Thesis title: Experimental investigation on multiphase water plunging jets
Water plunging jets are encountered in many industrial and civil applications like chemicals stirrers, steel industry, food industry, water sewage systems, as well as in natural processes like waterfalls and rivers self-purification. The reason of such a success is connected to two main characteristics of these jets: significant mixing properties and air entrainment. The former are related to specific features due to ambient fluid entrainment, turbulent production, jet geometry and outlet type. Therefore, these features play a key role in every application where a fast and diffuse turbulent mixing is required, as those previously recalled. In parallel, air entrainment depends on intrinsic properties of both air and water, namely density, viscosity, surface tension, along with jet impact velocity and disturbance level. For these reasons, plunging jets are important in applications where a cheap and effective oxygenation of liquids is required. Nevertheless, air entrapment can also lead to negative effects such as choking of sewer ducts and manhole due to the free cross section area reduction for the flow rate discharge. Thus, the study of this class of jet flows is relevant not only for practical application but also from the fundamental research point of view.
This work aims investigating a sharp-edged orifice’s plunging jet which first issues horizontally in air and then plunges in a water pool. In order to shed light onto the role of jet geometry in this configuration, we focus on different orifice shapes: circular, rectangular, triangular, elliptical and square. All tests are conducted through 2D high-speed Particle Image Velocimetry, so that statistical results in space and time, along with time-resolved information are extracted.
In order to get widespread understanding on the jets’ mixing properties and on how much they are affected by air bubbles interactions, the work is structured into two main steps. In the first one, circular and rectangular plunging jets with a limited generation of air bubbles are considered. In this way, jet inner mixing ability is addressed by looking at its almost undisturbed evolution below the free surface, so far representing our ground for the following steps. In the second one, higher plunging velocities are considered, thus letting bubble entrapment to get higher relevance. So far, air and liquid phases are separated using a dedicated image processing algorithm, so that we perform PIV cross-correlation for liquid and air phase. Therefore, we obtain velocity fields for both phases and their mutual interaction.
Strictly speaking, in the first part of the study circular and rectangular orifices are chosen with the aim to consider an axisymmetric and a clear asymmetric configuration. Three Reynolds numbers are investigated for both the jets: 11000, 18000 and 25000. In each of these cases, three laser planes are analysed: two horizontal planes at about 2.25 D and 3.5 D from the pool floor (where “D” stands for the orifice’s equivalent diameter) and one vertical passing through the jet axis. We are able, in this way, to gain an average three-dimensional description of jets behaviour. Results point out an asymmetrical evolution for both circular and rectangular jets, with an opposite dependence on Reynolds number. Moreover, rectangular jets exhibit axis switching before impacting on the free surface, along with a double jet-like shape of its cross-sections which is Reynolds number dependent. Axial velocity decays, potential core lengths and spreading rates are evaluated for both circular and rectangular jets and highlight an opposite trend between the two geometries, thus suggesting a higher mixing for the lowest circular Reynolds number and the highest rectangular ones. However, axial velocity rms profiles show an overall greater turbulent production for the highest Reynolds number circular jet. Ambient mass entrainment points out the different interactions of the two plunging jets with the ambient flow: in circular case, it entrains fluid from the surroundings, from horizontal to vertical planes in streamwise direction, while in rectangular one it ejects flow from vertical to horizontal planes. Moreover, rectangular jets reveal to be the most affected by three-dimensional effects due to a counter-clockwise rotation around its centerline which is more evident as Reynolds number increases. Finally, circular lowest Reynolds number jet findings suggest it is the most promising in terms of mixing with respect to round free and vertical plunging jets.
In the second part of the study, we considered circular, triangular, rectangular, elliptic and square orifice shapes at Reynolds numbers equal to 18000, 25000 and 30000. They revealed to correspond to three different air entrainment regimes: no air bubbles entrapment, intermittent entrainment and fully developed air capture. One vertical laser plane passing through jet axis is investigated for all jets. In order to separate the air phase from the liquid one, a dedicated algorithm is developed and presented. It is based on morphological operations, image reconstruction, filtering, watershed segmentation and adaptive binarization. Results point out a weaker dependence of liquid jet velocity field on the presence of bubbles when intermittent entrainment regime occurs, and a stronger one when air entrainment is fully developed, for all jet geometries. The only exception is the square jet, which exhibit an earlier transition to the stronger dependence even at intermittent regime. Generally speaking, bubbles show lower velocities than the liquid phase till about half of the jet length. When their amount within the jet is large enough, they succeed in dissipating liquid phase momentum rapidly, so that a drop in jet velocity is observed. Void fraction, bubble count rate and mean bubble size are investigated both globally and locally, i.e. in the entire pictures’ field of view and along jet cross-sections. Finally, rectangular jets reveal to have the highest global void fraction in our measurement plane, followed by the square jet at Reynolds number ≈30000.