Abstract of PhD Thesis

Experimental and Modelling Study of Ethyl Propanoate and Diisobutylene

Wayne K. R. Metcalfe (2008), National University of Ireland, Galway

Ethyl propanoate

A shock tube ignition delay study was carried out for ethyl propanoate, a model biodiesel fuel. The study was performed over a range of reflected shock pressures (1 and 4 atm), reflected shock temperatures (1180− 1660 K) and equivalence ratios (f = 0.25 − 1.5). Jet-stirred reactor species profiles were obtained over a range of equivalence ratios (f = 0.3 − 2.0) at a pressure of 10 atm and in the temperature range, 700−1100 K. The experimental data was then used to validate a detailed chemical kinetic mechanism that was also developed during this study. The simulations performed very well over the complete range of data. Sensitivity analysis coupled with rate of production analysis highlighted the importance of the six-centered elimination reaction yielding propanoic acid and ethylene. The reactivity of ethylene, especially the reactions between vinyl radical and molecular oxygen, were shown to have a large effect on the reactivity of the system.

Diisobutylene

A shock tube study was carried out for diisobutylene, a compound chosen to represent alkenes in a surrogate fuel. Diisobutylene is a mixture of two conjugate olefins of iso-octane, 2,4,4-trimethyl-1- (and -2-) pentene, in a ratio of 3:1 in favour of the 1-pentene isomer. Ignition delay times were measured for 2,4,4-trimethyl-1-pentene over a range of equivalence ratios (f = 0.25−1.0) and pressures (1.0−4.0 atm) and in the temperature range 1150−1550 K. The reactivity of the 2-pentene isomer was also examined and it was found that it was faster to ignite across the temperature range studied. A detailed chemical kinetic mechanism was developed and it reproduced the experimental observations.

RCM simulation

A code written to simulate chemical kinetics inside the reaction chamber of a rapid compression machine (RCM) while accounting for post-compression heat losses was re-commissioned and used to simulate pressure profiles obtained in our RCM. The code was first validated by simulating unreactive pressure profiles and then applied to reactive propane mixtures. Once the important variables were identified and altered to match the compressed gas pressure and post-compression heat losses of a pure argon compression, the code re-produced very good results for other unreactive and reactive experiments.

Rate constant calculation

Transition state theory, implemented in the Polyrate program was used to calculated a rate constant from first principles, for the six-centered elimination reaction of propanoic acid and ethylene from ethyl propanoate. Gaussian’ 03 was used to calculate the molecular energies and structures of the species involved in the reaction, information required for the Polyrate calculation. The complete calculation was controlled by the interface program, Gaussrate. The final result obtained for the rate constant was disappointing as it differed considerably from the experimental value. In spite of this, the experience gained studying this reaction showed that the method could prove useful for rate constant estimation in the combustion regime, particularly for smaller systems.