WALL MODEL EFFECTS ON TWO PHASE FLOWS AND PRESSURE DISTRIBUTION IN NANOCHANNELS

Open Access
Author:
SELLAMI, NADIA
Graduate Program:
Aerospace Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 13, 2009
Committee Members:
  • Michael Matthew Micci, Dissertation Advisor
  • Michael Matthew Micci, Committee Chair
  • Robert Graham Melton, Committee Member
  • Deborah A Levin, Committee Member
  • Melik C Demirel, Committee Member
Keywords:
  • Molecular Dynamics
  • Nanoflows
  • Surface interactions
  • Virial Pressure
  • Nanojets
Abstract:
Molecular simulations of single phase bounded nanoflows, especially at high density, showed discrepancies from the classical Navier Stokes solutions: the failure of predicting the slip value at the wall, stratification of the density close to the wall and excessive heating which affects the natural thermal fluctuations of the atomistic system (NVE ensemble). These discrepancies are a direct consequence of the importance of the surface effects for such scales as the surface to volume ratio increases dramatically at the nanoscale. To alleviate some of these observed phenomena, the modeling of the solid boundaries progressed from implicit mathematical wall models to explicit multi-layered atomistic structure including temperature/pressure control mechanisms and heat transfer exchanges. However, the wall models used in molecular simulations vary greatly in physical characteristics such as the wetting property (under static conditions) or momentum and heat exchange (under flow conditions) and consequently, the equilibrium and steady state conditions reached depend on the complexity of the model and the application it is developed for. This work investigates the characteristics of different wall models found in the literature and compares their effects for the specific applications of single phase flows and nanojets (two-phase flows). It is found that the system thermodynamic pressure varies considerably depending on the parameters and complexity of the surface models and consequently alters both the flow and the jet behaviors. Assessments of these differences in terms of the system pressure, slip value at the surface and the injection velocity for different wall categories (atomistic, stochastic/diffuse and functional wall models) and parameters are provided. Another important consequence is the dependency of nanojet stability on the dense flow-surface interactions and liquid-gas-solid surface interactions. A new integrated and sinusoidal wall model was developed to satisfy the requirements of our main application: two-phase flow injection. It provides agreement with thermodynamic data and flow profiles as well as injection velocity. Other numerical tools were also introduced to better serve this specific numerical experiment. A new Lennard-Jones modified potential for long range corrections and a preliminary pressure driven flow method were developed and implemented and promising results for pressure, energy and flow profiles are found.