Systematic study of the impact of chemistry and radiation on extinction and carbon monoxide formation at low scalar dissipation rates

摘要

In fire flames scalar dissipation rates in the reaction zone are low and radiation becomes important for extinction. The chemistry mechanism also strongly affects extinction and CO formation. The present study applies the unsteady laminar flamelet approach (or, equivalently, the conditional moment closure (CMC) method in mixture fraction domain, without transport in physical space). Six chemistry mechanisms are analyzed. In addition to adiabatic conditions, three radiation models are considered: RADCAL-based gray gas; weighted sum of gray gases (WSGG); and prescribed radiative fraction model. The oxygen mass fraction values for extinction are discussed, diluting the oxidizer by adding nitrogen. Carbon monoxide profiles are reported as well. The radiative fraction model is not capable of predicting the strong radiative effects at low scalar dissipation rates, while the RADCAL-based and the WSGG model are able to do so. The analysis of extinction and CO reveals strong sensitivity to the mesh in mixture fraction space, as well as to the shape of the conditional scalar dissipation rate profile. Results with the ARM2 mechanism and GRI3 are in very close agreement, so that significant time savings are possible, using ARM2.