The Strain Exerted by a Vortex on a Flame—Determined from Velocity Field Images

Velocity field imaging techniques were used to observe how a single toroidal vortex, which represents one eddy in a turbulent flow, exerts aerodynamic strain on a premixed flame. By achieving dense seeding of the flow, the spatial derivatives of velocity were determined accurately, which allows the following to be measured as a function of space and time: the aerodynamic strain rate that is tangential to the flame, the rate of flame stretch, the vorticity field, the shear strain rate field and the flow pattern near the forward stagnation point. The vortex strength was sufficient to cause quenching of the flame. An unexpected result is that the maximum strain on the flame does not occur on the centerline near the forward stagnation point. Instead the strain rate distribution is significantly different from numerical simulations of Poinsot, et al., for which strain is maximum on centerline. The difference is due to the different vortex sizes considered, which indicates that small vortices exert a different strain rate distribution on a flame than larger vortices, and that the process cannot be modelled as being self-similar. During flame quenching, the maximum local strain rate is measured to be 35 sec^-1, which is similar to the value of 42 sec^-1 that is required to quench a steady, planar counterflow flame of the same equivalence ratio. The velocity field images also show how the flame creates vorticity in the products. This flame-generated turbulence results from the gas expansion and baroclinic torque terms in the vorticity transport equation. The velocity field ahead of the flame also is affected by the flame, but no vorticity is generated in the reactants; this validates the assumption made in many models that the turbulence in the reactants is undisturbed by the flame.