The problem of formation of the "hollow" agglomerates at burning of composite solid propellants

V. A. Babuk, A. A. Nizyaev

Baltic State Technical University «VOENMEH», named after D.F. Ustinov, Saint Petersburg, Russia

Abstract: Agglomeration is a part of aluminized propellants burning process. Agglomeration leads to formation of enlarged particles — agglomerates that consists mainly of molten drops of Al and Al$_2$O$_3$ and can include gaseous cavities. Differences in chemical composition and structure of agglomerates significantly affects on their evolution in combustion chamber and finally on two-phase flow properties.
In the present work the problem of formation of peculiar type of agglomerates that contain stationary large gaseous cavities is considered. Such type of agglomerates is called the "hollow" agglomerates. Formation of the bubbles in agglomerates leads to reducing of density agglomerates that affects on their motion and evolution as a part of two-phase flow.
It was experimentally established that formation of the "hollow" agglomerates occurs on final stages of agglomerating particles evolution on the surface of burning propellant. The key phenomena leading to formation of gaseous cavities is the chemical interaction between condensed Al and Al$_2$O$_3$ with formation of gaseous products. The reaction occurs at temperatures below Al boiling point in the following way:
\begin{equation*} \mathrm{Al_2O_3}^c +\nu_1 \mathrm{Al}^c \leftrightarrow 3 \mathrm{Al_2O}^g + 2 \mathrm{Al}^g, \end{equation*}
where $\nu_1$ = 4.75, $\nu_2$ = 0.75 at pressure 1.0 MPa, and $\nu_1$ = 4.6, $\nu_2$ = 0.6 at pressures above 2.0 MPa.
A physical and mathematical model of the "hollow" agglomerates formation process is presented. The model is based on analysis of equilibrium state between competing processes of gas formation on the surface between Al and Al$_2$O$_3$ drops and condensation of gaseous products on the bubble surface. The model takes into account diameter, chemical composition, structure parameters and temperature of agglomerating particle as well as environment pressure.
Parametric analysis of the model was carried out. The effects of various factors on the "hollow" agglomerates properties were determined. The conditions of formation of stationary gaseous cavities as well as conditions of the cavity collapse and gaseous products removal were found. The results qualitatively correspond to experimental data obtained for ammonium perchlorate and ammonium nitrate based propellants.
Analysis of the gaseous cavity growth dynamics and bubble stability was carried out. It allowed to make a conclusion that stationary gaseous cavity remains stable over time.
Developed model allows predicting the possibility of formation of the "hollow" agglomerates and determining their structure properties.

Keywords: agglomeration, structure, chemical interaction, combustion.

UDC: 544.023.55