Use of Advanced Particle Methods in Modeling Space Propulsion and its Supersonic Expansions

Open Access
Author:
Borner, Arnaud
Graduate Program:
Aerospace Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 26, 2014
Committee Members:
  • Deborah A Levin, Dissertation Advisor
  • Adrianus C Van Duin, Committee Member
  • Barbara Jane Garrison, Committee Member
  • Dennis K Mc Laughlin, Committee Member
Keywords:
  • Molecular Dynamics
  • Water
  • Electrospray
  • DSMC
  • Electric Field
  • Ionic Liquid
  • Coarse-Grained
Abstract:
This research discusses the use of advanced kinetic particle methods such as Molecular Dynamics (MD) and direct simulation Monte Carlo (DSMC) to model space propulsion systems such as electrospray thrusters and their supersonic expansions. MD simulations are performed to model an electrospray thruster for the ionic liquid EMIM-BF4 using coarse-grained potentials. Two coarse-grained potentials are compared, and the effective-force coarse-grained potential is found to predict the formation of the Taylor cone, the cone-jet, and other extrusion modes for similar electric fields and mass flow rates observed in experiments of a IL fed capillary-tip-extractor system better than the simple CG potential. Later, a fully transient three-dimensional electric field, solving Poisson's equation to take into account the electric field due to space charge at each timestep, is computed by coupling the MD model to a Poisson solver. The boundary conditions (BCs) are found to have a substantial impact on the potential and electric field, and the "tip" BC is introduced and compared to the two previous BCs, named "plate" and "needle", showing good improvement by reducing unrealistically high radial electric fields generated in the vicinity of the capillary tip. The influence of the different boundary condition models on charged species currents as a function of the mass flow rate is studied, and it is found that a constant electric field model gives similar agreement to the more rigorous and computationally expensive tip boundary condition at lower flow rates. Supersonic expansions to vacuum produce clusters of sufficiently small size that properties such as heat capacities and latent heat of evaporation cannot be described by bulk vapor thermodynamic values. Therefore, MD simulations are performed to compute the evaporation rate of small water clusters as a function of temperature and size and the rates are found to agree with Unimolecular Dissociation Theory and Classical Nucleation Theory. The heat capacities and latent heat of vaporization obtained from Monte-Carlo Canonical-Ensemble simulations are used in DSMC simulations of two experiments that measured Rayleigh scattering and terminal dimer mole fraction of supersonic water-jet expansions. Water-cluster temperature and size are found to be influenced by the use of kinetic rather than thermodynamic heat-capacity and latent-heat values as well as the nucleation model. Additionally, MD simulations of water condensation in a one-dimensional free expansion are performed to simulate the conditions in the core of a plume. We find that the internal structure of the clusters formed depends on the stagnation temperature conditions. Clusters of sizes 21 and 324 are studied in detail, and their radial distribution functions (RDFs) are computed and compared to reported RDFs for solid amorphous ice clusters. Dielectric properties of liquid water and water clusters are investigated, and the static dielectric constant, dipole moment autocorrelation function and relative permittivity are computed by means of MD simulations.