Investigations of interfacial waves at the inlet section in stratified oil-water flows

The formation and evolution of oil-water interfacial waves at the inlet section of a horizontal test pipe was investigated experimentally via high-speed imaging. Images were collected with a Phantom Miro 4 high-speed camera at a rate of 1000 fps. Wave velocity, amplitude, frequency and wave length at different oil-water flow rates (input ratios, r = 0.6-2.5; mixture velocities, Umix = 0.8-2 m s−1) were calculated from the images. The fluids used were tap water (ρ = 1000 kg m−3, μ = 0.001 kg m−1 s−1) and Exxsol D140 oil (ρ = 830 kg m−3, μ = 0.0055 kg m−1 s−1). The waves formed via a KH mechanism immediately after the junction where the two fluids joined and at a velocity roughly equal to half the mixture velocity with a frequency in the range 11-20 Hz for all flowrate combinations. Once formed, and at a short distance from the junction the wave amplitudes decreased while the wave velocities and the wavelengths increased. The frequency, however, remained constant. Experimental data was compared against predictions of the wave theory and the instability analysis. The propagation of interfacial waves at half the mixture velocity was predicted by the theory of dynamic waves. Results from the inviscid stability analysis at the inlet agreed qualitatively with the flow pattern map observed well further downstream the inlet, but quantitative differences were seen, which could be due to the viscosity of the oil phase.