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.
