and that formed by the tubes bundle can be attributed tothe following. In a solid wall column, a ‘no slip’ boundarycondition is valid at r/R = 1. However, for the current sys-tem under investigation, not all surrounding wall is solidmatter. Consequently, the ‘no slip’ condition is ‘weakened’.As a result, the liquid velocity at this locus is not zero butrather has a value which will be denoted here as ul, wall. Inother words, although the absolute value of the gas holdupat the wall is higher for the tubes bundle case, the liquidrecirculation is reduced as a result of a lower gas holdupFig