DEVELOPMENT AND APPLICATIONS OF REAXFF REACTIVE FORCE FIELD SIMULATIONS IN COMBUSTION: FROM KINETICS TO DYNAMICS

Restricted (Penn State Only)
Author:
Ashraf, Chowdhury Mahbub
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 01, 2018
Committee Members:
  • Prof. Adri van Duin, Dissertation Advisor
  • Prof. Adri van Duin, Committee Chair
  • Prof. Richard Yetter, Committee Member
  • Prof. Jacqueline O'Connor, Committee Member
  • Prof. Lasse Jensen, Outside Member
Keywords:
  • Molecular dynamics
  • ReaxFF
  • Combustion
  • Chemical Kinetics
  • Pyrolysis
  • Oxidation
Abstract:
From the last decades, the focus of the combustion research has shifted more and more from experiment to simulations. This is partly because of the difficulty to perform experiments under the conditions at which modern day jet and rocket engines operate and partly because of the advancement of computational technology required to perform theoretical calculations or simulations. Currently, computational fluid dynamics (CFD) based continuum simulations are considered to be an essential part of combustion research, as it can simulate turbulent reacting flows with significant accuracy. One integral part of CFD simulations is the chemical kinetic model and the fidelity of the simulation greatly depends on the chemical model it uses. These chemical models are normally developed and validated against experimental observations. The actual engine condition might vary significantly from the experimental conditions, which can severely restrict the accuracy of the chemical models under such condition. Thus, a computational method is required that can simulate the complex physics and chemistry of complex combustion process and capture the possible reaction pathways without any prior user input. The ReaxFF reactive force field method is one such method, as it can be used to simulate reactions in large (>> 1000 atoms) dynamic systems and can provide atomistic-scale insights of the critical reaction steps. In this dissertation, ReaxFF reactive force field simulations have been developed and used in the following research areas ranging from kinetics to dynamics: a) In this dissertation, the first atomistic-scale based method for calculating ignition front propagation speed has been developed with a hypothesis that this quantity is related to laminar flame speed. This method is based on atomistic-level molecular dynamics (MD) simulations with the ReaxFF reactive force field. Results reported in this study are for supercritical (P=55MPa and Tu=1800K) combustion of hydrocarbons as elevated pressure and temperature are required to accelerate the dynamics for reactive MD simulations. These simulations are performed for different types of hydrocarbons, including alkyne, alkane, and aromatic, and are able to successfully reproduce the experimental trend of reactivity of these hydrocarbons. Moreover, our results indicate that the ignition front propagation speed under supercritical conditions has a strong dependence on equivalence ratio, similar to experimentally measured flame speeds at lower temperatures and pressures, which supports our hypothesis that ignition front speed is a related quantity to laminar flame speed. In addition, comparisons between results obtained from ReaxFF simulation and continuum simulations performed under similar conditions show good qualitative, and reasonable quantitative agreement. b) The ReaxFF method provides an attractive computational method to obtain reaction kinetics for complex fuel and fuel mixtures, providing an accuracy approaching ab-initio based methods, but with a significantly lower computational expense. The development of the first ReaxFF combustion force field by Chenoweth et al. (CHO-2008 parameter set) in 2008 has opened new avenues for researchers to investigate combustion chemistry from atomistic-level. In this dissertation, two issues with the CHO-2008 ReaxFF description has been addressed. While the CHO-2008 description has achieved significant popularity for studying large hydrocarbon combustion, it has some significant limitations for C1 chemistry and properties for graphitic materials. In this dissertation, a newer version of ReaxFF combustion force field has been developed by addressing the limitations of CHO-2008 version, while retaining the accuracy of the previous description for larger hydrocarbons. Thus, ReaxFF CHO-2008 DFT-based training set has been extended by including reactions and transition state structures relevant to the syngas and oxidation initiation pathways and re-trained the parameters. c) Combustion devices such as rocket engines, gas turbines and HCCI engines frequently operate at a pressure higher than the critical pressure of the fuel or the oxidizer. This significantly limits the transferability of existing chemical kinetics models as they are developed and validated at low pressure/temperature conditions, considering only temperature dependence on the reaction rates while neglecting pressure dependence on combustion pathways. Since the experiments are difficult to perform at the supercritical region, this study demonstrates the capability of ReaxFF reactive force field method simulations to study combustion kinetics of fuels and fuel mixtures at these conditions with an objective to investigate how the presence of a highly reactive fuel can alter the properties of a much less reactive fuel during pyrolysis. Two different fuel mixtures are considered, namely JP-10/toluene and n-dodecane/toluene and find that they behave differently at different mixing conditions and densities. This study also compares results with continuum simulation results using a detailed chemical kinetic model and elaborate why the continuum results fail to capture the phenomena predicted by the ReaxFF simulations. d) Syngas (Synthesis Gas), a mixture of CO and H2 gas, has a very high availability and great flexibility of usage. As such, syngas is expected to play a significant role in future energy generation. However, the varying amount of mixture components makes it difficult to develop a reliable reaction mechanism for syngas combustion. The main variable to consider for direct syngas combustion include the carbon monoxide/hydrogen ratio, fluctuating level of carbon dioxide and water and presence of other species that can significantly affect the combustion process. This study investigates the effect of the presence of water molecules in syngas combustion. In these reactive molecular dynamics simulations, two different carbon monoxide/hydrogen ratios are considered and the effect of presence of water molecules in CO2 production is examined at different temperatures. This study indicates that a low amount of water molecules significantly increases carbon dioxide production, which is also observed experimentally. However, beyond that level, water molecules have limited effect on CO oxidation.