DROPLET VAPORIZATION OF N-HEPTANE IN SUBCRITICAL CONDITIONS USING MOLECULAR DYNAMICS
Open Access
- Author:
- Borner, Arnaud
- Graduate Program:
- Aerospace Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- August 12, 2009
- Committee Members:
- Michael Matthew Micci, Thesis Advisor/Co-Advisor
Michael Matthew Micci, Thesis Advisor/Co-Advisor - Keywords:
- n-heptane
vaporization
Droplets
molecular dynamics - Abstract:
- The vaporization of a n-Heptane (C7H16) droplet is investigated using Molecular Dynamics (MD). Inter-molecular and intra-molecular forces are incorporated using the Lennard-Jones 12-6 and a torsional potential. During each integration step the structure of the molecule is maintained by constraining bond lengths and bond angles iteratively. The RATTLE method, a variation of the Velocity Verlet algorithm, is implemented to maintain the constraints and advance the system through time. The system is initialized in a BCC lattice with each molecule in a helical structure and is allowed to evolve into a random arrangement, using velocity rescaling to ensure that the kinetic and internal temperature are maintained in this initial period. This rescaling is continued until the simulation has reached thermal equilibrium, then it is free to evolve. Due to the high computational costs of vaporization a polyatomic molecule using MD, a parallel implementation of the system has been developed. Ten simulations were done on systems of approximately 1500 molecules under pressures ranging from one to twenty-five atmospheres. The droplet was evaporated for a time ranging from one to two nanoseconds depending on the pressure. In all cases the droplet was initialized at 293.15 K and the vapor was initialized at either 471 K or 550 K. The latter case is above the supercritical temperature of n-Heptane. Cross-sectional contour plots looking at the kinetic and internal energy as well as the average force acting within the droplets are presented. Results show that the droplet loses its spherical shape very fast, especially when the pressure is increased. The Amsterdam method was used to analyze the evaporation rate for each case. It was found out that all cases adhere to the D² fairly well and that increasing the pressure accelerates the droplet vaporization. Finally, the evaporation constant was calculated and this study shows that it increases linearly with the reduced pressure. This work provides a foundation on which future polyatomic, in particular hydrocarbon, droplet vaporization studies can be done. The next step is to also increase the pressure above its critical value for n-Heptane, as well as increasing the size of the system.