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International audience; Acoustic damping plays a key role in the stability analysis of liquid rocket engines (LRE). Its characterization is a necessary step towards prediction of thermoacoustic instabilities. To our knowledge, current analysis of the damping is limited to experimental measurements in practical systems or theoretical models for simple configurations. This study presents large-eddy simulations (LES) of the NPCC setup, a cold-flow configuration representing a simplified LRE geometry, compared to experiments previously carried out at EM2C, to characterize the global acoustic damping of the rig. Steady-state simulations are first performed. Headlosses due to the injection plane and jet profiles are assessed. Then, simulations of the forcing of three eigenmodes give results in very good agreement with the experiment. It is found that the high forcing amplitudes reached in the experiment triggers a non-linear response of the rig. By imposing a lower modulation in the simulations, it is possible to stay in a linear framework. A low-order linear model for the pressure response using the acoustic damping as a key parameter is then compared to the LES and correctly predicts the transitory phase and the limit-cycle amplitude.

International audience; This work aims at a better understanding of damping in coupled-cavities configurations such as liquid rocket engines (LRE). A cold flow rig called NPCC (New Pressurized Couple Cavities) is employed. It consists of two cavities mimicking dome and combustion chamber, linked by three injectors. The cavities can be either continuously excited or successively excited then relaxed by two different perforated rotating wheels placed at the outlets. Eigenmodes are first characterized. The rig is then excited at three successive eigenfrequencies using continuous modulation. The modes structures are checked using pressure transducers. Velocities at the injector's exits are measured with hot wires to characterize the injector's response. Forcing followed by relaxation (alternating forcing) is then used to study the damping rate of each mode, which is retrieved from the pressure transducers signal. 2D unsteady numerical simulations are performed to retrieve these values, both for continuous and alternating modulation. In the damping rate study, at least 10 excitation/relaxation cycles are necessary to achieve good convergence. The results are shown to be independant from the location of the probe used to derive the damping rate. The results are in reasonable agreement with the experiments for the three modes studied with continuous modulation. The study of the damping rate will however require 3D simulations to achieve good results with the methods presented here.

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International audience; The present work introduces a method to treat interfaces in a multiphase flow with phase changes using the socalled Second Gradient theory introduced by Van Der Waals (1789) and later extended by Cahn and Hilliard (1958). In the prospect of simulating a full cryogenic rocket-engine ignition, the model is intrinsically designed so as to treat such flows with real gas thermodynamics for a wide range of regimes, from a subcritical two-phase flow where the capillary dynamics pilot the system through the surface tension to a supercritical fluid where such effects disappear. Its thermodynamic consistency, a key element, has been thoroughly investigated and demonstrated using canonical mono-dimensional cases. In order to reach a reasonable grid resolution, an interface thickening method has been developed, inspired by the work of Jamet et al. (2001). It allows to keep a typical direct numerical simulation (from the turbulence point of view) resolution for the interface. This method has been implemented in the code AVBP, jointly developed by CERFACS and IFPEN, and validated on two-dimensional simulations of out-of-equilibrium plane interfaces. The model has been used to simulate liquid-gas mixing layer and is currently applied to non isothermal liquid jet injection.

International audience; A two-staged swirling burner is numerically simulated using Large Eddy Simulation (LES). This combustor uses two types of injection: a multipoint system that consists in 10 holes in a crossflow configuration, and a pilot system that uses a pressure-swirl atomizer. The relation between the rate of fuel injected from each injection system was found to be related with flame shape transition and hysteresis phenomena[4]. Also, the pilot spray was found to have a major role on these transitons, so it is of paramount importance to correctly reproduce its behavior on the numerical modeling, if one is interested in simulating these flame bifurcations. To describe the spray, a point-droplet approximation is used in a Lagrangian framework with the FIM-UR model [1], that has already proven its accuracy for several configurations. However, in this application it fails to reproduce the droplet size distribution, especially in the Central Recirculation Zone (CRZ), as it uses an arbitrary expression to impose the spray opening limits (which are not input parameters). In the present work, the input parameters of the FIM-UR model are modified to enable the recovery of the right droplet size distribution and improve the description of the liquid velocity field, resulting in a better numerical representation of the experimental results, essential for further studies.

