Fundamentals of gravity level dependent two-phase flow and heat transfer—A tutorial

Abstract

Multiphase flow, the simultaneous flow of the different phases (states of matter) gas, liquid and solid, strongly depends on the level and direction of gravitation, since these influence the spatial distribution of the phases, having different densities. Many investigations concern behavior of liquid-solid flows (e.g. in mixing, crystal growing, or materials processing) or gas-solid flows (e.g. in cyclones or combustion equipment). But of major interest for aerospace applications are the more complicated liquid-vapor or liquid-gas flows, being characteristic for aerospace thermal control systems, life sciences systems and propellant systems. Especially for liquid-vapor flow in aerospace two-phase thermal control systems, the phenomena become extremely complicated, because of heat and mass exchange between the phases by evaporation, condensation, and flashing. Though very many publications (textbooks, conference proceedings, journal articles) concern two-phase flow and heat transfer, publications on the impact of reduced gravity are very scarce. This is the main driver for carrying out research in micro-gravity. Various heat and mass transfer issues of two-phase heat transport technology for space applications are discussed, focusing on the most complicated case of liquid-vapor flow with heat and mass exchange. Simpler cases, like adiabatic or isothermal liquid-vapor flow or liquid-gas flow, can be derived from this case, by setting various terms in the constitutive equations equal to zero. The discussions start with the background of the research, followed by a short description of two-phase flow and heat transfer phenomena. The impact of the gravity level will be assessed, including development supporting theoretical work: Thermal/gravitational scaling of two-phase flow and heat transport in two-phase thermal control loops, including gravity level dependent two-phase flow pattern mapping and condensation issues. Outcomes of theoretical work are compared with results of experiments, done on earth and in micro-gravity.