Vertically Downward Two-Phase Slug Flow

Vertically downward slug flow of air-water mixtures was experimentally investigated in circular pipes of diameter D = 2.60, 10.16, and 14.0 cm. The terminal velocity of descending, stationary, and ascending bubbles in downward flows was measured and correlated with the air and water volumetric flux densities. Except for very small pipe diameters, bubbles in downward flow are unstable and eccentrically located off the pipe axis in regions of lower fluid velocities. Consequently, the value of the flow distribution parameter C₀ is less than unity for downward flow, compared to C₀ = 1 for rising bubbles in quiescent liquid, and C₀ = 1.2 for ascending bubbles in upward flow. Above a certain pipe diameter stable Taylor bubbles are not possible for downward flows. The terminal velocity coefficient C₁ is no longer 0.35 for downward flows in relatively large pipes because the unstable bubbles ride the pipe wall, resulting in a higher value of C₁.