Experimental study of foam flow in horizontal pipes: Two flow regimes and its implications

Although foam has been widely used in many scientific and engineering applications, the current understanding of foam rheology in pipes is still very limited because of its complex nature. This experimental study, for the first time, investigates the flow rheology of foams in pipes by placing a special emphasis on two distinct foam flow regimes. A wide range of experimental conditions are examined in this study, which include five different surfactant formulations (Cedepal FA-406, Petrostep CG-50, Stepanform 1050, Aquet TD-600, and Ultra-Palmolive), three different surfactant concentrations (0.1, 1, and 5 wt.%), two different pipe diameters (0.5 and 1 in. nominal size stainless steel pipes), and two different filter opening sizes (50 and 90 μm) for upstream foam generation. The experiments revealed the following characteristics: (1) foam flow in pipes exhibited two different flow regimes called "high-quality" regime and "low-quality" regime, (2) the high-quality regime was characterized by unstable and oscillating pressure response which resulted from repeating free gas and foam slug, whereas the low-quality regime was characterized by stabilized pressure response which resulted from the flow of uniform and homogeneous foams, (3) different patterns of pressure contours were observed-the pressure contours were relatively steep in the high-quality regime but relatively gentle, or even almost horizontal, in the low-quality regime, and (4) the value of foam quality (f_g^*) that splits the two flow regimes was shown to be affected by experimental conditions such as surfactant formulations and concentrations. These observations imply that the rheology in the high-quality regime is primarily governed by dynamic mechanisms of lamella creation and coalescence during the flow, and the rheology in the low-quality regime is primarily governed by interactions between individual bubbles and/or interactions between bubbles and pipe wall. Therefore, the high-quality regime is likely to expand (or, the low-quality regime is likely to contract, equivalently) with a reduction in surfactant foamability.