Progression of localized extinction in high Reynolds number turbulent jet flames

Abstract

A transported joint probability density function (PDF) approach closed at the joint composition-enthalpy level has been applied to investigate piloted partially premixed CH 4/H 2/Air turbulent jet diffusion flames at high Reynolds numbers ( Re ≈ 60,000 and 67,000) for two equivalence ratios ϕ = 2.1 and ϕ = 3.2. The flames were studied experimentally at Sandia National Laboratories and are well-characterized close to the nozzle through multi-scalar measurements for 1 ⩽ x/ d ⩽ 4. The chemical closure is obtained through a systematically reduced C/H/N/O mechanism featuring 16 independent, 4 dependent, and 28 steady state scalars. The velocity field is computed using the second moment closure of Speziale et al. and molecular mixing is modeled using the modified Curl’s model. The current flames offer the opportunity of computational investigations of their thermochemical structure close to the burner. Moreover, the data sets are finely spaced through the local extinction regime. The aim of the computational study is to provide an assessment of the impact of closure approximations for the scalar dissipation rate. The effects of variations in the time scale ratio (2.3 ⩽ C ϕ ⩽ 4) were thus investigated along with the impact of an extended algebraic relationship that accounts for small scale properties. Comparisons with experimental data show that the modified Curl’s model fails to reproduce the correct temperature patterns for the customary values of C ϕ due to excessive extinction. The results are more favourable for larger values of C ϕ and the extended closure produces good agreement with experimental data for the current flames.