Analysis of thermofluids in flameless (MILD) combustion: assessment, improvement and development of combustion models by CFD

Abstract

Flameless combustion, also called MILD combustion (Moderate or Intense Low Oxygen Dilution), is a technology that reduces NOx emissions and improves combustion efficiency. It is based on the aerodynamic recirculation of flue gas inside the furnace diluting air and/or fuel streams. Therefore, appropriate turbulence-chemistry interaction models are needed to address this combustion regime via computational modelling.In this Thesis the applicability of two different turbulence-chemistry interaction models, the Eddy Dissipation Concept (EDC) and the Flamelet Generation Manifold (FGM) models, are studied and then some extensions of both models, The Generalized New Extended EDC model and the Dilued Air FGM, are developed and implemented in ANSYS Fluent for better predict flameless combustion.Models are validated comparing modelling results with experimental data of the Delft Lab Scale furnace (9kW) burning Natural Gas (T=446 K) and preheated air (T=886 K) injected via separate jets, at an overall equivalence ratio of 0.8.It could be concluded that both models improved modelling results respect the existing models. The Generalized New Extended EDC model provides better mean temperature results close to the burner and at the mid height of the furnace, and the Diluted Air FGM model shows better consistency with experimental data on the highest height of the furnace, where the dilution effect is more noticeable.