Alain Cartellier

Effect of preferential concentration on the settling velocity of heavy particles in homogeneous isotropic turbulence

The behaviour of heavy particles in isotropic, homogeneous, decaying turbulence has been experimentally studied. The settling velocity of the particles has been found to be much larger than in a quiescent fluid. It has been determined that the enhancement of the settling velocity depends on the particle loading, increasing as the volume fraction of particles in the flow increases. The spatial and temporal distribution of the particle concentration field is shown to exhibit large inhomogeneities. As the particles interact with the underlying turbulence they concentrate preferentially in certain regions of the flow. A characteristic dimension of these particle clusters is found to be related to the viscous scales of the flow. Measurements of the settling velocity conditioned on the local concentration of particles in the flow have shown that there is a monotonic increase in the settling velocity with the local concentration (the relation being quasi-linear). A simple phenomenological model is proposed to explain this behaviour

Simultaneous void fraction measurement, bubble velocity, and size estimate using a single optical probe in gas–liquid two‐phase flows

Optical probes are now commonly used in industrial conditions as well as in laboratory experiments. Although they have been primarily devoted to void fraction measurements, additional information could be extracted from the raw signals they deliver. For a stretched optical probe, it is shown that the modulus of the ensemble velocity of a bubble ‖V0‖ could be inferred from the rise time Tu associated with the liquid/gas transition. These two parameters are correlated thanks to piercing experiments in which the interface curvature R and the angle β between the probe and the normal to the interface are controlled. While the influence of R is negligible, the rise time is very sensitive to β. A one‐to‐one relation between Tu and ‖V0‖ is ensured for quasiperpendicular impactions which must be accordingly distinguished among the bubble signatures. The software which includes the adequate signal processing criteria is described. It gives access to void fraction and to the chord‐velocity joint distribution which p...

Preferential concentration of heavy particles: A Voronoï analysis

We present an experimental characterization of preferential concentration and clustering of inertial particles in a turbulent flow obtained from Voronoi diagram analysis. Several results formerly obtained from various data processing techniques are successfully recovered and further analyzed with Voronoi tesselations as the main single tool. We introduce a simple and nonambiguous way to identify particle clusters. We emphasize the maximum preferential concentration for particles with Stokes numbers around unity and the self-similar nature of clustering and we report new unpredicted results concerning clusters inner concentration dependence on Stokes number and global seeding density. Some of these experimental observations can be consistently interpreted in the context of the so-called sweep-stick mechanism. Finally, we stress the great potential of Voronoi analysis that offers important openings for new investigations of particle laden flows in terms, for instance, of simultaneous Lagrangian statistics of particle dynamics and local concentration field.

Optical probes for local void fraction measurements: Characterization of performance

Optical probes are widely used in two phase gas‐liquid flows, especially in laboratories for experimental research. However, despite all the work previously done on the subject, the performance of these sensors is not yet firmly established. To clarify this point, well controlled experiments dealing with the response of various optical probes during the piercing of an interface are presented. The latency length concept L*, which is the spatial resolution of the interface detection by a given probe, is introduced. The minimum value recorded is about 130 μm. Moreover, this parameter is closely related to the hydrodynamic response time of the sensor, and as such, to the geometric displacement velocity of each interface detected. Hence, the latency length is proposed as an objective basis for the comparison of the different probes. Finally, the relation between the response time and the signal processing techniques is discussed, and it is shown that the shortest length L* must be sought in order to ensure a b...

Single and double optical probes in air-water two-phase flows : real time signal processing and sensor performance

Abstract To progress in the understanding of phase detector capabilities, a first requirement has been fulfilled by manufacturing reproducible sensors. A second objective, investigated in this paper, is to diminish the sensitivity of the processing technique to the various criteria required to perform the signal analysis. A new real-time signal processing technique, based on optical probe responses during probe–interface interactions, is presented, and its objectiveness is demonstrated. When used with mono-fiber or dual-fiber sensors, it provides local void fraction and gas velocity measurements. The performance of these measuring chains is evaluated in various air–water two-phase flows for different probes. Possible mechanisms responsible for void fraction uncertainties are discussed. It is also shown that monofiber probes with a conical–cylindrical–conical extremity are well suited to measure gas velocities in dispersed two-phase flows.

Turbulent transport of material particles: an experimental study of finite size effects.

We present experimental Lagrangian statistics of finite sized, neutrally bouyant, particles transported in an isotropic turbulent flow. The particles diameter is varied over turbulent inertial scales. Finite size effects are shown not to be trivially related to velocity intermittency. The global shape of the particles acceleration probability density functions is not found to depend significantly on its size while the particles acceleration variance decreases as it becomes larger in quantitative agreement with the classical k(-7/3) scaling for the spectrum of Eulerian pressure fluctuations in the carrier flow.

