Launching Liquid Slugs Through Tubes: Investigating Multiphase Liquid Slug Acceleration Behaviors in Spacecraft Propulsion Systems using 3D Multiphase VOF CFD Simulations and High-Speed Flow-Visualization Validation Experiments

Propellant management in crewed and robotic spacecraft propulsion systems is complex. Unintended and unavoidable scenarios can lead to liquid "slugs" of propellant accumulating in unwanted locations and thereafter being rapidly accelerated through complex tubing networks otherwise filled with vapor or gas. To understand and characterize the multiphase fluid dynamics of these "slug launch" scenarios, NASA Marshall Space Flight Center's ER42 branch – propulsion fluid dynamics research – conducted 3D, transient, multiphase Volume of Fluid CFD simulations using an in-house tool called LOCI-Stream VOF on canonical tube geometries with straight sections, bends, and bend complexes. In addition, high-speed flow visualization experiments were conducted on similar tube geometries at the NASA White Sands Test Facility. These simulations and experiments found a variety of unique multiphase fluid dynamic effects involving the leading and trailing edges of the slug in straight sections and bends as well as the progressive erosion, acceleration, and eventual "punch-through" of the liquid slug. Understanding these multiphase effects has led to significant and counter-intuitive observations of critical relevance to important engineering concerns associated with these "slug launch" scenarios.