Pressure Oscillations in Solid Rocket Motors
By M. Telara, Avio Spa, Colleferro (Rome), Italy, andF. Stella, F. Paglia, and M. Giangi, Dipartimento di Meccanica e Aeronautica Università di Roma “La Sapienza”, Rome, Italy
Pressure oscillations are a well-known problem of large solid rocket motors (SRMs). In this kind of motor, pressure oscillations lead to thrust oscillations and to significant dynamic loads that often cause the need for damperson the payload, reducing the payload mass capacity and launcher usability. The problem has attracted the attention of several researchers during the last few decades and, although pressure oscillations cannot yet be completely controlled, the physics of the mechanism that causes them in SRMs is now well understood. Pressure oscillations originate in the combustion chamber of an SRM after most of the fuel has been burnt and after vortices have developed in the flow of the gaseous combustion products. A feedback loop can cause these vortical perturbations to grow until a self-sustained pressure oscillation becomes established.
Ariane 5 geometry – the PTFs placed on the frontal surfaces of solid propellant segments are circled in red
Copyright ESA/CNES/ARIANESPACE – D. DUCROS
A solid rocket motor consists of a few elements, including:the solid propellant, called grain, a case, thermal protections, a nozzle, and an igniter. For large SRMs, which for technological reasons are characterized by a segmented grain geometry, vortices in the combustion chamber flow can be produced in three different ways:
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By obstacle vortex shedding behind the tip of frontal thermal protection devices
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By parietal vortex shedding from the propellant surface
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By angle vortex shedding off angles in the grain geometry
The three mechanisms of vortex shedding on the full P230 geometry
Frontal thermal protection (PTF) devices are present on the front surfaces of propellant segments in Ariane 5 P230boosters. Made of different types of ethylene-propylene-diene-monomer (EPDM) based rubbers, the PTFs are used to prevent the combustion of the frontal face of the grain segments. During flight, the inert material of the PTFs burns at a slower rate than the propellant, leading to annular ring protrusions in the combustion chamber that act as obstacles to the flow. After much of the propellant has burned, the flow past these rings produces regions of high shear and causes periodic vortex shedding. The PTF rings are, in the authors’ opinion, the most important source of vortex shedding in the Ariane 5 P230 booster rocket.
The interaction between aerodynamics and acoustics that is at the heart of self-sustained pressure oscillations in the SRM combustion chamber is essentially a feedback loop. When the vortex shedding frequency is close to the natural frequency of the chamber, pressure oscillations grow and reach a maximum.The numerical simulation of such complex phenomena is a challenging task, since every one of the steps involved in the feedback loop has to be properly accounted for. In the past,codes devoted to the prediction of pressure oscillations have been developed and extensively applied. Several authors have presented numerical results for SRM pressure oscillations, but in some cases the results are substantially different from experimental data [1].
Pressure spectrum of the full Ariane 5 P230 geometry, computed using the mid-sized mesh
One of the goals of the present study is to evaluate FLUENT for its ability to estimate the frequencies and amplitudes of pressure oscillations in the Ariane 5 P230 booster. Before attempting numerical simulations of the full scale P230 motor,a number of other tests were conducted on simpler geometries,ranging from cold flow tests to small scale SRM (called LP6)simulations. For the sake of conciseness, in the present paper only one cold flow test and the P230 results are presented.Both analyses share the same idea in approaching the problem, which is that the pressure oscillation is basically a fluid dynamics phenomenon that can be adequately reproduced using a pure CFD approach. For the simple cold flow case, the fluid flow was assumed to be in the axial direction toward the SRM nozzle. For the small scale and full scale SRM models, inlet conditions were imposed normal to the side walls, simulating solid propellant combustion. Axisymmetric models were used for all cases studied. Each time-dependent simulation began with a long initial transient during which time the feedback mechanism became established. After this period, data acquisition began for the analysis of the chamber acoustics.
In all of the numerical simulations performed, an impressive agreement between the results and experimental data was observed both in terms of oscillation frequency and amplitude.
In the cold flow test case, errors lower than 2% on the frequency and lower than 13% and 6% on the amplitudes of the first and second modes were obtained. A previous work [2], on the other hand, reported an error of one order of magnitude on the amplitude.
The most significant results were obtained for the full scaleP230 SRM, and these details are described in Reference 3. To conduct this analysis, the flow and geometrical conditions corresponding to the “second peak” of the Ariane 5 MPS P230 motor was chosen. The second peak occurs at about 70% of the total combustion time, so most of the fuel has been burned at this point. During the numerical simulations, control points were located at the motor head where the static pressure was sampled during bench/flight tests. In order to test the grid dependence of the results, three different meshes were adopted,ranging from a coarse mesh (100,000 cells) up to a very fine one (1,200,000 cells). All results from the three meshes studied predicted the main peak at 20.7 Hz, which is in good agreement with the values measured during the firing test and/or obtained from flight data (around 21.2 Hz). It is worthy to observe that this difference is inside the error bar of the numerical simulation, or frequency resolution of the analysis,which is derived from the time step and the number of samples taken. These results demonstrate the ability of the method to predict oscillation frequencies even with a coarse mesh.
Feedback loop leading to self sustained pressure oscillations in SRMs
FLUENT numerical simulation of the simplified cold flow test; vortex shedding behind the PTF rings and changes in the propellant grain geometry are shown
On the contrary, the amplitude of oscillations was found to change considerably with mesh density, going from 40 mbar (coarser mesh) to 151 mbar (finer mesh). This result showed that a mesh sensitivity analysis is needed whenever the amplitude of oscillations is of interest. The amplitudes of oscillations obtained using the two finer meshes were very close to each other (149 and 151 mbar) showing that the numerical solutions had reached asymptotic behavior. The results for oscillation amplitude were compared with experimental data obtained from bench/flight data of the P230booster. The comparison was conducted on the basis of energy in the frequency band 17-25Hz,and showed a difference from averaged flight data lower than 5%.It is worth noting that all mechanisms of vorticity production in SRMs have been observed and properly reproduced using CFD. The results suggest that vortices initially produced from the shear layer on the PTF, are merged in the initial phase of the process and then mixed with the vortices extracted from the side (propellant) walls. In this manner, large, non-homogeneous vortex structures are produced and transported along the motor to the nozzle exit. In spite of the complexities of this flow pattern, numerical simulations conducted have shown an impressive agreement with bench and flight experimental data, showing that the adopted methodology can properly resolve such fine details of the flow.
Details of obstacle vortex shedding
Details of parietal vortex shedding
References
- Kourta, K.: Computation of vortex shedding in solid rocket motors using time dependent turbulence model., J. of Prop. And Power, Vol. 15, No.3, pp.390-400, 1999.
- Anthoine, J.; Buchlin, J.M.; Guery, J.F.: Effect of Nozzle Cavity on Resonance in Large SRM:Numerical Simulations. J. of Prop. and Power, Vol.19, No. 3, pp 374-384, 2003.
- Stella, F.; Paglia, F.; Giangi, M.; Telara, M.: Numerical Simulation of Pressure Oscillations in Solid Rocket Motors. European Conference in Aerospace Sciences(EUCASS) Moscow, 2005.
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