Monofiber optical probes for gas detection and gas velocity measurements: conical probes

Abstract Optical probes are a very common tool for the investigation of gas–liquid flow dynamics. A single sensitive tip gives access to the phase indicator function, while using two tips some distance apart provides estimates of the gas velocity. Recently, it has been shown that the gas velocity could also be inferred from a monofiber probe provided that its latency length is known. To improve the capabilities of this new technique, an optimisation of the probe geometry, based on optical simulations and controlled piercing experiments, has been undertaken. In this first paper, conical probes (1C) produced using a new manufacturing technique are considered. Although they are effective for simultaneous gas velocity measurements, their actual response is sensitive to small geometrical defects occurring at their tips. Therefore, calibrations on well controlled interfaces appear necessary both to check the presence of pre-signals responsible for incorrect phase detections, and to establish a significant rise-time/velocity correlation.

Liquid sloshing and wave breaking in circular and square-base cylindrical containers

Near resonance sloshing in containers, filled with a liquid to a given depth h, depends on three parameters, which are the viscous damping, the frequency offset that contains the forcing amplitude and the fluid depth. Experiments have been conducted with low-viscosity liquids mainly in circular cylindrical containers of radius R subjected to harmonic horizontal forcing; complementary experiments on wave breaking have been performed in a square-base container. The fluid depth was kept large (h/R > 1) so that it was no longer a variable parameter. The bounds of existence of the different wave regimes, namely planar waves, swirling waves, chaotic sloshing as well as breaking waves, have been determined as a function of forcing frequencies relative to the lowest natural frequency ω1 and for a wide range of forcing amplitudes. It is shown that when the forcing frequency ω is slightly larger than the lowest natural frequency ω1, planar wave motion bifurcates to a swirling wave mode at finite wave amplitude, the value of which depends on the offset parameter. The swirl wave amplitude grows exponentially and saturates at a certain value. The swirl has a hard-spring behaviour, is very robust and can generate a vortical flow of the liquid column. Chaotic sloshing and wave breaking occur quasi-periodically: growth of planar wave amplitude at a rate depending on the forcing amplitude, collapse, irregular swirl and again growth of planar wave amplitude. The details and periodicity of the chaotic behaviour and breaking depend on the frequency-offset parameter. Close to the natural frequency, chaotic wave motion is possible without breaking. Planar wave breaking is, in general, associated with spilling caused by the encounter of nearly freely falling lumps of fluid with the upward moving wave crest, in a way demonstrated previously in two-dimensional wave breaking. In three dimensions, the wave crest is destabilized in the crosswise direction so that spilling is not uniform along the wave crest and an irregular swirl is generated following breaking; free fall of fluid lumps occurs over many wave periods. The complementary experiments, performed in a square-base container of base dimension L, show four different wave patterns of wavelengths L and L/2 crosswise to the primary wave. This cross-wave instability is interpreted in terms of parametric instability.

Bubble-induced agitation and microstructure in uniform bubbly flows at small to moderate particle Reynolds numbers

The bubble-induced agitation has been quantified in dilute and homogeneous bubbly flows. To characterize the microstructure, experimental techniques have been developed that provide the average perturbed liquid velocity and the pair density. For particulate Reynolds numbers Rep about unity and contaminated bubbles, the liquid agitation is shown to increase as α/Rep where α is the local void fraction. Although in partial agreement with the scaling proposed by Koch (Phys. Fluids, 1993), the pair-density distribution does not exhibit any deficit. On the opposite, experiments at Rep=O(10) and clean bubbles reveal a strong deficit in the rear of test bubbles as well as a moderate accumulation both in a horizontal plane and all along the deficit zone. The extent of the deficit slowly decreases with the void fraction, roughly as α−0.3±0.1. It is shown that the agitation originates from this near wake region, the structure of which strongly differs from the one due to a single inclusion both in terms of its later...

Monofiber optical probes for gas detection and gas velocity measurements: optimised sensing tips

Abstract In order to perform gas velocity measurements using a single optical probe, an optimisation of the probe geometry has been undertaken. The responses of conical probes, analysed in a previous article, were found to be strongly sensitive to deviations from an ideal geometry. To render the technique much sounder, two new shapes, namely, cone+cylinder (2C) and cone+cylinder+cone (3C) are considered. They are both effective for simultaneous gas detection and gas velocity measurements, but the latter, free of proximity detection, appears to be the most promising due to a calibration curve, i.e. the relationship between signal rise time and interface velocity, weakly sensitive to uncontrollable parameters such as the interface impact angle. In addition, its latency length can be controlled during the manufacturing process, allowing good reproducibility of probe tips. Analysis of the signal transients is used to provide guidelines for effective signal processing. Finally, possible extensions of the monofiber technique are discussed as well as remaining limitations.