Turbulent combustion of jet flames in a hot diluted coflow of
combustion products is conducive to the transition from conventional
flamelet combustion to a regime of moderate or intense low oxygen
dilution (MILD) combustion, which is commonly characterized by a very
low emission and noise. MILD combustion is also characterized by
distributed combustion where the net heat release is positive across
the entire combustion domain. The turbulence/chemistry interactions in
this regime that determine the flame structure, local temperature, and
species distribution critically depend on the mixture fraction and
scalar dissipation fields. However, there are no experimental tools to
measure the mixture fraction field in a distributed (MILD) combustion
regime. The present work offsets this limitation by demonstrating a
Rayleigh scattering-based approach to measure mixture fraction in a
turbulent ethylene MILD combustion zone. 1D counterflow flame
simulations enabled mapping the locally calibrated Rayleigh scattering
fields to mixture fractions in the fuel-rich regions. This approach
also shows very low sensitivity to the local temperature and
composition. Overall, the results provide compelling evidence that the
distributed heat release does not significantly impact the turbulent
processes of the flow-field for the conditions examined. The
measurement uncertainty from this approach and its extension to more
complex fuels are also discussed. The present technique is limited to
mildly turbulent, fully MILD/distributed flame with representative
scalar dissipation rates.