Ph.D. Defense by Samah Mohamed

May 03 2018 10:00 AM - May 03 2018 11:00 AM

Join us in supporting Samah Mohamed as she presents her Ph.D. defense titled "Simulating Low Temperature Combustion: Thermochemistry, Computational Kinetics and Detailed Reaction Mechanisms".

Time: 10:00 - 11:00 am, Thursday, 3 May 2018
Location: Building 5, lev. 5. Room 5220

Samah Mohamed completed her M.Sc. in Chemical Engineering from the University of Nottingham, United Kingdom in 2012 and worked as a lecturer in the University of Khartoum, Sudan before joining KAUST for Ph.D. in 2014. She has already co-authored more than 10 research papers under the guidance of Prof. Mani Sarathy. Her research interests are in Chemical kinetic modelling, quantum chemistry calculations.


Detailed chemical kinetic models are important to understand and predict combustion properties. Better estimations require an accurate description of thermochemistry and kinetic rate parameters. This study identifies important reaction pathways at the low temperature chemistry of branched conventional and alternative fuels. Rate constants and branching ratios for important reactions are provided and important phenomena are investigated. 

The thermochemistry and kinetics of the 2-methylhexane model, an important component in gasoline surrogate, is updated using recent group values and rate rules from the literature. New reactions, such as hydroperoxyalkylperoxy (OOQOOH) alternative isomerization, which are competing pathways to the most important low temperature chain branching reaction (OOQOOH conventional isomerization) are also added to the model. The results show that both conventional and alternative isomerization pathways significantly affect the model reactivity. 

In light of the above, high levels computational chemistry calculations were performed to provide site-specific rates rules for OOQOOH conventional isomerization considering all possible reaction sites. This is also one of the first studies to investigate the effect of chirality on calculated rate constants. Results indicate that chirality is important when two chiral centers exist in the reactant. 

OOQOOH alternative isomerization rate constants are usually assigned in analogy to the isomerization of an alkylperoxy (RO2) radical which may introduce some uncertainty in the branching ratio and thus the importance of this pathway. This study calculates the rate constants for selected alternative isomerization reactions in order to compare and test the validity of using analogous RO2 isomerization rates. The effect of intramolecular hydrogen bonding in the calculated energies and rate constants for different reaction pathways is investigated. The result shows that alternative isomerization is a competing pathway only when it proceeds via a less strained transition state relative to the conventional isomerization transition state. A detailed analysis of the hydrogen bonding effect helped to identify cases where assigning rates in analogy may not be valid. The provided rates in this study will improve the predictions of the kinetic models and thus, the understanding of the combustion chemistry.