Emptying of large-scale pipeline by pressurized air

Emptying of an initially water-filled horizontal PVC pipeline driven by different upstream compressed air pressures and with different outflow restriction conditions, with motion of an air-water front through the pressurized pipeline, is investigated experimentally. Simple numerical modeling is used to interpret the results, especially the observed additional shortening of the moving full water column due to formation of a stratified water-air "tail". Measured discharges, water-level changes and pressure variations along the pipeline during emptying are compared using Control Volume model results. The CV model solutions for a non-stratified case are shown to be delayed as compared with the actual measured changes of flow rate, pressure and water level, but by considering water-column mass loss due to the water-air tail and residual motion, the calibrated CV model yields solutions that are qualitatively in good agreement with the experimental results. A key interpretation is that the long air-cavity celerity is close to its critical value at the instant of minimum flow acceleration. The influences of driving pressure, inertia and friction predominate, with the observed water hammer caused by the initiating downstream valve opening not significantly influencing the water-air front propagation. Keywords: Air-water interactions, Drainage, Experimentation, Pipes, Transient flow, Unsteady flow