Mixture fraction measurements via WMS-ITAC in a microgravity vortex ring diffusion flame

Flame-vortex interaction experiments conducted under microgravity conditions provide a carefully controllable environment in which fundamental combustion science issues relevant to turbulent flame processes can be investigated. The present study provides results from microgravity measurements of mixture fraction fields and differential-diffusion effects associated with mixing and combustion in a vortex ring diffusion flame. The experiments are based on a new extension of wavelength modulation spectroscopy (WMS) that uses iterated temperatures with assumed chemistry (ITAC) based on chemical equilibrium state relations. The high degree of axisymmetry achieved in these vortex ring experiments permits Abel inversion to yield the space- and time-varying mixture fraction field Ζ(x, t) throughout the fuel-rich portions of the flame, from which major species and temperature are obtained. The WMS-ITAC method uses the measured local temperature-dependent absorbance of a single major species, in this case methane, to find the local mixture fraction f that corresponds to mutually consistent values of the local temperature T(Ζ) and the concentration Y(Ζ) of the absorbing species. Comparisons of the resulting fields obtained with two different sets of state relations, one of which accounts for the differing diffusivities of individual molecular species and temperature, permit assessment of differential-diffusion effects in the vortex ring diffusion flame. The results show how differential diffusion alters the spatiotemporal structure of such flame-vortex interactions.