In this thesis Direct Numerical Simulation (DNS) is used to investigate the development
of turbulent premixed flame kernels during the early stages of growth typical of the
period following spark ignition. Two distinct aspects of this phase are considered: the
interaction of the expanding kernel with a field of decaying turbulence, and the
chemical and thermo-diffusive response of the flame for different fresh-gas
compositions. In the first part of the study, three-dimensional, repeated simulations with
single-step chemistry are used to generate ensemble statistics of global flame growth.
The surface-conditioned mean fluid-velocity magnitude is found to vary significantly
across different isosurfaces of the reaction progress variable, and this is shown to lead to
a bias in the distribution of the Surface Density Function (SDF) around the developing
flame. Two-dimensional simulations in an extended domain indicate that this effect
translates into a similar directional bias in the Flame Surface Density (FSD) at later
stages in the kernel development. Properties of the fresh gas turbulence decay are
assessed from an independent, non-reacting simulation database.
In the second part of this study, two-dimensional simulations with a detailed 68-step
reaction mechanism are used to investigate the thermo-diffusive response of pure
methane-air, and hydrogen-enriched methane-air flames. The changes in local and
global behaviour due to the different laminar flame characteristics, and the response of
the flames to strain and curvature are examined at different equivalence ratios and
turbulence intensities. Mechanisms leading to flame quenching are discussed and the
effect of mean flame curvature is assessed through comparison with an equivalent
planar flame. The effects of hydrogen addition are found to be particularly pronounced
in flame kernels due to the higher positive stretch rates and reduced thermo-diffusive
stability of hydrogen-enriched flames.
| Identifer | oai:union.ndltd.org:CRANFIELD1/oai:dspace.lib.cranfield.ac.uk:1826/3486 |
| Date | January 2008 |
| Creators | Dunstan, T. D. |
| Contributors | Jenkins, K. W. |
| Publisher | Cranfield University |
| Source Sets | CRANFIELD1 |
| Language | English |
| Detected Language | English |
| Type | Thesis or dissertation, Doctoral, PhD |
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