Stability of a swirled liquid film entrained by a fast gas stream

International audienceWe study the liquid flow inside a recessed gas-centered swirl coaxial injector, where a swirled liquid flowing against an outer wall is destabilized by a central fast gas stream. We present measurements of the liquid intact length inside the injector, as a function of swirl number and dynamic pressure ratio. We propose a simple model to account for the effect of these parameters.We next study the surface instability inside the injector: its frequency ismeasured for several swirl angles, and as a function of gas velocity. Results are first confronted to the predictions of an inviscid linear stability analysis including swirl, and second to the predictions of a viscous linear stability analysis where swirl is not included. The viscous analysis captures the experimental frequency

Flapping instability of a liquid jet

International audienceIn air assisted atomization, small droplets arise from the stripping of a liquid jet (or a film) by a fast gas stream (Lasheras & Hopfinger 2000, Eggers & Villermaux 2008). Yet, the incoming liquid jet is seemingly never fully atomized by the stripping process alone. Instead, the remaining jet experiences a flapping instability, similar to the instability observed on liquid sheet configurations: the resulting large scale structures break into large liquid lumps some distance downstream the injection. Little is known on the underlying mechanism of this instability and on the characteristics of the large drops it produces, though these large drops probably control flame extent in combustion devices. We suggest in the present study that this instability could be triggered by non-axisymmetric Kelvin-Helmholtz modes. Indeed, in coaxial injector configuration, non-axisymmetric modes of the KH instability can be observed. First, we study the dependence of KH modes upon two control parameters, namely the liquid and gas velocities, and discuss the symmetry of these modes. Secondly, we investigate a possible link between non-symmetric modes of KH instability and the large scale instability. Finally, amplitude of the large scale oscillation is measured as a function of gas and liquid velocity

Influence of Gas Turbulence on the Instability of an Air-Water Mixing Layer

International audienceWe present the first evidence of the direct influence of gas turbulence on the shear instability of a planar air-water mixing layer. We show with two different experiments that increasing the level of velocity fluctuations in the gas phase continuously increases the frequency of the instability, up to a doubling of frequency for the largest turbulence intensity investigated. A modified spatiotemporal stability analysis taking turbulence into account via a simple Reynolds stress closure provides the right trend and magnitude for this effect

3D Acoustic Lagrangian Velocimetry

International audienceWe report Lagrangian measurements obtained with an acoustic Doppler velocimetry technique. From the Doppler frequency shift of acoustic waves scattered by tracer particles in a turbulent flow, we are able to measure the full three-component velocity of the particles. As a first application, we have studied velocity statistics of Lagrangian tracers in a turbulent air jet at Rλ∼320 and at various distances from the nozzle. The choice of an air jet is motivated by the fact that jets produce a well characterized high level tubulence and open air flows are well suited to simultaneaously achieve classical hot wire Eulerian measurements. Therefore, we are also able to explicitly address the question of the differences between Eulerian and Lagrangian statistics. As Lagrangian tracers we use soap bubbles inflated with Helium which are neutrally buoyant in air and can be assimilated to fluid particles. Velocity statistics are analysed. We show that the Lagrangian autocorrelation decays faster in time than its Eulerian counterpart. Finally we present Lagrangian time velocity increments statistics which, as already reported by previous work, exhibits stronger intermittency than Eulerian velocity increments

A Study of the Internal Two-Phase Flow in Gas-Centered Swirl Coaxial Injectors

International audienceAn effective atomization of liquid is of importance in the performance of combustion engines. For liquid hydrocarbon rocket engines with a staged combustion cycle for high-power application, the Gas-Centered Swirl Coaxial (GCSC) injector is widely employed. Gaseous oxidizer at high velocity enters directly through the center of the injector and is surrounded by a swirled liquid film injected along the periphery of the injection element. The swirled liquid film is stripped and fragmented into drops by the high velocity gas stream. The understanding of the atomization characteristics of the injector should be improved for the design of more reliable and efficient injectors dedicated to liquid rocket engines. In order to effectively evaluate atomization performances, it is essential to precisely predict liquid film dynamics inside the injector. The liquid film thickness and length are a function of the injector recess length, and they affect the atomized drop size. Internal flow visualization with a LIF (Laser Induced Fluorescence) method was conducted to investigate the overall form and the interface corrugation of the liquid flow at various swirl strength conditions. The swirl strength is varied by changing the inlet angle of tangential entry holes. The experimental results show clearly that the intact liquid length increases with increasing the swirl strength at the same dynamic pressure ratio. We also measured the frequency of the surface perturbations with a spectral method. We find that this frequency increases steadily with gas velocity, and appears to be independent of the initial swirl number

Instability regimes in the primary breakup region of planar coflowing sheets

International audienceThis article investigates the appearance of instabilities in two planar coflowing fluid sheets with different densities and viscosities via experiments, numerical simulation and linear stability analysis. At low dynamic pressure ratios a convective instability is shown to appear for which the frequency of the waves in the primary atomization region is influenced by both liquid and gas velocities. For large dynamic pressure ratios an asymptotic regime is obtained in which frequency is solely controlled by gas velocity and the instability becomes absolute. The transition from convective to absolute is shown to be influenced by the velocity defect induced by the presence of the separator plate. We show that in this regime the splitter plate thickness can also affect the nature of the instability if it is larger than the gas vorticity thickness. Computational and experimental results are in agreement with the predictions of a spatio-temporal stability analysis

., Induced agitation and the prediction of phase distributions in laminar bubbly flows, CD Proc.

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Bubble columns hydrodynamics revisited according to new experimental data

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Some aspects of bubbly flows dynamics as revealed by advanced measuring techniques combined with hybrid modeling

International audienceBuoyancy driven bubbly (or particulate) flows are commonly exploited in industry. Owing to the strong mechanical coupling between the two phases, their structure is still difficult to predict in spite of the numerous contributions that have been dedicated to determine and/or to improve closure laws. Nowadays, direct numerical simulations of a large set of freely moving inclusions are becoming more and more realistic, and it is sometimes argued that such a direct approach offers the best option for predictions. In this lecture, we will focus on simple prototype bubbly flows formed from spherical monodispersed bubbles in a laminar continuous phase. We will first review key behaviours of such flows, as well as what can be learned from axial momentum balances. Then, we will illustrate the diversity and the complexity of the coupling mechanisms that must be accounted for in order to acccurately predict the transverse distributions. The discussion will be grounded on a hybrid formalism combining kinetic theory concepts for the dispersed phase and a continuous approach for the carrier fluid. Measuring techniques issues will also be evoked during the lecture, both in terms of uncertainty associated with usual variables (such as void fraction, bubble velocity...), and also in terms of the determination of new quantities as required by the hybrid model. We will show how the proposed averaged formalism can lead to a closed system of equations when combined with direct simulations involving only a few test inclusions. We will conclude on a possible numerical implementation of that scheme in order to predict simple laminar bubbly (or particulate) flows