Hide/Show Apps

Phase change heat transfer from nano and micro size droplets on a non-flat substrate

2019
Rezaeimoghaddam, Mohamma
Heat transfer via condensation/evaporation is a major contributor to heat removal in numerous engineering applications. When the dimensions of a liquid droplet or film reaches the order of micrometers the shape of the liquid-vapor interface is dominated by the capillary forces and the effect of gravity subsides. As the dimensions of the phase change liquid approaches nanometer size, the interface shape is strongly affected by dispersion effects, predominantly in the neighborhood of the contact line. At the close proximity of the contact line, continuum mechanics breaks down due to the extreme thin liquid films, requiring the use of molecular dynamics to assess the micro contact angle at the intersection of the solid-liquid-vapor boundary. The rate of phase change is a strong function of the temperature and pressure difference at the interface as well as the liquid film thickness. In heat pipe applications, the majority of heat transfer is due to latent heat and understanding the dynamics of phase change is crucial to improving the performance of heat pipes. Thin film evaporation and condensation, droplet condensation and evaporation, unsteady growth and coalescence of condensing droplets and films are phenomena encountered in grooved heat pipes. In this thesis a methodology for solving the augmented Young-Laplace equation to determine the shape of the liquid interface during evaporation and condensation phase change is developed, the results of which are extrapolated to the limit of continuum to match the contact angle predicted by molecular dynamics simulations. This approach is also used to simulate the growth and coalescence of liquid droplets using a quasisteady assumption, the results of which can be applied to the condensation on the fin top of grooved heat pipes. Finally, the effects of surface waviness and roughness on the shape and phase change rate of liquid films are investigated for surfaces generated using manufacturing techniques in common use.