flowing in a 6.9 mm diameter vertical return bend with a curvature ratio, 2R/D = 3. For two-phase flow entering at the lower tube with a total mass flux of 50 kg/m2·s at a low quality like 0.01, a flow reversal phenomenon occurs at the entrance region of the return bend. Further, a very special localized annular flow is seen for x = 0.009 and violent oscillations of the liquid slug inside the return bend is encountered. The flow reversal disappears when the mass flux is increased to 300 kg/m2·s. For two-phase flow entering the upper tube at G = 50 kg/m2·s, the occurrence of the flow reversal across the return bend is not seen even for a very low quality like 0.001. Instead of the flow reversal phenomenon, a very special "freezing slug" phenomenon is seen due to die interaction of the buoyancy force and inertia force. At a very high vapor quality like 0.8, the concave part of the return bend exhibits a "dry out" characteristic. However, this special phenomenon also disappears when the mass flux is further increased to 300 kg/m2·s.
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