APPLICATION OF REAXFF BASED REACTIVE MOLECULAR DYNAMICS TO SIMULATE LASER INDUCED INCANDESCENCE OF SOOT AND CHAR OXIDATION

Open Access
Author:
Kamat, Amar
Graduate Program:
Mechanical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 09, 2010
Committee Members:
  • Prof Adri Van Duin, Thesis Advisor
  • Adrianus C Van Duin, Thesis Advisor
Keywords:
  • laser induced incandescence
  • soot
  • reaxff
  • molecular dynamics
  • accommodation coefficients
Abstract:
Laser Induced Incandescence (LII) of soot has developed into a popular method for making in-situ measurements of soot volume fraction and primary particle sizes. However, there is still a lack of understanding regarding the generation and interpretation of the cooling signals. To model heat transfer from the heated soot particles to the surrounding gas, knowledge of the collision-based cooling as well as reactive events, including oxidation (exothermic) and evaporation (endothermic) is essential. In this thesis, LII of soot has been simulated using the ReaxFF reactive force field for hydrocarbon combustion. Soot was modeled as a stack of four graphene sheets linked together using sp3 hybridized carbon atoms. To calculate the thermal accommodation coefficient of various gases with soot, graphene sheets of diameter 40 Angstroms were used to create a soot particle containing 2691 atoms, and these simulations were carried out using the ReaxFF version incorporated into the Amsterdam Density Functional (ADF) program. The reactive force field enables us to simulate the effects of conduction, evaporation and oxidation of the soot particle on the cooling signal. Simulations were carried out for both reactive and non-reactive gas species at various pressures, and the subsequent cooling signals of soot were compared and analyzed. To correctly model N2 – soot interactions, optimization of N-N and N-C-H force field parameters against density functional theory (DFT) and experimental values was performed, as described in this thesis. Subsequently, simulations were performed in order to find the thermal accommodation coefficients of soot with various monatomic and polyatomic gas molecules like He, Ne, Ar, N2, CO2 and CH4. For all these species, good agreement was found between our ReaxFF results and previously published accommodation coefficients. Initial simulations were also performed to study the oxidative pyrolysis of an Illinois char model containing about 7000 atoms. Analysis of the reactive behavior of char was carried out based on the temporal change of the concentration of oxygen, and the rate of change of five and six member rings present in char as a function of temperature. The utility of ReaxFF based Molecular Dynamics to simulate the physical and chemical behavior of carbonaceous materials is thus illustrated through this research.