Multi-scale Eulerian-Lagrangian simulation of a liquid jet in cross-flow under acoustic perturbations

[EN] The design of modern aeronautical propulsion systems is constantly optimized to reduce pollutant emissions while increasing fuel combustion efficiency. In order to get a proper mixing of fuel and air, Liquid Jets Injected in gaseous Crossflows (LJICF) are found in numerous injection devices. However, should combustion instabilities appear in the combustion chamber, the response of the liquid jet and its primary atomization is still largely unknown. Coupling between an unstable combustion and the fuel injection process has not been well understood and can result from multiple basic interactions. The aim of this work is to predict by numerical simulation the effect of an acoustic perturbation of the shearing air flow on the primary breakup of a liquid jet. Being the DNS approach too expensive for the simulation of complex injector geometries, this paper proposes a numerical simulation of a LJICF based on a multiscale approach which can be easily integrated in industrial LES of combustion chambers. This approach results in coupling of two models: a two-fluid model, based on the Navier-Stokes equations for compressible fluids, able to capture the largest scales of the jet atomization and the breakup process of the liquid column; and a dispersed phase approach, used for describing the cloud of droplets created by the atomization of the liquid jet. The coupling of these two approaches is provided by an atomization and re-impact models, which ensure liquid transfer between the two-fluid model and the spray model. The resulting numerical method is meant to capture the main jet body characteristics, the generation of the liquid spray and the formation of a liquid film whenever the spray impacts a solid wall. Three main features of the LJICF can be used to describe, in a steady state flow as well as under the effect of the acoustic perturbation, the jet atomization behavior: the jet trajectory, the jet breakup length and droplets size and distribution. The steady state simulations provide good agreement with ONERA experiments conducted under the same conditions, characterized by a high Weber number (We>150). The multiscale computation gives the good trajectory of the liquid column and a good estimation of the column breakup location, for different liquid to air momentum flux ratios. The analysis of the droplet distribution in space is currently undergoing. A preliminary unsteady simulation was able to capture the oscillation of the jet trajectory, and the unsteady droplets generation responding to the acoustic perturbation.Thuillet, S.; Zuzio, D.; Rouzaud, O.; Gajan, P. (2017). Multi-scale Eulerian-Lagrangian simulation of a liquid jet in cross-flow under acoustic perturbations. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 782-789. https://doi.org/10.4995/ILASS2017.2017.4697OCS78278

An improved multiscale Eulerian-Lagrangian method for simulation of atomization process

International audienc

Toward direct numerical simulation of high speed droplet impact

International audienceWhen a liquid drop impacts a solid surface or a liquid film, several outcomes are possible. In particular, splashing phenomena can exhibit complex behaviors, like the formation of very thin crowns and emission of small droplets. Underlying mechanisms are hard to elucidate, partly due to the difficulty for current experimental devices to access the very small length scales involved. Here, we use direct numerical simulation to explore low and high velocity drop impact phenomena. We show that classical incompressible two-phase methods can be sufficient to address low energy impacts and take into account wetting phenomena. However, dedicated robust and conservative methods are needed to simulate splashing phenomenon at higher velocities. In our test cases, we show that an impact on a thick liquid film exhibits thick crown formation and delayed splashing. On a dry wall, on the other hand, splashing phenomenon can be difficult to reproduce even with high velocity impacts. We show however how a higher value of the surrounding air density may trigger splashing. The presence of a very thin liquid film on the wall strongly modifies impact outcome, forcing the ejection of a thin crown and subsequent secondary droplet emission

Regularization of the Lagrangian point force approximation for deterministic discrete particle simulations

International audienceThe current article presents a regularization procedure of the Lagrangian point-force approach commonly used to account for the perturbation of a fluid phase by a dispersed particle phase. The regularization procedure is based on a nonlinear diffusion equation to naturally ensure parallel efficiency when the regularization length scale extends over several grid cells. The diffusion coefficient thus becomes a function of the particle source term gradient and expressions allowing to approximately adjust the regularization length scale according to the local particle to mesh size ratio are proposed, so that mesh refinement or polydisperse sprays may be handled. Elementary numerical test cases confirm the convergence of the present procedure under mesh refinement and its ability to locally adapt the regularization length scale. Furthermore, the chosen regularization length scale allows to match the leading order term of the perturbation flow field set by the particle beyond approximately two particle diameters in the Stokes regime. When applying the presented source term regularization procedure, the terminal velocity of a particle settling under gravity in the Stokes regime becomes relatively insensitive to mesh refinement. However, errors with respect to the theoretical settling velocity remain substantial and removal of the particle's self induced velocity appears necessary to recover the undisturbed fluid velocity at the particle location and correctly evaluate the drag force. As the current regularization procedure yields source terms that are close to c 2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Gaussian, an analytic expression from the literature is used to estimate the particle's self induced velocity. When combining source term regularization and removal of the particle's self induced velocity, good results are obtained for the terminal settling speed in the Stokes regime. Results obtained for horizontally separated particle pairs settling under gravity in the Stokes regime show equally good agreement with theoretical results. Because analytic expressions for the particle's self-induced velocity are no longer available at finite particle Reynolds numbers, correlations recently proposed in the literature are used to obtain correct settling velocities beyond the Stokes regime

Multi-scale simulation of the atomization of a liquid jet in cross-flow in the presence of an acoustic perturbation

