Pressure Drop of Taylor Flow in Capillaries: Impact of Slug Length

In a single capillary, the frictional two-phase pressure drop in Taylor flow has been measured using various liquids, and a correlation to predict the friction factor has been developed. A carefully designed inlet section for the capillary allowed the independent variation of gas bubble and liquid slug length. Gas and liquid superficial velocities were varied in the range 0.04–0.3 m/s. If the slug length was lower than 10 times the capillary diameter, the frictional pressure drop in the liquid slug increased drastically from the single phase limit (f = 16/Re). The slug length dependence is caused by a larger contribution to the pressure drop of the end effects near the bubble caps. Increased pressure drop at the ends of the slug is caused by two separate effects: (1) near the bubbles the circulation inside the liquid slug induces extra friction, and (2) the difference in curvature of the gas-liquid interface at the front and at the rear of the bubble gives rise to extra pressure drop. The use of different liquids allowed the independent variation of the Reynolds number Re and the Capillary number Ca, and an expression for the frictional pressure drop as a function of Re, Ca and the slug length was developed. The results of this work allow the determination of slug length from pressure drop measurements in closed equipment where the slug length cannot otherwise be measured easily. The applicability of the pressure drop model to estimate mass transfer is demonstrated by combined pressure drop and gasliquid measurements in a monolith, which is essentially an array of capillary channels.