A PIV investigation of stratified gas–liquid flow in a horizontal pipe

Simultaneous Particle Image Velocimetry (PIV) measurements of stratified turbulent air/water flow in a horizontal pipe have been performed using small water droplets, d p ‾ = 6 μ m , as tracer particles in the gas-phase. This seeding technique ensures that the surface tension of the water layer remains unaffected upon contact with the tracer particles in the gas-phase and thus allows small scale interfacial structures, such as capillary waves to occur and evolve naturally. Experiments have been conducted in a 31 m long, 100 mm in diameter PVC pipe using air and water at atmospheric pressure as test fluids. For the purpose of validation of the experimental set-up and the suggested seeding technique, gas single-phase measurements were performed at Re D = 45 , 000 and compared to existing DNS results from the literature with similar Re-number, showing very good agreement. Two stratified flow cases, i.e. smooth and wavy, are extensively discussed with emphasis on the effect of the interface pattern on the gas streamwise turbulence profile u g ′ . A simple analysis using the u g ′ -profiles of 17 stratified flows suggests the presence of a correlation between the turbulence structure of the gas-phase and global flow conditions such as the pressure drop and the bulk velocity.