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Two-phase flow investigation in a cold-gas solid rocket motor model through the study of the slag accumulation process

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VKI PHDT 2008-03, Two-phase flow investigation in a cold-gas solid rocket motor model through the study of the slag accumulation process, ISBN 978-2-930389-30-3

Two-phase flow investigation in a cold-gas solid rocket motor mo

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Two-phase flow investigation in a cold-gas solid rocket motor model through the study of the slag accumulation process
By Balázs Tóth
PhD Thesis from the von Karman Institute/Université Libre de Bruxelles, Belgium, January 21, 2008
ISBN 978-2-930389-30-3
Abstract

The present research project is carried out at the von Karman Institute for Fluid Dynamics (VKI, Rhode-Saint-Genèse, Belgium) with the financial support of the European Space Agency (ESA).
The first stage of spacecrafts (e.g. Ariane 5, Vega, Shuttle) generally incorporates large solid propellant rocket motors (SRM), which often consist of a segmented structure and a submerged nozzle. During the combustion, the regression of the solid propellant surrounding the nozzle integration part leads to the formation of a cavity around the nozzle lip. The propellant combustion generates liquefied alumina droplets coming from the chemical reaction of the aluminium of the propellant grain.

The alumina droplets being carried away by the hot burnt gases flow towards the nozzle. Meanwhile, the droplets may interact with the internal flow. As a consequence, some of the droplets are entrapped in the cavity forming an alumina puddle (slag) instead of being exhausted through the throat. This slag reduces the performance of the motor.

The aim of the present study is to characterize the slag accumulation process in a simplified model of the MPS P230 motor using primarily optical experimental techniques. Therefore, a 2D-like cold-gas model is designed, which represents the main geometrical features of the real motor (the presence of an inhibitor, a nozzle and a cavity) and allows to approximate non-dimensional parameters of the internal two-phase flow (e.g. Stokes number, volume fraction). The model is attached to a wind-tunnel that provides quasi-axial flow (air) injection. A water spray device in the stagnation chamber realizes the models of the alumina droplets, which accumulate in the aft-end cavity of the motor.

To be able to carry out experimental investigation, at first the VKI Level Detection and Recording (LeDaR) and Particle Image Velocimetry (PIV) measurement techniques had to be adapted to the two-phase flow condition of the facility. A parametric liquid accumulation assessment is performed experimentally using the LeDaR technique to identify the influence of various parameters on the liquid deposition rate. The obstacle tip to nozzle tip distance (OT2NT) is identified to be the most relevant quantity. It indicates how much a droplet passing just at the inhibitor tip should deviate transversally to leave through the nozzle and not to be entrapped in the cavity.

As LeDaR gives no indication of the driving mechanisms, the flow field is analysed experimentally, which is supported by numerical simulations to understand the main driving forces of the accumulation process. A single-phase PIV measurement campaign provides detailed information about the statistical and the instantaneous flow structures. The flow quantities are successfully compared to an equivalent 3D unsteady LES numerical model.

Two-phase flow CFD simulations suggest the importance of the droplet diameter on the accumulation rate. This observation is confirmed by the two-phase flow PIV experiments as well. Accordingly, the droplet entrapment process is described by two mechanisms. The smaller droplets (representing a short characteristic time) appear to follow closely the air-phase. Thus, they may mix with the airflow of the recirculation region downstream the inhibitor and they can be carried into the cavity. On the other hand, the larger droplets (representing a long characteristic time) are not able to follow the air-phase motion. It is indicated e.g. by the large mean velocity difference between the droplets and the air-phase in the two-phase flow measurement data. Therefore, due to the inertia of the large droplets, they may fall into the cavity in function of the OT2NT and their velocity vector at the level of the inhibitor tip.

Finally, a third mechanism, dripping is identified as a contributor to the accumulation process. In the current quasi axial 2D-like set-up large drops are dripping from the inhibitor. In this configuration they are the main source of the accumulation process. Therefore, additional numerical simulations are performed to estimate the importance of dripping in a more realistic configuration. The preliminary results suggest that the dripping is not the main mechanism in the real slag accumulation process. However, it may still lead to a considerable contribution to the final amount of slag.

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