Modeling and Experimental Investigation of Bubbly Flows in Liquid Metal for CNTP

Centrifugal Nuclear Thermal Propulsion (CNTP) is an advanced form of nuclear spacecraft propulsion currently under development which alleviates the temperature limitation of solid-fueled Nuclear Thermal Propulsion (NTP) designs by operating the nuclear fuel in the liquid phase. The fuel is contained by rotating the cylindrical fuel elements at a high enough speed to hold the molten fuel against the porous fuel element cylinder walls by centrifugal force. Propellant passed through these pores then bubbles radially inward through the molten fuel before exiting axially through the bottom of the cylindrical fuel element and being expelled through a nozzle to generate thrust. Because of the high temperature of the fuel and propellant, CNTP offers significantly greater specific impulse compared to solid-fueled NTP designs while keeping the thrust-to-weight ratio near unity. Such a system could significantly reduce round-trip times for human missions to Mars and enable robotic missions to more distant destinations in the solar system. However, in spite of the clear benefits such a system would offer, there are significant technological challenges that must be overcome. The thermal-fluid design of the reactor calls for propellant to be bubbled through a liquid fuel annulus which is rotating at several hundred RPM. The current concepts also call for a temperature of 1500 K at the exterior radius of the annulus and a temperature of 5500 K at the interior radius of the annulus. Of particular interest in this study is the challenge of demonstrating adequate heat transfer to the propellant bubbles in the molten fuel, as well as demonstrating that such a large temperature gradient can be maintained in the molten fuel annulus. The authors are creating a finite-difference model to characterize the temperature distribution in the liquid fuel annulus and to model the heat transfer between the bubbles and molten uranium fuel. During the 1960s and '70s a handful of experimental studies were performed for similar liquid-fueled nuclear rocket engine designs. These studied heat transfer between bubbles and the surrounding fluid and the bubble motion in rotating liquids. The liquids used in these studies were typically non-metallic, such as water or glycerol, with very different fluid properties than liquid uranium in the temperature range anticipated for CNTP. In addition to the modeling effort, a current experimental study is being performed which focuses on studying the bubble motion in a liquid metal system using the eutectic alloy galinstan, which has thermophysical properties much closer to molten uranium and should yield results which are more relevant to the nuclear engine. This experiment studies bubble rise in the liquid galinstan with X-ray imaging techniques to understand the bubble size, velocity, and frequency. Further experiments are planned to study bubble flow in a rotating container of liquid metal and to study the heat transfer between bubbles and the surrounding liquid to verify the numerical model.