  The predictive numerical simulation of supersonic turbulent combustion, in
which the turbulent intensity is high and the fuel/air mixture is near the
flammability limit, remains challenging. An investigation of subgrid mixing
timescale modelling in large eddy simulation (LES) of supersonic turbulent
flames was carried out in this study. In the Partially Stirred Reactor (PaSR)
combustion model, the dissipation-rate-based models estimate the mixing
timescale by modeling dissipation rate, which may lead to incorrect estimation
of mixing times at different grid resolutions. To solve the existing problem, a
gradient-based mixing timescale estimation method was proposed. Subsequently,
different approaches of subgrid mixing timescale in PaSR are compared and
validated on a supersonic mixing layer, including a dissipation-rate-based
model and the proposed gradient-based model. Sensitivity analysis of the model
constants and grid resolution was also studied and compared with detailed DNS
data. The results highlight the importance of mixing models to correctly handle
turbulence/chemistry interactions in supersonic combustion and clearly indicate
the proposed model can correctly estimate the subgrid mixing timescale at
coarse grids and decline rapidly as it tends to DNS limit.
