A methodology to predict thermoacoustic stability in rocket engines is presented. It is based on a divide and conquer principle. The central elements, consisting of the combustion chamber and the nozzle are calculated together directly by a hybrid approach using an extended version of the DLR's acoustic solver PIANO. Beside these central elements, the different components affecting the overall thermoacoustic stability are simulated separately and their properties are lumped into an adequate mathematical description, which is then integrated into PIANO. Each component is analyzed and optimized in its individual environment to reduce the complexity of the interaction processes, which govern thermoacoustics. The challenging step however is the incorporation of all components into a complete stability analysis and thereby keep the computational cost within reasonable limits to make this approach attractive for industrial purposes. In this report the fundamental approach is explained as well as the different components are described by means of their relevance for thermoacoustics and used modelling approaches are shown. Finally the strengths of the approach are confronted with its disadvantages. Especially its realizability and future prospects are discussed.
High pressure §uctuations coupled with unsteady heat release can a¨ect a rocket engine seriously. Especially when the oscillations match eigenmodes such as T1, T1L1 and T2, T2L1, the acoustic pressure amplitude can reach a critical level. This paper deals with the investigation of the nozzle admittance, which is an important value to characterize the in §uence of the nozzle on the pressure inside the combustion chamber. Two di¨erent nozzle geometries are investigated experimentally at high frequencies. A method to decouple the acoustic modes is presented. The results are compared against an existing theory and simulated data.
The efficient prediction of combustion noise by azonal approach is discussed. Forthe so called propagation zone an optimized finite difference method adopted from fantone noise propagation is applied together with alinear perturbation approach based on the Euler equations. Special attention is payed to the indirect noise generation in the propagation zone, where initially quiet perturbations of the fluid state radiate noise when accelerated or decelerated in av arying base flowregime. Anon-isentropic mathematical model is chosen. The approach is compared with apreviously published experiment for the assessment of indirect combustion noise. The numerical results showthe strong influence of reflections from the inflowboundary of the plenum on the resulting pressure response in the outlet duct. By assuming reasonable reflections from the plenum, the observed time signal of the pressure reaches ar easonable agreement between numerical simulation and experiment. Furthermore, the acoustic intensity is used to showthe good quality of the numerical solution. Finally,the intensity based source location identifies the nozzle as adominant source of sound, with large magnitudes of generation and annihilation of acoustic energy.The source power found in the heated volume is more than twoorders of magnitude smaller than in the nozzle. This result again underlines the importance of indirect combustion noise. PACS no. 43.20.Mv,43.28.Kt, 43.60.Jn ACTA ACUSTICA UNITED WITH ACUSTICA Richter et al.:M odel experiment forindirect combustion noise Vol. 95 (2009)
In this paper the benchmark problem of radiation from a semi infinite duct is discussed. The duct is modeled by an infinitesimal thin wall. Comparisons are made between the analytic solutions of R. M. Munt and the 2 nd CAA Workshop with our CAA-code. In all cases a 3D axisymmetric approach is chosen to describe the propagation of a single mode. The procedure consists of two steps.Step 1 concerns a numerical simulation of the near-field-region.Step 2 calculates the farfield-solution by means of the acoustic-analogy of Ffowcs Williams and Hawkings. Since the wall is of infinitesimal thickness, the edge needs to be treated in a special way. Two different approaches are persecuted and the results are compared.
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