LES and finite-volume CMC modelling of a turbulent lifted H2/N2 flame: effects of CMC mesh resolution and numerical scheme

TL;DR

This study uses LES and finite-volume CMC modeling to analyze how mesh resolution and numerical schemes affect the prediction of a turbulent hydrogen/nitrogen lifted flame, revealing limitations of coarse meshes and differences in convection schemes.

Contribution

It provides a detailed assessment of the impact of CMC mesh resolution and convection schemes on flame simulation accuracy and unsteady flame dynamics.

Findings

Coarser CMC meshes underestimate lift-off height and over-predict reactive scalars.

Refinement of CMC mesh beyond a certain point does not significantly improve results.

Different convection schemes cause limited differences in time-averaged predictions but affect unsteady details.

Abstract

Large eddy simulations with three-dimensional finite-volume Conditional Moment Closure (CMC) model are performed for a hydrogen / nitrogen lifted flame with detailed chemical meachanism. The emphasis is laid on the influences of mesh resolution and convection scheme of finite-volume CMC model on predictions of reactive scalar distribution and unsteady flame dynamics. The results show that the lift-off height is underestimated and the reactive scalars (e.g. temperature, H2 and OH) are over-predicted with coarser CMC mesh. It is also found that further refinement of the CMC mesh would not considerably improve the results. The time sequences of the most reactive and stoichiometric OH mass fractions indicate that finer CMC mesh can capture more unsteady details than coarser CMC mesh. Moreover, the coarse CMC mesh has lower conditional scalar dissipation rate, which would promote the ealier auto-ignition of the flame base. Besides, the effects of the convection schemes in the CMC equations on the lifted flame characteristics are also investigated. It is shown that different convection schemes lead to limited differences on the time-averaged temperature, mixture fraction and species mass fractions. Moreover, the RMS values of H2 and OH mass fractions show larger deviation from the measurements with hybrid upwind and central differencing scheme, especially around the flame base. Furthermore, the distributions of the numerical flux on the CMC faces also show obvious distinction between the upwind scheme and the blending scheme. The budget analysis of the individual CMC terms shows that a sequence of CMC faces has comparable contributions with upwind scheme. However, with the hybrid schemes, the instantaneous flux is dominantly from limited CMC faces. The reactivity of a CMC cell is more easily to be affected by its neighbors when the upwind scheme is used.