Abstract
As an attempt to improve the performances of multi-entry gullies with applications to drainage system of a building, the hydrodynamic characteristics of air-water flows through the gullies with and without swirl generation vanes (SGV) are experimentally and numerically examined. With the aid of present Charge Coupled Device (CCD) image and optical systems for experimental study, the mechanism of air entrainment by vortex, the temporal variations of airflow pressure, the trajectories of drifting air bubbles and the self-depuration process for the gullies with and without SGV are disclosed. The numerical simulations adopt Flow-3D commercial code to attack the unsteady two-phase bubbly flows for resolving the transient fields of fluid velocity, vorticity and pressure in the gullies with and without SGV. In the twin-entry gully without SGV, air bubbles entrained by the entry vortex interact chaotically in the agitating bubbly flow region. With SGV to trip near-wall flows that stratify the drifting trajectories of the air bubbles, the air-bubble interactions are stabilized with the discharge rate increasing more than 7%. The reduction of the self-depuration period by increasing discharge rate is observed for the test gullies without and with SGV. Based on the experimental and numerical results, the characteristic hydrodynamic properties of the air-water flows through the test gullies with and without SGV are disclosed to assist the design applications of a modern drainage system in a building.
Highlights
To facilitate the efficient water supply and discharge for a building remains as a difficult task due to the complex flow bifurcations in water supply networks as well as the dynamic and unsteady interfacial air-water flow mechanisms developed in a drainage system
After the downward vortical air-water stream impinging onto the base plate of the test gully, the radially spreading air-water flow turns to be up-lifted through the 180°. sharp bend into the annular pathway between primary and secondary drums
With the stabilized air-bubbles drift in the annular pathway among the upward flows for the test gully with swirl generation vanes (SGV), the maximum discharging rates at present pressure potentials tested are increased more than 7% from those through the test gully without SGV
Summary
To facilitate the efficient water supply and discharge for a building remains as a difficult task due to the complex flow bifurcations in water supply networks as well as the dynamic and unsteady interfacial air-water flow mechanisms developed in a drainage system. The momentum changes of air-water flows caused by varying flow direction, expansions and contractions, bifurcations and/or chocking the airways incur the locally positive or negative transient airflow pressures that propagate throughout the entire drainage system at the sonic speed [2]. The impacts of such transient propagation—including the effects on acoustic resonances, discharging capacities and local negative or positive pressures—depend on the air-water interfacial structures and on the reflection and transmission of pressure waves on the interfacial and solid boundaries. With the presence of entrained air to add the damping effect on the collapsing bubbles, the damages caused by cavitation were alleviated, recommending the installation of aeration devices to entrain air for alleviating the cavitation effect
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