Swirling flows allow a fast and efficient mixing of fiiel and oxidizer and are. thus, often applied in practical combustion systems. On the other hand, a number of problems arise in the numerical simulation of these flames and the improvement of Computional Fluid Dynamics (CFD) codes is a challenge in modem combustion research. The main goal of this work has been a qualitative and quantitative experimental characterisation of confined swirling natural gas/air diffusion flames (with a thermal load of 150kW) using different laser techniques in order to generate a database for the validation of CFD codes and for a better understanding of swirl stabilized flames.
In order to visualize the structure of the flames, qualitative two-dimensional laser-induced fluorescence (LIF) was used. The single-pulse 2D LIF images of OH show strong corrugations of different zones hi the flames, i.e., an inner and outer recirculation zone and a mixing zone. The fuel distribution and fuel/air mixing were investigated by 2D LIF of NO. which was seeded to the natural gas.
The mam part of the investigation concerned quantitative single-pulse laser measurements of spontaneous Raman scattering to determine simultaneously the temperature T. the mixture fraction f. and the species concentrations of CO2, O2, CO, N2, CH4, H2O, and H2. The mam problems in adapting the measuring technique to the flames and combustion chamber are described and solved in this work, i.e., the mfluence of the correction of signal background caused by laser-induced fluorescence from polycyclic hydrocarbons and the heat transfer to the surrounding optics. The results consist of the joint probability density functions (PDFs) of the measured quantities. The spatial distributions of the mean values and rms fluctuations reflect the flame behaviour in the different zones: the inner recirculation zone is characterized by hot, neat stoichiometric combustion products, the zone of mixing and reaction by strong turbulent fluctuations of T and f. and the outer recirculation zone by a homogeneous mixture of exhaust gas at chemical equilibrium but reduced temperature.
A deeper insight into the turbulence-chemistry interaction and the thermochemical state of the flames was gamed from the correlations between various quantities. The scatterplots of temperature versus mixture fraction revealed, for instance, the coexistence of unreacted fuel and oxidizer, even for stoichiometric mixtures. Furthermore, the temperature reduction due to radiation, flame stretch, and wall contact could be quantified from these correlations.