Experimental and numerical investigations of nozzle geometry effect on the instabilities in solid propellant boosters

Abstract

Introduction The nozzle design effect on sound production is investigated at VKI to improve the understanding of the aeroacoustic coupling that occurs in the Ariane-5 booster. Acoustic measurements performed by means of PCB piezoelectric transducers are taken in a 1/30scale axisymmetric cold flow model of the Ariane5 SRM with purely axial injected flow. Flowacoustic coupling is observed for the nozzles including cavity and an analytical model of the resonance occurrence is developed. Furthermore, the maximum resonance amplitude is highly dependent on the nozzle design. Without cavity, the fluctuations are damped by at least one order of magnitude. The experimental pressure spectra are compared to numerical simulations performed using the code CPS. The frequencies are well simulated by the numerical code even if the pressure levels are overestimated. The nozzle design effect on sound production is also observed numerically. Furthermore, the nozzle cavity modifies the flow field around the nozzle head. With cavity, the recirculation bubble is shorter and the flow close to the nozzle head presents high amplitudes of radial mean velocity and fluctuation. That explains why the vortices break up when interacting with the nozzle head generating acoustic pressure fluctuations. *Ph.D. Candidate, AIAA Student Member t Professor *Head of CFD Group, AIAA Member Copyright © 2000 by the von Karman Institute for Fluid Dynamics. Published by the American Institute of Aeronautics and Astronautics, with permission. The aeroacoustics of solid propellant booster is currently being investigated at the von Karman Institute (VKI) as a part of the ASSM program (Aerodynamics of Segmented Solid Motors), initiated by the CNES to support the development of the Ariane-5 solid propellant motor (EAP). This accelerator has a segmented combustion chamber consisting of three cylindrical segments and a submerged nozzle (figure 1). Two inhibitor rings ensuring thermal protection separate the three segments. The hot burnt gas flow originates radially from the burning surface of the combustion chamber and then develops longitudinally before reaching the exhaust nozzle. During the combustion, the regression rate of the burning surface is faster than those of the inhibitor rings. Then, the lasts become obstacles into the flow-field and generate a significant risk of hydrodynamic instabilities. Pressure oscillations have been already observed for solid rocket motors, such as the U.S. Space Shuttle, the Titan SRM and the Ariane-5 EAP [1, 2, 3]. Similar results were obtained on sub-scale models of the Ariane-5 booster [4].