Numerical Investigation of Moderate or Intense Low-Oxygen Dilution Combustion in a Cyclonic Burner Using a Flamelet-Generated Manifold Approach

摘要

Innovative solutions in terms of energy efficiency and pollutant emission abatement require the development of new combustion technologies. In particular, several solutions imply a process based on mixture dilution and preheating that can lead to very peculiar combustion regimes. New combustion concepts, such as moderate or intense low-oxygen dilution (MILD) combustion, rely on a local self-ignition mechanism as a result of the obtainment of burned gas/fresh reactant mixtures that lead to a process mainly stabilized by means of a distributed autoignition. Despite the very interesting features related to such a concept, several modeling issues have to be properly investigated, to permit the development of MILD combustion concepts through a computationally driven design. The evaluation of characteristic times, on both micro- and macroscales, is strongly influenced by the emerging characteristics related to a strong coupling between mixing and kinetic times as a result of the high dilution levels of such technologies. To include detailed chemistry in computational fluid dynamics simulations, the flamelet-generated manifold seems to be a promising choice. Specifically, the aim of this work is to prove the reliability of the tabulated chemistry method with respect to a cyclonic burner that was used as a test case for validation purposes. Reynolds-averaged Navier Stokes simulations were realized, and a chemistry tabulation approach was used to take into account detailed chemistry effects. Finally, an assessment of the heat transfer mode was carried out by including both convection and radiation heat exchange in the modeling and comparing experimental and numerical results to temperature and gas concentration measurements obtained in several locations of the experimental apparatus. The computational tool was able to catch in a satisfactory manner the main features of the combustion regime in the cyclonic burner, and the validation was strongly improved when radiative heat transfer is included in the numerical model.