Interfacial relations in liquid-vapor phase change processes : an atomistic and continuum study

Abstract

Multiphase fluid phenomena involving phase transformations are not only abundant in nature but also prevalent in various engineering applications. Such liquid-vapor phase change processes (evaporation/condensation) have been studied over several decades but certain important aspects are yet to be understood. For instance, quantitative theories which accurately predict mass fluxes or characterize the temperature jump at liquid-vapor interfaces during phase transformations are still subjects of active research. Along with better fundamental `theories' for describing evaporation/condensation rates, continuum numerical methods capable of utilizing these `theories' to model macroscopic phenomena are essential. Exploring some of the fundamental questions regarding liquid-vapor phase transformations and proposing better continuum numerical methods for modeling macroscopic phase change problems are the two broad objectives of this thesis.

Recently molecular dynamics (MD) simulations have shown that the Schrage relations predict phase change rates for non-polar atomic fluids quite accurately. However, applicability of this finding for real fluids, such as water, is still debatable. Here, using molecular dynamics simulations we study steady state evaporation and condensation processes of molecular polar fluids, i.e. water. In a one-dimensional heat-pipe geometry, non-equilibrium mass flow is driven by controlling the temperatures of the source/sink. Resulting mass fluxes (as a function of driving forces) are evaluated and validity of Schrage relations is investigated.

To describe phase change processes at the continuum scale, along with bulk conservations laws, certain interfacial relations are necessary. The origin of these relations might be theoretical/empirical or a combination of both. In this context, the Schrage equation and expressions describing interfacial temperature jumps can be treated as `inputs' to a continuum formulation. Typical continuum numerical methods modeling liquid-vapor phase change assume local thermal equilibrium at the liquid-vapor interface -- continuity of temperature at phase interfaces, and a relation between interfacial saturation pressure and temperature based on phase co-existence curves. Several standard macroscopic problems have been solved accurately by adhering to this assumption. However, in micro-scale and certain non-standard macro-scale applications, significant jumps in temperature are observed at liquid-vapor interfaces during phase change, and vapor pressure can be far from equilibrium values. To examine the importance of some these assumptions, we develop a locally discontinuous arbitrary Lagrangian Eulerian finite element formulation capable of modeling temperature discontinuities at interfaces. Furthermore, we use the kinetic theory based Schrage relationship to evaluate the rate of phase change. We apply our methodology to solve the problem of flowing vapor in a planar heat pipe and explore the effect of temperature discontinuities at liquid-vapor interfaces.