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Experimental Study of Transient Supersonic Gas Release from a High Pressure Vessel: Visualization and Quantitative Analysis of Muzzle Flow

par Moumen, Abdelhafidh
Président du jury Parente, Alessandro
Promoteur Hendrick, Patrick ;Gallant, Johan
Publication Non publié, 2023-09-08

Thèse de doctorat

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Résumé :

This thesis presents a comprehensive study of the qualitative and quantitative aspects of the transient supersonic release from a high-pressure vessel. Experimental contributions are provided to examine the muzzle flows: highly transient, supersonic flows discharged in the vicinity of the firearm muzzle during shooting.About 70% of the chemical energy developed by a conventional ballistic launcher is not converted into kinetic energy of the projectile but is contained in a combustion gas-particle mixture. This highly compressed gas is rapidly discharged from the muzzle in a few milliseconds after the projectile launch. These muzzle flow fields exhibit the structure of an extremely underexpanded supersonic jet encapsulated by an outer blast wave. As it expands, this flow interacts with a precursor one: an identically shaped flow formed by the air initially located upstream of the accelerated projectile and expelled outwards by a piston effect. These phenomena affect (i) the firing signature, (ii) the projectile-flow interaction, and (iii) the launcher accuracy. Hence, an advanced understanding of the muzzle flow is crucial for both launcher designers and users. Even so, the findings in the dynamics of muzzle flows are limited and poorly understood. Only qualitative and intrusive measurementtechniques are performed. This is due to (i) the harsh environment, (ii) the short duration, and (iii) the explosive course during the launch phase.This study aims to improve the probing capability during the intermediate ballistics phase, which takes place in the vicinity of the muzzle between the instants of the in-bore acceleration of the projectile and its free-flight, and gain a better understanding of muzzle flow fields generated by small caliber firearms. Two non-intrusive and whole-field measurement set-ups are performed based on the background oriented schlieren (BOS) and particle image velocimetry (PIV) methods to achieve this goal.First, using the high-speed and time-resolved BOS system, the salient features of the precursor and propellant flow fields and their mutual interactions are accurately resolved. The captured structures, such as vortex rings, barrel shock, Mach disk, and blast wave, are compared to that issued from realized conventional schlieren set-ups. Good agreements are observed.• The BOS capability is proven to visualize complex density flow fields. Quantitatively, through free-flight visualization of conical-nosed projectiles, the BOS technique can accurately reconstruct the density step change across thin physical discontinuities, such as shock waves.• The structures of muzzle flow fields and dependence on the launcher are described qualitatively and in terms of density profiles.• To further expand the application of the BOS technique to the ballistics field, we provide three essential improvements to the above analyses. These contributions consist of novel methodologies for the (i) reconstruction of three-dimensional axisymmetric refractive index fields, (ii) automatic masking of combustion products that interfere with the raw BOS images, and (iii) a posteriori estimation of uncertainty when using BOS measurements to reconstruct the refractive index/density field.Second, the suitability of the PIV technique to provide accurate velocity measurements in the challenging environment of the propellant flow is studied The flow structure is revealed to comprise two distinct regions: (i) the first region, located within the extremely underexpanded supersonic jet, consists of spatially dispersed particles, and (ii) the second region, downstream of the Mach disk, is formed by large structures. The study demonstrates that cross-correlation and particle tracking velocimetry (PTV) algorithms can determine the flow velocity in both regions. Consequentially, through an extensive experimental effort, the following conclusions are drawn:• PIV using unburned powder particles as PIV tracers is feasible• The combustion products contain sub-micrometric particles which can follow with high fidelity the gas. However, the muzzle flow cannot be treated as homogeneous. • An innovative approach was identified to seed the main propellant flow by coating powder grains with 1% of 100 nm-sized ZrO2. The seeding of these inert particles is recommended, especially under subsonic conditions or when unburned particles are scarce. • The use of TiO2 as tracer particles to seed the muzzle flow was found to be detrimental and potentially hazardous to the equipment and personnel. The TiO2 particles intended to act as tracers, surprisingly, not only melted but also functioned as a combustion accelerator anddecreased the number of particles in the propellant gas.• A comprehensive analysis of the muzzle flow fields resulting from the launch of subsonic and supersonic .300 Blackout projectiles is presented. This analysis provides valuable insights into the behavior of the propellant flow in different projectile launch scenarios. The study’s findings represent an advancement in the intermediate ballistic literature, offering valuable insights into the dynamics of muzzle flow. The experimental set-ups employed in this research mark a significant step forward in the probing capability within this field. The comprehensive and quantitative characterization of the muzzle flow features provides essential data for the optimization of muzzle devices and the validation of numerical codes. Notably, the study addresses several phenomena that were previously only qualitatively approximated in the literature, now offering a more precise and quantitative understanding. These phenomena include the exploration of possible mechanisms of projectile acceleration and disturbance, as well as the examination of the interactions between the main and precursor flows and their impact on the dynamics of muzzle flow fields. Such detailed and quantitative analyses contribute to the advancement of knowledge in this area and providea solid foundation for future research and applications.