a b s t r a c tIn the present study the flame structure of a piloted partially-premixed dimethyl ether flame (DME-D), which is based on the Sydney/Sandia piloted jet burner flame series, is investigated using an LES-flamelet-progress variable approach (LES-FPV). Simulation results are used together with a comprehensive experimental data set including multi-scalar measurements of temperature and major species from Raman/Rayleigh scattering, intermediate species CH 2 O and OH from laser induced fluorescence (LIF) and velocity data from particle image velocimetry (PIV). The comparison between numerical and experimental data includes the mean and the root mean square (RMS) radial profiles for velocity, mixture fraction, temperature and mole fractions of major species at different downstream locations. Furthermore, species distributions conditioned on the experimentally accessible mixture fraction are compared and differences between DME and methane flames are discussed. In addition to this comparison, the computation of CH 2 O-LIF and OH-LIF signals as well as the effective Rayleigh cross-section was incorporated into the flamelet-progress variable approach. The filtered and time-averaged numerical results are then directly compared with the corresponding experimental signals at different axial positions. The characteristic separation of instantaneous CH 2 O and OH fields, which was previously observed in simultaneous LIF measurements, is discussed and analyzed based on the underlying flamelet structures. Finally, modeling assumptions from the experimental post-processing for the effective Rayleigh cross-section, which were introduced to account for the experimentally inaccessible intermediate hydrocarbons, are evaluated using the detailed species composition from the numerical simulations.

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The effect of differential diffusion of two passive scalars having Schmidt numbers of unity and 0.25, respectively, is investigated using direct numerical simulation of a temporally evolving jet. The objective of the research is twofold: (i) to compare the turbulent/non-turbulent (T/NT) interface position using the scalar criterion between the unity- and low-Schmidt-number scalar; and (ii) to determine the impact of the T/NT interface on differential diffusion. For the latter, the T/NT interface is detected using the vorticity criterion. To quantify the effect of differential diffusion, a normalised differential diffusion parameter is analysed, clearly showing the dominance of differential diffusion at the T/NT interface. A transport equation for the scalar differences is then evaluated, which shows that differential diffusion originates at the interface. Further, the separation between the passive scalars, arising due to differential diffusion, is studied using conventional and conditional statistics with respect to the interface distance. Since differential diffusion is known to be present at large and small scales, the connection between them is analysed using the scalar dissipation rate. Moreover, the physical mechanism responsible for the departure of the two scalars is analysed using the scalar gradient alignment, the ratio of the diffusive fluxes and by a transport equation for the scalar gradients.