Design and development of a lab-scale burner for MILD/flameless combustion

This paper discusses the design and development of a burner for MILD/Flameless Combustion processes. Computational Fluid Dynamics was used to simulate preliminary designs for the burner and to obtain a characterization of the combustor for non-reactive conditions. MILD or Flameless combustion is a stable combustion process without the presence of visible flame, defined by the recirculation of hot products of combustion inside the chamber volume. The combustor presented in this work was built in vermiculite and it has a prismatic shape to ensure the optical accessibility. A preheated main flow of diluent and oxygen and the fuel flow are fed inside the combustion chamber from one side. Diametrically opposed the feeding configuration is reproduced, thus realizing a spiral flow field inside the combustion chamber. The system is provided with a quartz window. The oxidation process of fuels/oxygen mixtures diluted in N2 can be studied varying external parameters of the system, namely inlet temperatures (up to 1200K), equivalence ratio (lean to reach mixtures), residence times and mixture dilution levels. Temperature measurements inside the chamber are realized. Rapid mixing between the injected fuel and hot oxidizer has been carefully explored for autoignition of the mixture to achieve volumetric combustion reactions. Distributed reactions can be achieved for a non-premixed configuration with sufficient entrainment of hot species present in the flame and their rapid turbulent mixing with the reactants. Experimental tests, realized for C3H8/O2 mixtures diluted in N2, showed that for inlet temperatures higher than 900 K, MILD combustion condition is established for an overall dilution level of 90 % in nitrogen. Increasing the preheating temperature, MILD occurs and combustion becomes invisible and homogeneous.