FLOW CHARACTERISTICS OF UPWARD TWO-PHASE FLOWS IN A ROD BUNDLE GEOMETRY

In the present paper, the local two-phase flow parameters were measured with a four-sensor optical probe in adiabatic upward air-water two-phase flows in a 6 x 6 vertical rod bundle with rod diameter of 10 mm, pitch of 16.7 mm, square channel box side length of 100 mm and hydraulic equivalent diameter (D-H) of 18.7 mm. The local measurements were performed in an octant triangular region of the rod bundle cross-section at the axial position with height-to-diameter ratio (z/D-H) of 149 under a total of 16 flow conditions. The local void fraction, interfacial area concentration (IAC), bubble diameter and bubble velocity were obtained in the four-sensor probe measurements. Both of the measured void fraction and IAC show their radial local distributions with the core-peaking and wall-peaking shapes which are closely linked with the superficial velocity of two phases. The distribution shapes tend to change from the core-peaking to the wall-peaking when the superficial liquid velocity (<j(f)>) increases and the superficial gas velocity (<j(g)>) decreases. The measured diameters of local bubbles keep the similar values in the measuring cross-section, increase when the <j(g)> increases and decrease when the <j(f)> increases. The bubbles behave with the velocities whose main flow direction component keeps a typical radial power-law (core-peaking) profile and whose cross-sectional velocity components show a significant trend of bubbles migrating from the center to the wall region of the channel box especially under high <j(f)> conditions. The area-averaged results of void fraction and IAC were obtained by a cross-sectional area-averaging scheme. The resultant area-averaged void fraction and IAC were used to check the void fraction predicting capability of two drift-flux correlations and the IAC prediction performance of two IAC correlations respectively. The applicability of these correlations to the rod bundle geometry was discussed and concluded finally in this paper.