Influence of strain fields on flame propagation
Flame propagation can be highly influenced by the presence of strain fields. Two regimes of turbulent flame propagation can be identified: (a) a strain-dominated regime which occurs when the smallest eddies are larger than the laminar flame thickness; and (b) a mixing-dominated regime found when the smallest eddies are smaller than the laminar flame thickness. Therefore, flame propagation in a low to moderate intensity turbulent medium, and initial stages of flame growth from a point ignition source, may be dominated by straining effects. This is because in these cases it is very likely that the laminar flame thickness is smaller than the Kolmogorov length scale, which is a measure of the smallest structures of the turbulence. In this dissertation theoretical and experimental work is reported on the influence of a uniform strain field (together with heat loss) on laminar flame propagation. The theoretical results show that, in general, the laminar burning velocity decreases when the strain rate is increased. It is also shown that the Lewis number is a very important parameter in this phenomenon. The decrease in burning velocity is enhanced with increasing Lewis number. Heat loss is also shown to be important, with further decrease in burning velocity when the heat loss is increased. Experimental work is carried out on an axisymmetricstagnation point flow, in which a laminar flame is established. Different values of the strain rate are imposed on the flame. Also, different fuels and mixture strengths are used. The velocity and temperature fields are measured, allowing the strain rate and burning velocity to be quantified. Reasonable agreement is obtained between the theoretical and experimental results. The consequences of the results obtained are discussed for the general case of turbulent flame propagation, and for the particular case of cyclic variations in combustion in spark ignition engines. It is shown that variations in the turbulent strain rate from cycle to cycle can cause cyclic variations in combustion.