Laminar premixed flames have applications in many residential, commercial, and industrial devices and processes. Of primary importance in their description is the burning rate which directly affects the heat release and pressure development in energy conversion systems. In the present study, fundamental properties of laminar premixed flames have been studied experimentally and numerically with an objective to understand the factors which influence their burning rate measurements. Spherically expanding flames following central ignition were employed to quantify the flame structure and propagation rate. The experiments were carried out in fan- stirred Leeds combustion bombs, while the numerical solutions were obtained by solving the reactive Navier-Stokes equations. In the early stages of flame propagation, ignition energy was found to significantly affect the flame development. The effect of flame stretch rate, as quantified by the Markstein length (Lb) of the mixture, proved critical in the ignition energy dependency of flame propagation. For relatively high values, an underlying . common variation of self-sustaining flame speed with radius could be indentified by the rapid convergence of curves for different ignition energies. This allowed the quantification of an optimum ignition energy at which the flame became independent of ignition at a smallest radius. For relatively low values, low energy spark ignition was found to give rise to a distorted and wrinkled flame kernel. This problem was found to worsen as the Lb decreased, such that its apparent measured value became increasingly influenced by any distortions produced by the spark. In the later stages of flame propagation, the walls of the bomb started to influence the propagation such that assumption of flame propagation at constant pressure was invalidated. Due to this effect, the maximum flame radius, below which accurate measurements could be gathered in an experiment, was severely restricted. If this effect was ignored, the resulting choice of the flame radius range used for extracting the burning velocity would be too small. If this effect was not ignored, the resulting choice would be inaccurate. A correction factor was developed to correct the obtained measurements, to that of a freely propagating flame under constant pressure assumption. Linear and non-linear relationships between flame ( speed and stretch rate were examined for their usage in extracting the burning velocity. It was found that the size of the bomb significantly influenced the accuracy of burning rate measurements, which had not been recognized before.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:588993 |
| Date | January 2012 |
| Creators | Tripathi, Navanshu |
| Publisher | University of Leeds |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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