International audienceThe reduction of pollutant emissions is currently a major concern in the aerospace sector. Among the proposed solutions, lean combustion appears as an effective technology to reduce the environmental impact. However, this type of technology may also favour the appearance of combustion instabilities. These instabilities, resulting from a thermo-acoustic coupling, can lead to irreversible damage to the combustion chambers.Experimental studies previously conducted at ONERA on a multipoint injector by Apeloig highlighted the importance of atomization on the instabilities loop. Indeed, the fuel vapour concentration near the injection zone has been shown to fluctuate in accordance with the imposed acoustic perturbation. The driving mechanism would then result from a flapping motion of the liquid jets in the multiple injection points, induced by the gas flow oscillations. This would in turn affect the characteristic convective timescales of the fuel, in the form of a spray or even of thin liquid films on the duct walls.In order to characterize this interaction, this work focuses on the unsteady simulation of a round liquid jet in the presence of a transverse gas flow in a rectangular section duct. Following an experimental study, the multi-scale numerical approach for multi-phase flows, implemented in the ONERA CEDRE code, has been tested in presence of an imposed acoustic perturbation. This approach consists of the coupling of three models: a multi-fluid model able to capture the largest scales of the liquid column atomization; a dispersed phase approach for the atomized spray, and a “Shallow Water” approach for wall films. The coupling of these approaches is provided by dedicated atomization and impact models, which ensure liquid transfer between the three models.Simulation results show that the multi-fluid solver is able to correctly capture the largest scales of the liquid jet. The simulated liquid jet trajectories match the experimental ones, as well as their dynamic response to the imposed acoustic perturbation. As the liquid is transferred to the dispersed phase solver, the jet motion deeply affects the spray formation and behaviour. Good agreement was found on the particle resulting mean velocity, but only partial agreement on the phase delay. An important wall deposition has been detected for particular jet positions as well

Simulation numérique directe d'écoulements diphasiques avec maillage auto adaptatif

The object of this thesis was to study the direct numerical simulation of two phase, incompressible, isothermal immiscible flows with the adaptive mesh refinement technique. The Navier-Stokes equations are solved by an explicit projection method. The interface tracking is realized via Level Set method. The Ghost Fluid method used for the jump conditions allows the coupled resolution of the two fluids. The adaptive mesh has been implemented by the use of the PARAMESH package. This package builds a quad-tree like structure of mesh blocks of different cell size, which are recursively bisected in each direction. The blocks share the same number of cells, allowing an efficient workload balance among the available processors. A set of numerical tools have been developed to assure the correct resolution of the equations, starting from an elliptic solver, a fast and robust multigrid preconditioned BiCG-stab solver. The code has been proven on a set of academic test cases to maintain the fine mesh accuracy. The performances in terms of adaptive mesh and parallel speed-up have been shown as well. The code has finally been applied to the primary atomization of a liquid sheet sheared by two parallel high speed air flows. Results showed the capability to capture physical phenomena as the longitudinal sheet oscillation, and the ability to perform multi scale computations, which allow simulations with conditions closer to the actual injectors.L'objet de cette thèse a été d'étudier la simulation numérique directe d'écoulements diphasiques de fluides non miscibles, incompressibles et isothermes avec la technique du raffinement adaptatif local de maillage. La résolution des équations de Navier-Stokes incompressibles est faite par le biais d'une méthode de projection explicite. La capture de l'interface est réalisée explicitement par la méthode Level Set. L'utilisation de la méthode Ghost Fluid pour le traitement des conditions de saut permet la résolution couplée des deux fluides. Le maillage adaptatif a été implémenté avec les librairies parallèles PARAMESH. Celles-ci gèrent la création d'un maillage construit par blocs qui peuvent être bissectés de façon récursive afin d'obtenir la résolution désirée. Les blocs ont tous le même nombre de cellules, ce qui permet une répartition efficace de la charge de travail en parallle. Un ensemble d'outils nécessaires à une correcte résolution des équations ont été développés, à partir d'un robuste solveur elliptique BiCG-stab préconditionné par méthode multigrille. Le code a été vérifié sur des cas tests académiques capable de maintenir la précision sur les grilles plus fines. Les performances du code en termes de réduction de charge de travail et d'efficacité de la parallelisation ont été également illustrées. Le code a été enfin testé sur la désintégration assistée d'une nappe liquide bidimensionnelle cisaillée par des courants gazeux à haute vitesse. Le code s'est avéré capable de retrouver certains phénomènes physiques comme l'oscillation longitudinale de la nappe, ainsi que de permettre une simulation multi échelles, ce qui permet de plus se rapprocher des conditions des injecteurs réels

A Parallel Adaptive Projection Method for Incompressible Two Phase Flows

International audienceDirect numerical simulation of high-density ratio multiphase flows requires a lot of computational resources. We have developed a parallel algorithm for solving the incompressible Navier-Stokes equation in two-phase flows on an adap-tive hierarchy of mesh. The interface between the two fluids is treated by a coupled Level-Set/Ghost-Fluid method; the parallel AMR is handled by the PARAMESH package, which assures good scaling. A robust solver for the variable density pressure equation has been developed, including an AMR levels-based multigrid pre-conditioner. The code has been applied to the study of the oscillation of a two dimensional liquid sheet sheared by an air flow, and global oscillation frequencies have been computed for a simplified test case. A high resolution computation has been performed to show two dimensional ligament formation and break-up

Caractérisation expérimentale des transferts de chaleur dans un canal d'eau avec changement de phase

International audienceUne expérience de canal chauffé avec picots et alimenté en eau liquide a été conçue et fabriquée. Il s’agit d’étudier les phénomènes liés à l’ébullition de l’eau chauffée par la paroi inférieure du canal. Lors de la campagne d’essais les visualisations de l’écoulement montrent la génération de la vapeur dans l’écoulement fluide autour des picots. Des mesures de température dans la paroi et les valeurs de la puissance fournie ont conduit à identifier le flux de chaleur et la température à l’interface fluide/paroi. A partir du bilan thermique dans le fluide, sa température est calculée ce qui permet d’identifier le coefficient d’échange pariétal en présence de l’ébullition