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  • Supersonic Liquid Projectiles: a Novel Materials Processing Tool


    Mechanical Engineering Department New Jersey Institute of Technology

    University Heights, Newark, NJ 07102 USA

    Key-Words: - launcher, supersonic, projectile, impact, forming, microforming, welding, rocks boring, coal combustion Abstract: Formation and practical applications of supersonic liquid projectiles were investigated. The projectiles were generated in the course of unsteady liquid (water) acceleration in a converging nozzle. The liquid was driven by a moving piston or by powder combustion (explosion). Several versions of the launcher were tested and process conditions were evaluated. Numerical modeling of fluid acceleration using a method of characteristics and a commercial package (FLUENT) was carried out. The results were applied to analysis of the mechanism of the supersonic acceleration and to evaluation of the effect of process conditions on the projectiles properties. The high-speed movie was used to determine velocity of a projectile. The major effort, however, involved the study of the projectile-target interaction and, thus, potential practical applications of the proposed technology. It was shown that the projectile affects a target similarly to an explosive deposited on the target surface. Explosion-free neutralization of non-dischargeable explosive setup demonstrated the rate of target deformation by the impacting projectiles, while crashing of heavy reinforced concrete plates showed the intensity of the impact. The further experiments showed feasibility of the projectiles applications for various forming and micro forming operations, welding and rock boring. It is suggested that potential applications will include nanoimprint technology, solid free form fabrication of heterogeneous parts and emission-free coal combustion. Key-Words: supersonic, projectile, converging nozzle, impact, forming, microforming, welding, rocks boring, coal combustion

    1 Introduction One of the most effective avenues in the material processing is creation of high (> 1 GPa) hydrostatic stresses in a work piece. Tresca [1, 2] established that at a high hydrostatic pressure a solid could experience large homogeneous strains and behave like a liquid. This notion was experimentally demonstrated by P.W. Bridgeman [3, 4]. Bridgeman showed that under some conditions both brittle and ductile materials exhibit extremely high plasticity. For example, at a hydrostatic pressure of 3 GPa plastic deformation of B2O3 glass results in the reduction of the cross section area of a sample by 87%. This experimental study demonstrated that when subjected to a sufficiently high hydrostatic pressure any material, including extremely brittle ones, becomes ductile and can flow infinitely.

    The effect of the hydrostatic pressure on the workpiece properties is utilized in high energy rate forming (HERF) processes such as explosive, magnetic-pulse and electro-hydraulic forming which are widely used for materials shaping, improvement

    of materials properties, welding, sintering, composites fabrication, etc [5]. In HERF processes the punch is replaced by a compression waves emanating from the explosion site. Thus, only one die is needed. More important, high rate of deformation caused by these waves enables us to generate complex parts out of hard-to-process materials.

    One of reasons impeding the adoption of HERF is complexity of the process control. The energy of an explosion is evenly distributed within a sphere surrounding an explosion site and directing of this energy is a complicated engineering problem. The only engineering device where the control of the explosion energy is attained at an acceptable cost is a gun. In a gun products of the powder combustion (explosion) are confined by the gun barrel, thus the direction of the forces exerted on a projectile are precisely controlled.

    Unfortunately projectiles generated by a gun have no manufacturing use. First of all, the cost of a single bullet is unacceptable for industrial applications. Then, the energy efficiency of a gun is not sufficient.

    Proceedings of the 3rd IASME / WSEAS International Conference on CONTINUUM MECHANICS (CM'08)

    ISBN: 978-960-6766-38-1 Page 172 ISSN: 1790-2769

  • The most important, however, is the mechanism of a solid-solid impact, which makes precise control of the workpiece deformation difficult if not impossible. The shortcomings above are addressed if a solid projectile is replaced by a liquid one, e.g. by water projectile. The energy efficiency of the process can be dramatically improved. The liquid-target interaction can be controlled in a wide range so that a desired process accuracy can be attained.

