Mechanisms of turbulent flame propagation

Abstract

Measurements are reported of premixed hydrogen-air turbulent burning velocities, made by the double kernel method during explosions. Turbulence was created by four high speed fans driven by electric motors within the explosion vessel. This arrangement created a central region of uniform, isotropic turbulence in which all measurements were made. The ratio of turbulent to laminar burning velocity correlates well with both the turbulent Reynolds number of the reactants and the ratio of laminar burning velocity to r. m. s. turbulent velocity. The use of hydrogen-air mixtures has extended the data on premixed turbulent combustion to regimes with higher values of the last dimensionless ratio. At high values of the ratio there is evidence of a wrinkled laminar flame structure, but at lower values a small scale eddy structure seems to be dominant. A two eddy theory of turbulent combustion is presented. This rests-upon the assumption, supported by a good deal of experimental evidence, that two scales of eddy are particularly important. One is associated with the integral scale of turbulence, the other with the Kolmogorov microscale. It is assumed that all the material in the large eddies is used in the formation of the smaller dissipative eddies. It is assumed that laminar flame propagation occurs through the large eddies, whilst two approaches are considered in the case of dissipative eddies. In the first approach, laminar flame propagation across a vortex tube is employed, whilst in the second the concept of reaction time in the vortex tube is used. It is shown that the rate of burning in small eddies can be many times greater than that in large eddies. Theoretical values are obtained for the ratio of turbulent to laminar burning velocity, in terms of turbulent Reynolds number and the ratio of laminar burning velocity to r. m. s. turbulent velocity. These are in fair agreement with experimental values, but more data are required on the intermittency and chemical lifetimes of small eddies. Experiments are reported on the effect of turbulence upon flammability limits. These are narrowed as turbulence increases, but counter-action may be taken by increasing the spark iginition energy and by establishing the initial flame in a shielded region where the turbulence is reduced. The relevance of the theory to these results is discussed. Finally, the application of these findings in practical combustion chambers is discusse