Effects of trapped air on flow transients in rapidly filling sewers

This thesis presents the results of experimental and analytical investigations on the effects of trapped air on flow transients in pipelines, especially for sewer trunks during the rapid filling stage.; The experimental study consists of rapid filling of different pipeline configurations containing trapped air, including a single horizontal pipe which was initially empty, a single horizontal pipe which initially had a tailwater, and a horizontal pipe with a vertical pipe segment positioned at different locations along the horizontal pipe. The pipe end was outfitted with orifices of different sizes to study the effects of air leakage on the pressure. The effects of varying the driving head, initial water column length, and orifice size on the maximum pressure peaks and pressure oscillation patterns were investigated. The air-water flow patterns in a horizontal pipe during rapid filling stage were also observed with a high speed camera. Pressure histories synchronously recorded illustrate the relation between the air-water phase evolution and the pressure oscillation pattern.; The experimental study revealed three types of pressure oscillation patterns in a rapidly filling pipe system, depending on the size of the orifice. When no air is released or when orifice sizes are small, waterhammer effects are negligible because of the cushioning effect of the air pocket. When the orifice size is very large, the air cushioning effect vanishes and the waterhammer pressure is dominant. For intermediate orifice sizes, the pressure oscillation pattern consists of long period oscillations followed by short period pressure oscillations. The maximum peak pressure under no air release condition could be 4 times the driving head. Under air leakage condition, this peak pressure could be up to 15 times the upstream head. The pressure oscillation pattern and magnitude of peak pressure in an L-shape pipe system were close to those in a single horizontal pipe. It was found that the T-shape pipe system could mitigate the peak pressure significantly when the air release through the end of horizontal pipe was significant.; An analytical model, based on rigid water column theory, was developed to simulate the pressure transients in rapidly filling pipe systems containing trapped air. The model integrates the calculation of air pocket pressure oscillation with the magnitude of maximum waterhammer peak pressure. The analytical model was calibrated using the experimental data and was found to be able to predict pressure oscillation patterns for no or small air release situations. The model also is able to predict the maximum pressure magnitude for a wide air leakage range. The model study verified the ability of rigid water column theory in exploring the trapped air induced pressure transients during rapid filling.*; *The dissertation includes a CD that is compound (contains both a paper copy and a CD as part of the dissertation). The CD requires the following application: Labview data acquisition program.