High Resolution Void Fraction Measurements for the Validation of Flow Maps and CFD Codes

The present work is aimed at the investigation of two-phase flow in vertical pipes. Even under these very simple boundary conditions, strong 3D effects are observed. The distribution of the gas phase over the cross section varies significantly between the different flow patterns, which are known for the vertical two-phase flow. The paper presents a new device for measuring the void distribution with a high resolution (242 mea-suring points in a cross section area, measuring frequency approx. 1 kHz). This high resolution allows the calculation of local and averaged void fractions, a flow pattern visualization and the determination of bubble size distributions. Bubble flow appears with void center and boundary maximum distribution. The bubble size distribution allows the classification of the transition from homogeneous to inhomogeneous bubble flow and bubble to slug flow. These criteria are compared with selected empirical or theoretical flow pattern maps. The measurements are also used to assess the capability and the accuracy of the code CFX-4.2. The 3D calculations were still only successful for bubble flow, while the experiments were carried out in a wide range of superficial velocities, including other flow patterns like plug or churn turbulent flow.