Three-dimensional evolution and interaction of disturbance waves on a gas-sheared liquid film on a horizontal plane near the transition region

Disturbance waves are complex three-dimensional large-scale structures appearing on liquid films under strong gas shear. They strongly intensify heat, mass and momentum interchange between the phases, thus affecting the efficiency of industrial equipment. Here we analyze inception and three-dimensional spatiotemporal evolution of disturbance waves on a gas-sheared liquid film flowing along the bottom of a wide horizontal duct. The experiments are carried out using Brightness-Based Laser-Induced Fluorescence technique in a large region of interrogation with length of 380 mm and width of 166 mm (equal to the width of the duct). The range of gas superficial velocities is from 6 to 24 m/s with a step of 2 m/s; the range of liquid wetting densities is from 1 to 8 cm²/s with a step of 1 cm²/s. Three working liquids with different viscosity and surface tension are employed. Transition from small-scale 3D-waves to disturbance waves is identified; it is shown that the transition becomes less sensitive to liquid properties and flow rate as the liquid flow rate increases. An intermediate stage of the transition is the formation of smooth-interface patches with low amplitude and large longitudinal and transverse size. Quantitative criteria to identify the transition are proposed. The side walls of the duct mainly affect the amplitude and local longitudinal size of disturbance waves by distorting the shape of disturbance wave fronts. As a result, local time-averaged film thickness increases near the side walls. The disturbance waves are essentially three-dimensional with curved fronts and a wide range of transverse size. The width of a wave exerts a strong influence on its amplitude, velocity, and longitudinal size. Coalescence of disturbance waves is a three-dimensional phenomenon leading to elimination of narrow waves; as the flow develops, it becomes more uniform in transverse direction.