Experimental Investigation of Gaseous Oxyacetylene Blast Enhancement by the Combustion of Suspended Multimodal Spherical Aluminum Powder

Abstract

Multimodal micron-sized spherical aluminum powders were subjected to the detonation products of a gaseous oxyacetylene mixture. The objective was to analyze the blast enhancement from the combustion of non-uniform-sized aluminum particles. These multimodal aluminum powders consisted of a 50/50 mixture by mass of larger (~30 μm) and smaller (~1-10 μm) particles. Experiments were conducted at the large-scale Virginia Tech Shock Tube Research Facility to measure blast pressure, impulse, and heat release efficiency during combustion in these detonations. These results were compared against oxyacetylene detonations conducted with the addition of unimodal aluminum particles approximately 1, 10, 30, and 95 μm in diameter. These experiments were controlled by maintaining a particle mass concentration of 200 g/m3, a constant volume of air for particle dispersion, and a consistent size for the gaseous oxyacetylene explosive charge of 0.11 m3. This approach ensured that any variations in explosive output were due to the characteristics of the aluminum powder.

For unimodal aluminum, the combustion of 1 μm aluminum powder yielded the highest increase in blast pressure, impulse, and heat of combustion efficiency whereas H-95 provided the least amount of blast enhancement. These results showed an inverse relationship where decreasing aluminum particle size resulted in increased blast output, a phenomenon driven by the shorter combustion times of smaller particles. For multimodal aluminum combustion, the performance of these powders exceeded the pressure and impulse performance of their unimodal counterparts.

The heat of combustion efficiency—defined as the ratio of energy driving the shock wave to the total energy available—was estimated using a two-part blast scaling methodology. The first step in this process used Sachs' blast scaling laws to infer time-dependent energy release contributing initially to blast pressure and impulse. The second step introduced a new modified Sachs scaling technique to account for late-time energy release contributing solely to blast impulse. This scaling approach addressed the previously neglected impact of delayed aluminum combustion on blast behavior. This two-part scaling approach revealed that the combustion of multimodal aluminum powders in oxyacetylene detonations resulted in 75.1%-85.3% of the available heat of combustion contributing to blast pressure and impulse compared to the 30.8%-74.6% provided by unimodal aluminum powders. These results suggest that the combustion of multimodal aluminum powder results in more powerful and efficient detonations, providing a technique to improve and optimize energetic performance.