Effect of binary mixtures of surfactants on horizontal gas–liquid flow with low liquid loading

The interaction between anionic and zwitterionic surfactant (sodium dodecyl sulphate/dodecyl betaine) brings about the synergism in the surface tension reduction and the micelle formation. The binary mixtures of surfactants were added to air–water flow with low liquid loading in a horizontal 50.8 mm pipe. Two concentrations with a 6:4 molar ratio of zwitterionic to anionic surfactant were tested to investigate their effect on the frictional pressure drop. Parallel ring conductance technique was used to trace the liquid layer fluctuation near the pipe wall and examine the gas–liquid interfacial behavior. A modified two-fluid model with the consideration of a curved interface is introduced to obtain the closure relationship for scaling the gas–liquid interfacial roughness. It was found that the solution with low concentration did not have drag-reducing effect although the reduced surface tension, while the mixed surfactants solution with the concentration higher than the critical micelle concentration reduced the interfacial roughness, leading to pressure drop reduction at low and medium gas flow rates. At high gas flow rates, the effect of mixed surfactants became worse because of the degraded surface activity and the break-up of the mixed micelle under the high gas shear conditions.