Unsteady analysis of hydrogen/air mild combustion by means of Large Eddy simulation

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.Combustion processes are essential for power generation, since an overwhelming majority of energy-producing devices rely on the combustion of fossil or renewable fuels. Thus the development of a combustion technology able to accomplish improvement of efficiency with reduction of pollutant emissions, such as NOx, is a main concern. MILD combustion is one of the promising techniques proposed to achieve these goals. In this combustion regime the reactants are preheated above the self-ignition temperature and enough inert combustion products are entrained in the reaction region. As a result, the characteristic times of chemical kinetics and turbulent mixing are comparable and the combustion region is no longer identifiable in a flame front but extended over a wide region, so that MILD combustion is often denoted as flameless combustion. Importantly, pollutants emissions can easily reduce because of the small temperature difference between burnt and unburnt and of the lean conditions in the combustion chamber. In this work Large Eddy Simulation (LES) of a Hydrogen/Air burner operating in the MILD combustion regime is performed. Turbulent mixing controls most of the global flame properties, so computing large scale structures by means of LES is an important key to capture mixing properties. The filtered mass, momentum, energy and species equations are discretized with a 2nd order accurate central finite difference scheme over a cylindrical non-uniform grid. Unclosed terms due to subgrid-scales are modeled using a fractal model approach (FM). Radiant transfer of energy is taken into account. The predictions of temperature and pollutant formation are compared with available experimental results.vk201