Date of Award
Doctor of Philosophy (PhD)
Mechanical Engineering & Applied Mechanics
Jennifer R. Lukes
Boiling, evaporation, and liquid-vapor phase change are inherently multiscale processes. Current continuum-based numerical models fail to capture the atomistic nature of the local density fluctuations that lead to vapor nucleation. Atomistic methods, such as molecular dynamics (MD) simulations, are capable of fully resolving the effects of individual atomic interactions and nanoscale surface structure on the incipience of liquid-vapor phase change. Macroscopic problems, however, are still well beyond the reach of MD simulations due to the prohibitively large computational expense of modeling discrete particles. Hybrid atomistic-continuum (HAC) models offer a solution. HAC models limit the use of MD simulations to only a small region where atomistic-level resolution is necessary, such as near a wall or heater surface, and use continuum methods away from this region.
In this work a fully parallelized hybrid atomistic-continuum model is developed to resolve nanoscale features of liquid-vapor phase change. The domain is decomposed into an atomistic domain, where individual atomic interactions are computed, and a continuum domain, where the Navier-Stokes equations are solved. The two domains are coupled through an overlap region in which the solutions in both domains are consistent. The accuracy of the HAC model is demonstrated through the simulation of sudden start Couette flow, unsteady heat transfer, and the bulk flow of a liquid-vapor interface. The new HAC model is used to model vapor nucleation at a heater surface and compares well to analytic solutions for evaporation. Unlike continuum-only methods, the new HAC model is able to nucleate vapor from liquid naturally, given the correct thermodynamic conditions, without any assumptions on the nucleation location or frequency. The new highly-parallelized HAC model is shown to reduce computation time by a factor of five for Couette flow in a 78 nm channel as compared to a fully-atomistic simulation. This speedup is expected to become even greater for larger systems. A general discussion on the performance of the new HAC model is included along with a discussion of the advantages and disadvantages, specific to HAC models, of the volume-of-fluid (VOF) method used to track interfaces within the continuum domain.
Cosden, Ian A., "A Hybrid Atomistic-Continuum Model for Liquid-Vapor Phase Change" (2013). Publicly Accessible Penn Dissertations. 846.