Joint scalar transported PDF modeling of nonpiloted turbulent diffusion flames

A transported joint probability density function (JPDF) approach closed at the joint scalar level has been applied to investigate two nonpiloted CH4/H2/N2 turbulent (Re≃15200 and 22800) jet diffusion flames. The flames have been studied experimentally at the Deutsches Zentrum für Luft- und Raumfahrt (DLR) and at Sandia National Laboratories and are well characterized experimentally through extensive velocity and scalar measurements. The flames offer the opportunity of computational investigations of turbulence-chemistry interactions in CH4/H2 flames in the absence of both partial premixing with air and with a smaller stoichiometric mixture fraction (Zst=0.167) than in the corresponding piloted Sandia flames. The two flames also offer different levels of local extinction. Comparatively few theoretical studies have been performed of these flames and the present work provides an assessment of the ability of the transported PDF approach to reproduce their detailed thermochemical structure. The chemical closure is obtained through a systematically reduced C/H/O/N 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. It is shown that velocity and scalar fields are generally well reproduced for 10⩽x/D⩽80 though uncertainties in boundary conditions have an impact closer to the burner exit.