Investigation of Elliptical Cooling Channels for a Naval Electromagnetic Railgun

摘要

The future Naval Electromagnetic Railgun will use a mega-ampere electrical current to generate an electromagnetic force which accelerates a projectile to hypersonic velocities. The applied current can raise the bulk temperature of the rails by over 100 degrees Celsius, necessitating an active cooling system for the rails to sustain high rates of fire without incurring permanent damage to the gun. The electromagnetic force on the projectile and the rails creates a complicated stress state that varies as the projectile passes along the rail, first uniaxial then biaxial compression acts on the rails. In this study, a system of cooling channels for fluid flow down the length of the rails was considered, and channels with elliptical cross sections were examined. Elliptical shapes were considered due to the high surface area available for convection, relatively low impact on the stress distribution, and low stress concentration effect. By treating an elliptical channel as a variable area fin and varying the size and aspect ratio of the ellipse and the distance between channels, the heat transfer capability of a channel array was maximized based on given flow conditions and applied heat flux. The optimal channel design was further constrained by the applied compressive stresses. It was found that ellipses of different aspect ratios are optimal for the uniaxial and biaxial stress states, and the optimal channel design was limited by the competing effects of these two structural constraints. In order to test the thermal aspect of the design, a representative set of channels were machined into one third scale copper rails using wire electrical discharge machining. Tests were performed using both a steady state heat flux to determine the overall heat transfer coefficient and transient conditions to determine the system thermal relaxation time.