Large eddy simulation and experimental analysis of a swirl-stabilized flame under preheated and/or stratified conditions

Abstract

The ever increasing global energy consumption and environmental concerns combined with the lack of energy resources have put designers of combustion devices to a difficult test in order to come up with new technologies that are more energy efficient and less polluting. At the present time lean premixed (LPM) combustion is the most promising technology for environmentally friendly combustion systems since operating under fuel lean conditions can have low emissions and high efficiency. As mixing of fuel and air before combustion is not perfect in many practical premixed applications, the mixture is not spatially and temporally uniform and burns in partially premixed regime. Depending on the degree of mixture non-uniformity, the structure, amount of emissions and even flammability limits of a flame in this intermediate regime could be totally different from a premixed one. In order to investigate this effect on LPM combustion, a reduced scale swirl-stabilized LPM burner is built and analyzed experimentally and numerically in premixed and two different partially premixed configurations in this thesis. Experiments are done first visually using a digital camera for all configurations from stoichiometry to lean blow out and are repeated for an open and confined flame in a wide range of swirl numbers and two Reynolds numbers. Next, CO and NO/NOX measurements are carried out for the confined cases at the exit of the combustor using a Nondispersive Infrared (NDIR) and a Chemiluminescence analyzer (CLA) respectively. To further investigate the experimental results, the confined cases are, then, numerically analyzed by solving a system of partial differential equations including mass, momentum, species and enthalpy balance for turbulent reacting flows using finite rate chemistry combustion model and large eddy simulation. CFD and chemistry simulations are carried out using standard and customized solvers and libraries of two open source codes, OpenFOAM and Cantera respectively. Finally, the flowfield and flame structure of the premixed and stratified swirl-stabilized LPM flame is studied in a more practical situation and over a wide range of preheating levels by simulating six cases in three different combustion modes, namely conventional, HiTC and flameless mode.