[en] In order to clarify the mechanisms governing the two-phase external flow structure across the inverted U-tube bundle in a PWR steam generator, measurements of local void fraction and bubble impaction rate have been conducted in a vertical air-water flow across inclined tube bundles. The in-line and staggered 9x6 tube bundles with transverse pitch-to-diameter ratio 2.0 and longitudinal ratio 2.5 were used in the present work. It has been drawn from the present work that the phase distribution in inclined tube bundle is strongly affected by both mass velocity and the inclination angle of the tube bundle. The effect of inclination angle becomes more remarkable at lower mass velocity. (author)