Computational Fluid Dynamic (CFD) Modeling and Experimental Study of the Formation and Buoyancy-Driven Detachment of Bubbles in Variable Gravity Environments

Abstract

The United States and other nations plan to return humans to the surface of the Moon in this decade. On the horizon, the United States plans to send humans to the surface of Mars. Multiphase fluid systems, including boiling heat exchangers, chemical processes, cryogenic fuel management, life-support systems, In-Situ Resource Utilization (ISRU), and microfluidics, will be critical components of human missions to the Lunar and Martian surfaces. Both the Moon and Mars have reduced, or partial gravity environments (1/6 g and 3/8 g, respectively). While much is known about fluids in microgravity, the effects of partial gravity on fluid behavior are not well understood. In microgravity, surface tension dominates fluid behavior, whereas on Earth, buoyancy dominates. Modeling the transition from buoyancy-dominated fluid flows to surface tension dominated fluid flows is critical to understanding partial gravity heat and mass transfer.

Of specific importance, is understanding two-phase fluid systems in contact with a solid surface. This research investigates the adiabatic and isothermal formation, growth, and buoyancy-driven detachment of a gas bubble from an orifice submerged in a liquid. Specifically, the effect of gravitational acceleration and orifice plate material surface energy on a bubble's volume at detachment was measured. The research is presented in three phases. First, a theoretical force balance analysis was conducted, in order to isolate the forces acting on a bubble forming at an orifice. Secondly, a volume of fluid (VOF) Computational Fluid Dynamic (CFD) model was developed to model bubble growth and detachment as a function of gravitational acceleration and orifice plate surface energy. Thirdly, the results of the CFD model were validated in 1 g by experiment.

The research presents three important results: (1) The volume of a gas bubble at the point of detachment from a submerged orifice, under gravitational accelerations ranging from microgravity to Earth's gravity (1g), is directly proportional to g^-1.5, where g is the acceleration due to gravity. (2) Bubble volume at detachment from an orifice is highly dependent upon the apparent surface energy of the orifice plate. This has significant implications for heat and mass transfer in reduced gravity. (3) A new dimensionless quantity, Bu, was derived, which describes submerged orifice bubble behavior across gravity levels and orifice plate materials. Additionally, a critical value of Bu, denoted Bu*, was derived. For a given material and fluid combination, Bu* predicts the point at which the bubble will detach from the orifice. Bu* is constant across all gravity levels and is entirely dependent upon the orifice plate's apparent surface energy and fluid surface tension. The results of this research demonstrate that the design of multiphase fluid systems for the Moon and Mars must involve the judicious selection of materials which are in contact with the fluids.