Thermo-acoustic instabilities driven by fuel droplet lifetime oscillations

A new mechanism of thermo-acoustic destabilization of combustors powered by burning droplets is inves- tigated. Acoustic induced heat release disturbances are found to be driven by oscillations of droplet lifetime during the final stage of the droplet combustion before their extinction. The mechanism is the following. Syn- chronized acoustic disturbances alter the droplet evaporation rate and modify their diameter. Perturbations of the droplet diameter along the droplet trajectories lead to fluctuations of their lifetime. These fluctua- tions trigger in turn an oscillating motion of the burning droplet cloud boundary that is synchronized by the acoustic excitation. This leads to large heat release fluctuations before droplet extinction and constitutes a thermo-acoustic source. This mechanism is found to be particularly important in solid rocket motors in which aluminum droplets released from the propellant burn individually and are quenched as the droplet diameter falls below a critical residue diameter associated to an inert particle. Analytical models are derived in an idealized configuration where acoustic forcing takes place in the transverse direction to the droplet tra- jectories. Expressions are derived in the frequency space for the droplet diameter oscillations, the motion of the droplet cloud boundary and the resulting heat release disturbances, that take the form of a distributed Flame Describing Function. This model is used to reveal effects of the acoustic pressure level and of the residue diameter on the resulting motion of the combustion volume boundary and corresponding heat re- lease disturbances. It is further extended to consider the peculiar flow in a solid rocket motor and the heat release disturbance model is compared to results from numerical flow simulations of the motor. While high- lighted in the context of solid propulsion, the observations made are believed to be more general and the same mechanisms may persist in acoustically perturbed spray flames from hydrocarbon fuel droplets

An analytical model for acoustic induced aluminum combustion fluctuations in solid rocket motors

International audienceIn a Solid Rocket Motor, the combustion of aluminum droplets released by the solid propellant is often used to increase the thrust. The dynamics of this distributed combustion can drive thermo-acoustic instabilities. An analytical model for the local heat release rate fluctuations of the burning droplet cloud is derived and compared with previous low order models and with numerical flow simulations. This new model leads to close agreement with simulations and improve our understanding of the pressure oscillation source. Two contributions to heat release rate fluctuations are identified. The first one originates from the burning droplet cloud within the combustion volume and the second one lies at the combustion volume boundary between the burning cloud and the inert zone. The first contribution is the consequence of the response of the individual droplet dynamics to the flow oscillations and the second one is due to droplet lifetime oscillations. Both contributions depend on the droplet diameter, droplet velocity and gas velocity fluctuations. Models for diameter and droplet velocity fluctuations are derived by considering the peculiar structure of the acoustic boundary layer along the solid propellant surface with mass injection. The expressions for heat release rate fluctuations derived in this study can be used to predict the linear stability of a solid rocket motor at reduced computational costs

A numerical analysis of aluminum droplet combustion driven instabilities in solid rocket motors

International audienceAcoustic coupling with unsteady aluminum particle combustion may lead to self-sustained instabilities with large oscillation amplitudes in a solid rocket motor. A numerical analysis of this phenomenon is carried out for instabilities and at instability limit cycles in a set of generic configurations. It is found that the synchronized combustion oscillations can be split in two different contributions to heat release disturbances driving the instability. In the combustion volume, volumetric heat release rate fluctuations result from the cumulative contribution of burning rate oscillations of each individual aluminum droplet experiencing an oscillating drag. The second contribution to heat release oscillations in the SRM corresponds to the motion of the aluminum combustion zone boundary. In the configurations explored, this contribution may reach up to about 40% of the total heat release rate oscillation in the motor and is shown to depend on the way the end life of aluminum droplets is modeled

JAGUAR: a New CFD Code Dedicated to Massively Parallel High-Order LES Computations on Complex Geometry

International audienceLES of industrial flows is associated with geometrical complexity and requires high order schemes to minimize dissipation and dispersion. To tackle these two issues it is necessary to use unstructured grids and High Performance Computing algorithms. In this context, CERFACS initiated two years ago the development of a new CFD code called JAGUAR based on a mathematical framework leading to high-level capability for LES. In this paper, many topics for HPC are introduced and solved in order to obtain the best code performance

JAGUAR: a New CFD Code Dedicated to Massively Parallel High-Order LES Computations on Complex Geometry

International audienceLES of industrial flows is associated with geometrical complexity and requires high order schemes to minimize dissipation and dispersion. To tackle these two issues it is necessary to use unstructured grids and High Performance Computing algorithms. In this context, CERFACS initiated two years ago the development of a new CFD code called JAGUAR based on a mathematical framework leading to high-level capability for LES. In this paper, many topics for HPC are introduced and solved in order to obtain the best code performance