| Thermal management has become increasingly critical as certain electronic devices dissipate larger amounts of thermal energy. As a result, multiphase cooling technologies such as micro-channels, jet impingement, and spray cooling have become progressively more attractive as tools to cool these devices. Jets and sprays are capable of very high heat fluxes (> 100 W/cm2) because of the rapid formation and detachment of bubbles from the surface. Understanding the formation of bubbles might enable engineers to optimize the rate of heat transfer for multiphase cooling technologies.;This dissertation concerns the genesis of bubbles along solid surfaces, which is a phenomenon that occurs at the nanometer spatial scale and nanosecond temporal scale. Currently, three key issues hinder one's ability to completely understand the processes that govern vapor bubble inception: (1) experimental research under small spatial (< 10 nm) and temporal (< 10 ns) scales presents formidable instrumentation difficulties, (2) theoretical and numerical research using continuum-based approximations cannot resolve vaporization phenomena at the nanoscale, and (3) experimental challenges prevent the separation of the large number of variables that affect bubble nucleation.;Molecular dynamics simulation is particularly suited to address the three key issues presented above, and was utilized in this work. The research focus was to resolve the effect that nanometer-sized rectangular cavities of various sizes (∼0.54-9.36 nm) and surface wettability has on the rate of forming bubbles (i.e., nucleation rate). Non-equilibrium molecular dynamics simulations at constant wall temperature were carried out for the Lennard-Jones (LJ) fluid which was in thermal contact with LJ solid wall. Both two-dimensional (2D) and three-dimensional (3D) systems were considered. Nucleation was induced by expanding the system volume to a metastable state while keeping the temperature of the solid wall constant. The 2D and 3D models were validated by comparing calculated values of thermo-physical properties to those predicted by an accurate equation of state.;Nucleation of vapor bubbles was observed to occur along flat surfaces and surfaces with nanocavities. Nucleation occurred within the nanocavities provided that the cavity volume was larger than the volume of the critical bubble (> 500 A3) and that the solid-fluid attraction was not too high (epsilonsl < 1.0 x 10 -21 J). However, when the cavity size was at least 2.72 nm wide, nucleation occurred even for high solid-fluid attraction. For lower solid-fluid attraction, nucleation occurred within the nanocavities every time due to the stabilization provided by the cavity side walls.;For a nanocavity 2.72 nm wide and 8.17 nm deep, the nucleation rate increased over 170% compared to a flat surface when the solid-fluid attraction (epsilon sl) was 0.8 x10-21 J. The same cavity increased the nucleation rate 75% compared to a flat surface when the solid-fluid attraction was 1.0 x 10-21 J. Nucleation occurred on both flat surface and surface with nanocavities even though the system was initially well-wetted. The nucleation did not require pre-existing nuclei for activation, which is a finding that supports recent experimental work. These findings suggest the possibility of using nanocavities to both control location and rate of bubble nucleation, which might enhance heat flux capability of multiphase thermal management systems such as micro-channels, jet impingement and spray cooling. In addition, the ability to promote bubbles along nanocavities might also be used to reduce friction between solid-liquid interfaces (i.e., pipes and boats for increased energy efficiency). |
