Mixture fraction measurement in turbulent non-premixed MILD jet flame using Rayleigh scattering

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.