Flow Hydrodynamics in Open Cycle Designs
The shape of the fissile gas core can be either cylindrical, toroidal or counter flow toroidal. Since there are issues regarding the loss of fissile fuel with the cylindrical and toroidal designs, the counter-flow toroidal gas core geometry is the primary source of research. The counter flow toroid is the most promising because it has the best stability and theoretically prevents mixing of the fissile fuel and propellant more effectively than the aforementioned concepts. In this design, the fissile fuel is kept mostly in a base injection stabilized recirculation bubble by hydrodynamic confinement. Most designs utilize a cylindrical gas core wall for ease of modeling. However, previous cold flow tests have shown that hydrodynamic containment is more easily achieved with a spherical internal wall geometry design.
The formation of the fuel vortex is complex. It basically comes down to flow over a projectile shape with a blunt base. The vortex is formed by placing a semi-porous wall in front of the desired location of the fuel vortex but leaves room along its sides for hydrogen propellant. Propellant is then pumped inside the reactor cavity along an annular inlet region. A dead space then develops behind the semi-porous wall; due to viscous and shear forces a counter toroidal rotation develops. Once the vortex develops, fissile fuel can be injected through the semi-porous plate to bring the reactor critical. The formation and location of the fuel vortex now depends on the amount of fissile fuel that bleeds into the system through the semi-porous wall. When more fuel bleeds into the system through the wall, the vortex moves farther downstream. When less bleeds through, the vortex moves farther upstream. Of course, the upstream location is constrained by the placement of the semi-porous wall.
Read more about this topic: Gas Core Reactor Rocket, Open Cycle Versus Closed Cycle, Open Cycle Designs
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