Propagation and topology in turbulent premixed flames

In this work, the phenomenon of self-interaction in turbulent premixed flames is investigated. This refers to the self-intersection of a highly curved flame iso-surface, which leads to local annihilation at the point of intersection as the enveloped mixture is depleted. With a rise in turbulence intensity, the frequency of self-interaction greatly increases, while at the same time, the increase in turbulent flame speed becomes non-linear and is inhibited (known as self-inhibition). Accordingly, these interactions provide a plausible mechanism for the rapid destruction of flame surface area, especially at high turbulence intensities. By analysing their dynamics, self-interactions are investigated as a potential cause for self-inhibition of turbulent flame speed. First, a direct numerical simulation dataset is produced with methane-air as the reactant mixture burning in the thin reaction zone regime. Here, preliminary break-up (through self-interaction) of the reaction zone is seen with increasing turbulence intensity, as the flame speed is simultaneously inhibited. Critical points in the reaction progress field - where the scalar gradient becomes zero as a result of annihilation - are then used to establish self-interactions, while the local Hessian informs the topology of self-interaction. In total, four geometric types of interactions are identified, namely, reactant pocket, reactant tunnel, product tunnel, and product pocket - such that the former two locally enclose reactants, while the latter two envelop products. Each topology may further be formed through normal interaction (displacement along the normal), or conversely, counter-normal interaction (displacement against the normal), thus resulting in eight total types. Following, the dynamics of self-interaction in a turbulent premixed flame is examined. Out of the eight total types, six interactions are found to occur more frequently. In general, both types of pockets tend to propagate towards the centre, whereas the two tunnels can interact in either normal or counter-normal fashion. Local stretch rate profiles - that closely follow curvature in the vicinity of self-interaction - show that the process of pocket formation is locally destructive to flame surface area. In particular, the appearance of reactant pockets is only associated with local area destruction unlike any other configuration. The mechanisms that govern this area change for each topology are also highlighted. Finally, the change in structure and morphology of the global flame surface is addressed in light of local topologies. For a given intensity, a relative increase in the number of both reactant-enclosing topologies coincides with a globally destructive area balance. Here too, the uniquely inhibitive nature of reactant pocket formation is evident. Lastly, by examining reaction zone interactions, which increasingly favour destruction at higher intensities, the correlation between self-inhibition and self-interaction is established. Further implications of these findings for complex hydrogen-enriched flames are also discussed briefly as ongoing work. In this manner, novel insights into the propagation characteristics of turbulent premixed flames subject to self-interaction are provided in this thesis.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• Ahmed, Hassan

Advisor dc:contributor.advisor

• Cant, Stewart

• Turbulent premixed flames
• Topology
• Curvature
• Self-interaction
• Numerical combustion

Licence

• https://www.repository.cam.ac.uk/bitstreams/27dbfa7e-286b-4977-99bc-8edf5797178a/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

Author Identifier

0000-0002-8710-4675

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/397512