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The present work is motivated by the high-frequency combustion instabilities often encountered during liquid rocket engine developments. These instabilities are usually associated with transverse acoustic modes of the combustion chamber. In this framework, the objective of the present contribution is to gain comprehension on the dynamical response of coaxial two-phase flows submitted to transverse acoustic modulations. A formulation for the analytical acoustic radiation force experienced by a sphere submitted to a multi-harmonic stationary wave is first proposed. This formulation is then used as a reference to validate a compressible simulation code through a simple numerical case representative of what is experienced by the central liquid core in coaxial jets. Afterwards, the dynamics of a spray under acoustic radiation modulations are examined. It is found that the acoustic radiation force and the drag between the spray droplets and the gaseous phase play an important role in the size segregation mechanism. This last process may impact the triggering of combustion instabilities in liquid rocket engines at subcritical operating conditions.

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International audience; Because of increasingly stringent regulations on jet engines pollutant emissions, new ways of reducing them are needed. A promising way is the use of Lean Premixed Prevaporized (LPP) combustion where the liquid fuel is vaporized and mixed with air in excess before burning. In the present work, a model gas turbine combustor fed with liquid dodecane is studied experimentally. It is equipped with two fuel injection stages: a pressurized nozzle called the pilot and a multipoint device. When the split of the fuel injection between the stages is changed, several phenomena can be observed. The flame shapes can change drastically and present different behaviors, some of them showing a strong acoustic activity. In particular, two different flame shapes can be obtained for the exact same operating conditions depending on the burner history. The first flame, named sV, is stabilized thanks to an internal reaction zone that modifies the air flow compared to non-reacting conditions. The second state, associated with a Lifted flame, exhibits a strong thermo-acoustic instability linked to the quarter-wave mode of the chamber. A switch from the sV to the Lifted state can be triggered by air flow rate modulations while the opposite change can occur naturally as the fuel split is changed. These complex phenomena originate from multiple interactions between the fuel spray, the gaseous flow and the flame itself.

For a long time, engine noise has been dominated by fan and jet noise. With their reduction for modern turbojets, for instance, combustion noise is no longer negligible and efforts are concentrated on its understanding. The objective of the European project RECORD is to help to understand the fundamental mechanisms of core noise, in order to reduce it. In this paper, the recent advances achieved within the project for the noise generated through a nozzle are presented. This work includes the definition of a combustion test bench at the EM2C laboratory, equipped with various diagnostic techniques and an outlet nozzle to pressurize the chamber and generate entropy noise. Reactive simulations of this facility are performed by ONERA and CERFACS using LES. Finally, simulated flow fields are post-processed to determine direct (acoustic) and indirect (entropy) contributions to the global noise generated by the nozzle. This work is a first step before the modeling of a full engine, including the turbine stages., AerospaceLab Journal, Issue 11, June 2016; ISSN: 2107-6596

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Actes de: 6th International Conference on Multiphase Flow, ICMF 2007, Leipzig, Germany, July 9 – 13, 2007 Reference S4_Tue_C_23; International audience; The accurate simulation of the dynamics of polydisperse evaporating sprays in unsteady gaseous flow with large scale vortical structures is both a crucial issue for industrial combustion application and a challenge for modelling and scientific computing. Various appraoches have been developped in order to resolve the dispersed liquid phase starting from a “mesoscopic” level of description through the Williams equation. The usual Lagrangian approaches in polydisperse unsteady configurations lead to a very high computational cost and induce coupling difficulties due to the different type of description of the two phases involved. Several Eulerian models have been recently developed in order to avoid these two difficulties. However, the validation of the obtained models through detailed comparisons with measurements on well-instrumented experimental devices are seldom conducted in the litterature. In the present contribution, we consider the configuration of acoustically pulsated free jets with a polydisperse spray injection in a 2D axisymmetrical pulsated jet. It creates large vortical structure which are representative of the flow in more complex configurations and a strong interaction with the injected spray. In this context, we provide both a series of detailed experimental measurements through the coupling of laser diagnostics, as well as the multi-fluid modelling and associated numerical schemes in order to simulate accurately the dynamics of the spray. It allows us to study the physics of the coupling between the two phases and to validate the Eulerian multi-fluid model using the experimental measurements.