Marco Simiano

Turbulent transport and exchange mechanisms in an unstable oscillating bubble plume are investigated using a novel Large-Eddy PIV measurement technique providing simultaneous gas and liquid velocity fields. The flow was generated in a 2 m-diameter vessel with a water depth of 1.5 m for void fractions up to 4% and bubble size of the order of 2.5 mm. Sub-scale turbulent properties of the flow were estimated, revealing an enhanced anisotropic transport of turbulence and energy redistribution… Visualizza altro

Turbulent transport and exchange mechanisms in an unstable oscillating bubble plume are investigated using a novel Large-Eddy PIV measurement technique providing simultaneous gas and liquid velocity fields. The flow was generated in a 2 m-diameter vessel with a water depth of 1.5 m for void fractions up to 4% and bubble size of the order of 2.5 mm. Sub-scale turbulent properties of the flow were estimated, revealing an enhanced anisotropic transport of turbulence and energy redistribution mechanism and with strong shear-induced characteristics. It is shown that the plume dynamics contain distinct flow zones, each associated with specific flow events and featuring specific energy exchange mechanisms: the plume-core zone where the flow accelerates, the peak zone where the plume contracts, and a residual external zone where the stresses are relaxed. The turbulent kinetic energy is found to be dominated by the vertical stresses, along the flow direction, notably in the peak zone, a phenomenon associated with the meandering of the central bubble plume. The quantitative estimation of the production terms new explanations of the energy exchange mechanisms between the mean flow and the sub-scale motions within the three flow zones identified. During plume contraction the longitudinal normal- and cross-production terms act as pure generation terms extracting energy from the mean flow; when the contraction is terminated, the shear-induced contribution competes with the normal stress-induced term, which tends to restitute energy back to the mean flow. In the plume-core zone instead, both terms are negligible, except at lower elevations where, because of the strong vertical acceleration of the flow, the contraction occurs and the normal stress-induced restitutes energy to the mean flow. Meno dettagli

A large-eddy simulation of gas–liquid flow in a large scale bubble plume is presented. The Euler-Euler approach is used to describe the equations of motion of the two phase flow. The sub-grid scale modeling is based on the Smagorinsky kernel. All the non-drag forces (turbulent dispersion force (only for RANS), virtual mass force, lift force) and drag force are incorporated in the model. Overall, predictions are in good agreement with the experimental data at higher measurement levels but… Visualizza altro

A large-eddy simulation of gas–liquid flow in a large scale bubble plume is presented. The Euler-Euler approach is used to describe the equations of motion of the two phase flow. The sub-grid scale modeling is based on the Smagorinsky kernel. All the non-drag forces (turbulent dispersion force (only for RANS), virtual mass force, lift force) and drag force are incorporated in the model. Overall, predictions are in good agreement with the experimental data at higher measurement levels but discrepancies are observed in the region near the injector. The axial mean liquid velocity and gas velocity at all the measurement levels exhibit the expected Gaussian profiles and plume spreading. The predictions of gas void fraction, axial gas and liquid velocity are in good agreement with the experimental data except near the injector. Further, the detailed comparison of LES and RANS predictions along with experimental data is presented and discussed Meno dettagli

The detailed investigation of an unstable meandering bubble plume created in a 2-m-diameter vessel with a water depth of 1.5 m is reported for void fractions up to 4% and bubble size of the order of 2.5 mm. Simultaneous particle image velocity (PIV) measurements of bubble and liquid velocities and video recordings of the projection of the plume on two vertical perpendicular planes were produced in order to characterize the state of the plume by the location of its centreline and its equivalent… Visualizza altro

The detailed investigation of an unstable meandering bubble plume created in a 2-m-diameter vessel with a water depth of 1.5 m is reported for void fractions up to 4% and bubble size of the order of 2.5 mm. Simultaneous particle image velocity (PIV) measurements of bubble and liquid velocities and video recordings of the projection of the plume on two vertical perpendicular planes were produced in order to characterize the state of the plume by the location of its centreline and its equivalent diameter. The data were conditionally ensemble averaged using only PIV sets corresponding to plume states in a range as narrow as possible, separating the small-scale fluctuations of the flow from the large-scale motions, namely plume meandering and instantaneous cross-sectional area fluctuations. Meandering produces an apparent spreading of the average plume velocity and void fraction profiles that were shown to remain self-similar in the instantaneous plume cross-section. Differences between the true local time-average relative velocities and the difference of the averaged phase velocities were measured; the complex variation of the relative velocity was explained by the effects of passing vortices and by the fact that the bubbles do not reach an equilibrium velocity as they migrate radially, producing momentum exchanges between high- and low-velocity regions. Local entrainment effects decrease with larger plume diameters, contradicting the classical dependence of entrainment on the time-averaged plume diameter. The global turbulent kinetic energy was found to be dominated by the vertical stresses. Conditional averages according to the plume diameter showed that the large-scale motions did not affect the instantaneous turbulent kinetic energy distribution in the plume, suggesting that large scales and small scales are not correlated. With conditional averaging, meandering was a minor effect on the global kinetic energy and the Reynolds stresses. Meno dettagli

An extensive study of the most important hydrodynamic characteristics of fairly large-scale bubble plumes was conducted using several measurement techniques and a variety of tools to analyze the data. Particle image velocimetry (PIV), double-tip optical probes (OP) and photographic techniques were extensively applied to measure bubble and liquid velocities, void-fraction and bubble sizes. PIV measurements in a vertical plane crossing the centre of the injector provided the instantaneous… Visualizza altro

An extensive study of the most important hydrodynamic characteristics of fairly large-scale bubble plumes was conducted using several measurement techniques and a variety of tools to analyze the data. Particle image velocimetry (PIV), double-tip optical probes (OP) and photographic techniques were extensively applied to measure bubble and liquid velocities, void-fraction and bubble sizes. PIV measurements in a vertical plane crossing the centre of the injector provided the instantaneous velocity fields for both phases, as well as hydrodynamic parameters, such as the movement of the axis of the plume and its instantaneous shape. Statistical studies were performed using image processing to determine the distribution of the apparent instantaneous plume diameter and centreline position. An important finding was that there is little instantaneous spreading of the bubble plume core; the spreading of the time-averaged plume width (as measured from the time-averaged void-fraction and time-averaged liquid velocity fields) is largely due to plume meandering and oscillations. The liquid-phase stress tensor distributions obtained from the instantaneous velocity data indicate that, for the continuous phase, these stresses scale linearly with the local void-fraction in the range of 0.5% < α < 2.5%. The bubbles were found to be ellipsoidal, with shape factor e ≈ 0.5. Meno dettagli