    A device where the energy of an explosive media was converted into the kinetic energy of a liquid was suggested by Voitsekhovsky [6] and improved by other researchers [7-9]. However, while the extremely high projectile velocity (4650 m/s) and a high firing rate (2 Hz) were attained, none of the developed devices found practical applications.

    The Waterjet laboratory of New Jersey Institute of Technology (NJIT) initiated a study of the formation and application of supersonic projectiles in 1997. This work was a continuation of the research of Atanov [9]. The work of the Waterjet laboratory included numerical investigation of the water acceleration in a converging nozzle and an experimental study of the external and terminal projectile ballistics [10-18]. As a result, several launcher prototypes were designed and constructed and the feasibility of the use of liquid projectiles as a forming and welding tool was shown. While the work was initiated as an attempt to improve the conventional waterjet technology, the study showed that the supersonic liquid projectiles constitute a unique tool. The potential applications of this tool range from micro or, perhaps, nano-manufacturing to underwater rock boring and emission-free coal combustion. A summary of the performed research is given in this paper.

    2 Thermodynamics of the Launcher The operation of the proposed launcher is based on the use of a liquid, e.g. water, as a medium for conversion of available chemical, mechanical or electrical energy into the kinetic energy of a projectile. A liquid is an effective energy conversion medium. Because it is a condensed medium its energy density is much higher than that of a gas. At the same time, the liquid is a fluid, thus additional acceleration can be accomplished in a nozzle. Non- steady liquid acceleration in a converging nozzle is especially efficient technology. Unlike a steady flow, the acceleration of an unsteady stream in a nozzle is not limited to the sonic velocity. As it was shown by the previous studies internal-to-kinetic energy conversion and kinetic energy redistribution in an unsteady converging liquid flow enable us to achieve liquid velocity exceeding 3 M.

    One of the simplest devices for direct internal-to- kinetic energy conversion is a gun. However, the thermal efficiency of a gun is determined by the length of a barrel. At the same time a converging nozzle dramatically enhances the rate of the energy converging and thus increases thermal efficiency without complication of a device design. Launchers designed in the course of this study utilized the above feature of the converging nozzle.

    3 Launcher Operation A launcher used in this study for projectiles acceleration operates as follows. A liquid load is placed in the barrel as a conventional round and accelerated by the explosion of a propellant or any other energetic material. An electrical discharge, mechanical impact, vaporization of a cryogenic liquid, a laser, a magnetic field or any other energy sources suitable for rapid energy release can also be used for projectile acceleration. Propellant combustion (explosion) results in the formation of a rapidly expending gaseous cavity exerting driving forces at the gas-liquid interface. The driving forces can be generated at the liquid boundary by an impacting piston or magnetic field. The generated forces accelerate the liquid in a barrel and then in an attached nozzle. As a result a high-speed projectile is expelled from the launcher.

    A schematic of a launcher used in the performed study is shown in Fig. 1. As it is shown in this figure a water load was placed in a barrel and impacted by the products of the propellant combustion which expelled water from the launcher. In another type of a launcher, tested in this work, powder combustion was used to accelerate a piston which subsequently impacted the water load and expelled it from the launcher. While the rate of the energy injection in the liquid in the course of the piston impact by far exceeds that of the powder combustion no significant differences in the operation of both launchers was detected. However, due to the maintenance

    Figure 1. Schematic a launcher, water is driven by combustion products.

    Proceedings of the 3rd IASME / WSEAS International Conference on CONTINUUM MECHANICS (CM'08)

    ISBN: 978-960-6766-38-1 Page 173 ISSN: 1790-2769

  • difficulties the direct energy transfer from the combustion products to the liquid was used in further experiments.

    3 Numerical Modeling of the Projectile Formation Numerical modeling of the flow in the launcher was used to investigate water acceleration by powder combustion. In the course of modeling the launcher was approximated by a fluid system consisting of gaseous and liquid phases separated by a non- penetrable boundary. Expanding