The influence of reactant flow structure on flame front propagation
2010-11-12T09:31:19Z (GMT) by
The combustion of hydrocarbon based fuels is one of the worlds main sources of energy, with applications ranging from large scale industrial processes to transport. However, the use of these fuels has two keys problems, long term supply and emissions. In order to extend the use of hydrocarbon fuels and reduce their environmental impact, fundamental understanding of the combustion process is needed so that applications can be fully optimised. One of the most influential factors that effects the combustion processes is turbulence, a factor that significantly alters flame propagation and subsequent rates of heat release. It is this feature of combustion that is focused upon within this work. Initially flame-turbulence interaction is investigated using a fan stirred combustion bomb using high speed particle image velocimetry to examine the combustion of stoichiometric mixtures of methane and air. This study looks at how flame propagation effects turbulence and how different levels of turbulence effect flame structure. This work demonstrates that a flow field is significantly altered by a propagating flame, but that local turbulent structures are maintained ahead of it, structures that directly impact flame propagation. This section of work demonstrates that fundamental understanding is needed of how specific rotational flow structures, which characterise turbulent flows, effect local burning velocity. The rest of the work in this thesis details the study of the interaction between controlled toroidal vortices and a propagating flame front using a novel twin-chamber combustion bomb. As part of this study a new technique for the measurement of local burning velocity, using asynchronous particle image velocimetry, is developed and implemented; a technique which enables the quantification of local burning velocity within highly rotating flows. The information acquired using this new technique is then used to quantify the true local burning velocity by taking into account the translation of the flame via advection. Study of flame-vortex interaction in this manner is used to assess the impact of vortex structure on flame propagation rates. The burning velocity data demonstrates that there is a significant enhancement to the rate of flame propagation where the flame directly interacts with the rotating vortex. Away from this interaction with the main vortex core, the flame exhibits propagation rates around the value recorded for unperturbed combustion. Additional examination has shown that aspects of both the local flow field and the flame profile correlates with the local burning velocity; specifically flame curvature and the angle that the flame intersects the flame front.