Radiative flamelet/progress variable modelling of the UMD turbulent line fires under varying oxygen concentrations

Abstract

Advanced combustion models for fire simulations remain challenging due to the complex interaction of turbulence, combustion and radiation. Among these models, the flamelet-based model is a promising candidate to incorporate detailed chemistry at low computational cost. To this end, flamelet large-eddy simulations (LES) are performed to investigate extinction characteristics of a turbulent line fire, which is a target case in the Measurement and Computation of Fire Phenomena (MaCFP) workshop and has been experimentally studied at the University of Maryland. Two radiative flamelet formulations are investigated: (a) the radiative laminar flamelet model (RLFM-u) and (b) the radiative flamelet/progress variable (RFPV-u) model. In the normal air case where XO2 = 21%, RLFM-u and RFPV-u perform similarly, both yield reasonable agreement with experimentally measured temperature. However, in the under-ventilated case (XO2 = 13%), RFPV-u outperforms RLFM-u as it manages to capture the local flame extinction. Two dimensionless indices are introduced to quantify the role of radiation heat loss and local strain in flame weakening. It is revealed that radiative heat loss dominates the flame weakening in the normal air case while strain-induced local extinction contributes significantly to flame weakening in under-ventilated conditions, which is only captured by RFPV-u, highlighting its advantages in predicting fires with strong extinction.