טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentLidor Alon
SubjectTheoretical Study of the 3-Branched Explosion Limits of a
Flammable System
DepartmentDepartment of Aerospace Engineering
Supervisors Professor Eran Sher
Research Professor E Daniel Weihs


Abstract

The phenomenon of explosion (or self-ignition) of fuel mixtures is well known, as the ignition of fuels in Diesel engines, but also as unwanted phenomena like knocking in SI engines or the risk of fires on oil and gas platforms. The explosion limits are defined as the curve on a pressure-temperature diagram which separates the explosive and non-explosive regions of a fuel mixture. These limits are uniquely defined for a specific fuel-oxidizer pair, under a specific ratio and are also partially dependent on the vessel wall surface. For hydrogen and many hydrocarbon fuels distinctive 3-branched limits exist, where for a range of less than 200K there are two different pressure ranges for which ignition occurs, interspersed with two non-explosive regions. Although the explosion limits have been studied extensively for many years, there exists no model to date which can accurately predict the explosion limits over the complete range, capturing this unique branching behavior.

This research is composed of a theoretical investigation of the explosion limits of an H2-O2 mixture, with the objective of understanding the fundamental governing physics of the explosion limits. We also aim to develop a comprehensive explosion criterion for H2-O2 mixtures, and gain insights into the explosion process for other fuels. This study employs three distinct methods. The first method is an ignition delay time model, developed by combining elements of chain ignition theory and linking between the explosion limits to their characteristic time scales. By following this approach, a unified expression, capable of accurately predicting the explosion limits of the H2-O2 mixture has been developed for the first time. In the second part of this study, a detailed statistical thermodynamics analysis was performed on the upper and intermediate limits, a novel approach in the study of the explosion phenomenon. This analysis includes calculation of thermodynamic properties, their fluctuations and performing a thermodynamic stability analysis. The magnitude of fluctuations in both thermal energy and number of molecules is found out to be extremely small. However, the stability analysis reveals that the various species behave in different manners. The reactants exhibit instability above the explosion limit, while the products exhibit the opposite behavior, becoming unstable below it. The different chain-carries are on the verge of stability over the complete range of pressures and temperatures. It is shown that the phenomenon of self-ignition is due to thermodynamic instability, leading to the conclusion that an underlying comprehensive ignition criterion might be developed from thermodynamic considerations. In the last part, a short investigation using numerical simulations in CHEMKIN is performed. The effects of the ignition delay time, and of changes in the concentration of the different chain-carries, are evaluated. The main conclusion from this part is that the H2-O2 molecules are the dominant factor in the initiation of ignition at the intermediate limit. While it is shown that this type of analysis can be used for explosion limit prediction, several limitations exist, mainly in sensitivity to the typical runtime of the analysis. This further strengthens the conclusions that the self-ignition phenomenon is thermodynamic in nature.