Timescales distribution and reactive structures in MILD reactors for different energy carriers

Moderate or Intense Low-oxygen Dilution (MILD) combustion processes are purported to be fuel-flexible, thus offering a real opportunity to use a large palette of fuels. In this respect, the way the oxidative reactive structure may change by changing the specific fuel is a key issue to determine, to continue relying on high efficiency and low pollutant emissions of MILD combustion. In this work, the differences among the reactive structures of the MILD process at the macroscale were analyzed for pure NH3, CH4 and H2, with a peculiar attention to the interplay between chemical (τc) and turbulence (τI) timescales. Reactive structures were analyzed through a combined experimental and CFD approach. To this purpose, a cyclonic MILD burner was considered as reference experimental facility, while macroscale features were resolved through CFD simulations using the Partially Stirred Reactor (PaSR) model. Results were analyzed in terms of Heat Release, reactive cell number distribution and behavior. Experimental results highlighted the existence of a distributed reactive region for the cyclonic burner, independently of the considered fuel, while its spatial distribution and extension were found to be significantly affected by the fuel reactivity. As the fuel reactivity increases, reactive regions tighten and move upstream the burner exit. This results in a uniform heat release region for pure NH3 and more localized areas for H2, while CH4 shows and intermediate behavior. Furthermore, the chemical and fluid-dynamic timescales interplay covers a fundamental role in defining the global behavior of reactive regions. These approach a PSR-like condition as the fuel reactivity decreases (NH3>τI).