Vorticity-scalar alignments and small-scale structures in swirling spray combustion

Abstract

The effect of droplets, heat release, and swirl on the fine-scale structure of the turbulence in a full-scale gas turbine combustor is studied using large-eddy simulation. Alignment of vorticity and scalar gradient with the strain-rate field is examined in detail. Results indicate that the most likely strain state is axisymmetric extension corresponding to one of the two positive strain rates. Examination of the isotropic part of the strain tensor indicates that volumetric dilatation due to heat release significantly alters the strain-rate field. Analysis also shows that the vorticity tends to align with the intermediate strain rate, whereas the scalar gradient aligns with the most compressive strain rate. These results are in agreement with those obtained in isotropic turbulent flows. The magnitude of these alignments is found to decrease in the presence of droplets and with heat release and/or an increase in swirl. Probability density functions of the strain-rate eigenvalues and flow visualization are used to characterize the geometrical structure of the small scales. It is shown that both tube-like and sheet-like structures exist in the combustor and their relative abundance (or the lack thereof) is a function of spatial location and swirl magnitude. Tube-like structures are found to coincide with regions of intense vorticity gradients, whereas regions of increased scalar gradients form sheet-like structures that in turn wrap around the tubular vortical structures. The implication of these results on fuel/air mixing and combustion in a two-phase system is also discussed.