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PhD thesis defence on ignition on February 26

2020-02-05

On February 26th at 09.15, Frida Vermina Plathner will defend her PhD-thesis "Limiting Conditions for a Sustained Flame over Condensed Fuels - Analysis by Experiments and Stagnant Layer Theory".

The act will take place in lecture hall V:D (at John Ericssons väg 1 in Lund) and will start with a 30 minutes presentation followed by a short break before the defence starts. The entire dissertation normally takes 2-3 hours. The entire session will be performed in English.

After the seminar, lunch will be served and if you want to participate, please send an e-mail to johanna.kruse@risk.lth.se no later than February 14th. No notification is needed to only attend the defence act.

The thesis (excluding papers) can be found here.

Abstract

Ignition is one of the most important phenomena in unwanted combustion. In a fire several materials may ignite and burn simultaneously, and little is known about the combustion reaction kinetics of fuel gases that emerge upon heating. Simplified ignition thresholds have therefore traditionally been used in fire modelling to describe the ignition and extinction events. One of the thresholds that has been used by several researchers is the critical mass flux, which is the amount of fuel gases needed for sustained combustion. In fire safety engineering, this is a relatively new threshold, since ignition has commonly been described by a critical surface temperature or a critical heat flux upon the material.

The critical mass flux depends on measurable and controllable fuel and environmental properties, such as the oxygen mass fraction of the surrounding air, type of material, and flow field. The available experimental studies therefore provide varying results. To use these results in a model, knowledge is required about whether the modelled object is exposed to a similar environment as the tested specimen. This thesis therefore aims to investigate how the critical mass flux varies with a few key factors, namely its dependence on the type of material, the flow field, and the incident heat flux. Material type is represented in a simplified manner by the chemical heat of combustion and, by varying the fuel bed size, it is possible to correlate the critical mass flux to the convective heat transfer coefficient.

The results show that the critical mass flux is high for materials with low chemical heats of combustion, and vice versa. A closely related threshold for ignition – the critical heat release rate – is approximately constant for a large span of gas mixtures. For gases with a chemical heat of combustion below 10 kJ/g, however, a constant value cannot be assumed.

A smaller fuel bed infers a larger critical heat flux. The incident heat flux, however, has a negligible impact on the critical mass flux, despite its influence on the mass loss history.

The findings are summarized in an engineering correlation that can be used to approximate the critical mass flux value to use in modelling