Agglomeration in Solid Propellants loaded with inert particles Study of Physical Phenomena using Shadowgraphy Image Processing
Creators: name:  T. DECKER 
name:   R. DEVILLERS 
name:   S. GALLIER 
name:   J. ANTHOINE 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: PROPULSION PHYSICS
SOLID ROCKET MOTOR

description: Solid Rocket Motors (SRM) are now widely used in both civilian and military applications in propulsion. Most solid propellants used in space applications include oxidizing particles in a binder. Adding metal particles such as aluminum can increase propulsion performance, in the range of 10% in the case of aluminum. The addition of aluminum has consequences. The initial aluminum can agglomerate on propellant surface to form droplets of bigger size. Those droplets then interfere with the gas flow within the combustion chamber. The bigger the droplets, the longer the combustion time, leading to potentially enhanced interactions with the gas flow. A two-phase flow with burning aluminum in the motor chamber has consequences on performance and instabilities: two-phase losses, thermoacoustic instabilities, incomplete combustion, formation of alumina slags in the back of the chamber, nozzle erosion and plume signature. All this depends on aluminum particle size, hence agglomeration. Aluminum agglomerates sizes are mainly determined using empirical correlations or geometrical considerations [1]. The methods are moderately predictive and only yield mean diameters. The development of predictive models requires the understanding of the physical phenomena associated to agglomeration. While studies mainly focus on aluminum agglomeration, phenomena leading to bigger droplets are also observed with inert particles such as titanium dioxide TiO2 or magnesium oxide MgO [2]. Agglomeration of inert particles is easier to study because the droplets do not burn, releasing no smoke and keep their initial size, known a priori. Image processing used to study agglomeration is then easier to develop. The study presented here focusses on image processing to characterize inert particles agglomeration. Images were acquired in former studies at ONERA using a shadowgraphy set-up with high-speed acquisition up to 7.5kHz [2]. The combustion chamber is pressurized with pure nitrogen with three different initial pressure levels: 1.0, 2.0 and 3.0 MPa. Tested propellant compositions are all based on HTPB/AP mixtures with particles added in the range of 6% mass fraction. Various types of particles were used in order to compare various properties and size ranges (titanium oxide, magnesium oxide, glass-type particles). We use image processing in order to track a particle from the moment it is visible on the burning propellant-sample surface until it leaves it to enter the hot-gas flow, taking into account that various particles may bond to each other to form an agglomerate. The approach combines various image processing methods: an active contour method (Chan and Vese [3]) to segment the propellant-surface location and a non-fixed thresholding method to detect objects above the surface [4]. The detection of specific patterns on the propellant surface is investigated. Their evolution is studied in order to understand the physical phenomenon associated to agglomeration. We correlate the agglomerate size on the surface and in the flow to different parameters such as operating pressure or propellant composition. The evolution of agglomerates over time is also investigated by associating objects of interest over various successive images. This in-situ characterization of particle agglomeration in real propellant conditions is a first step towards the development of predictive models. References [1] J. Duterque Experimental studies of aluminum agglomeration in solid rocket motors. International Journal of Energetic Materials and Chemical Propulsion, vol. 4, no 1-6, 1996 [2] R. W. Devillers, G. Le Besnerais, M. Nugue, and N. Cesco. Experimental analysis of solid-propellant surface during combustion with shadowgraphy images: new tools to assist aluminum-agglomeration modelling. 7th European Conference for Aeronautics and Space Sciences, Milan (Italy), 2017. [3] T. F. Chan and L. A Vese. Active contours without edges. IEEE Transactions on image processing, 10(2):266277, 2001. [4] J. Matas, O. Chum, M. Urban and T. Pajdla. Robust wide-baseline stereo from maximally stable extremal regions. Image and vision computing, 22(10):761767, 2004