|Ph.D Student||Yavor Yinon|
|Subject||Characterization and Improvement of Aluminum Combustion|
in Solid Propellants
|Department||Department of Aerospace Engineering||Supervisor||Professor Emeritus Alon Gany|
|Full Thesis text - in Hebrew|
Solid rocket motors use composite propellants containing up to 20% aluminum powder for the enhancement of energetic performance. Aluminized solid propellants tend to experience the undesirable phenomena of agglomeration - forming large agglomerates from merging of small aluminum particles. This may lead to incomplete combustion, higher two-phase flow losses, and accumulation of slag which might damage motor insulation and add parasite mass.
This work investigates the combustion of aluminized propellants and in particular ways to reduce agglomeration, applying two technologies developed in-house: nickel-coating of aluminum particles and the use of porous aluminum (characterized by a large specific surface). Both methods should potentially lead to smaller agglomerates, as less particles can accumulate prior to ignition and ejection from the burning propellant surface.
Combustion tests of aluminized solid propellants of different compositions took place in a windowed strand burner at various pressures (1- 50 atm). A fast view video camera (1000-10,000 fps) was used during the experiments, and ejected particle distribution was obtained by measuring the particle images frame by frame and by their collection and sampling by a laser particle analyzer. The experiments revealed that the use of either nickel-coated or porous aluminum leads to smaller agglomerates. On average, using nickel-coated aluminum resulted in a 30% decrease in agglomerate median diameter, whereas porous aluminum lowered its size by 40%, corresponding to a significant decrease of 65% and 75% in agglomerate mass, respectively.
A theoretical model was developed, assuming accumulation of aluminum particles at a thin mobile layer formed at the propellant burning surface, which is interrupted by the ignition event associated with particle ejection. Hence, agglomeration number Nag defined as the ratio between aluminum particles ignition and accumulation times, indicates the amount of particle accumulation prior to ignition and ejection from the surface. The use of nickel-coated or porous aluminum is addressed through lower ignition temperature. Calculations show that Nag decreases (implying reduced agglomeration) when using nickel-coated or porous aluminum. Coarse AP particles cause discontinuity between aluminum particle sites, hence, increasing their loading results in reduced agglomerate size. A significant decrease in agglomerate diameter is also predicted for aluminum particles larger than the mobile layer thickness due to shorter ignition time resulting from particle exposure to extensive heating.
Predictions of the model for different propellant compositions and pressures reveal good agreement with experimental results of this research and data obtained by other scientists.