Raman-Rayleigh and LIF-OH measurements in turbulent H2/N2 flames with and without compositional inhomogeneity

This paper presents temperature and species measurements performed using Raman/Rayleigh scattering in turbulent partially premixed flames of H₂/N₂ = 40/60 by volume, issuing from the Sydney inhomogeneous piloted jet burner. Results are reported for three flames, with an equivalence ratio of φ = 4.76, one with a compositional inhomogeneous inlet, at 80 % of the blow-off velocity and the other two flames being compositionally homogeneous, one velocity is equal to the inhomgoeneous flame and the other at 80 % from blow-off. A key finding of this work is that the inhomogeneous flame is approximately half the length of the homogeneous flames, as verified using chemiluminescence and OH planar fluorescence imaging. Measurements of temperature and mass fractions of O₂, N₂, H₂O and H₂ reveal significant differences in the mixing, reactivity and differential diffusion between the three flames, particularly near the outlet. For the compositionally inhomogeneous case, there is a significant amount of unreacted and partially reacted fuel, at the outlet, and this is due to the short recess distance and large velocity ratio between the air in the annulus and fuel in the jet. These inhomogeneous mixtures, which may be stratified, interact with the hot pilot products and exhibit significant strain rates and hence local extinction. For both homogeneous cases, the flame burns robustly near the jet exit with only a modest increase in local extinction at higher jet velocities. The low velocity homogeneous flame has significant differntial diffusion close to the outlet, resembling a low strained multicomponent flamelet. Whilst both the inhomogeneous and high-velocity homogeneous flames have minimal differential diffusion, closely resemblimg a highly strained mutlicomponent and unity Lewis number flamelet simulation respectively. For all three flames, the mixture fraction, temperature, and species profiles approach similar scalar profiles downstream, reflecting a decay in the local extinction and differential diffusion effects.