Experimental Study of flow and Scalar Mixing in Swirl-stabilised burners

Instantaneous measurements of scalar, velocity and temperature in a swirl stabilized burner are presented. The scalar mixing is discussed in terms of mixture fraction distribution as well as the rate of scalar dissipation. The later quantity, occurring at the micro-scale of turbulent flow, characterizes the level of mixing between a fuel and an oxidizer. This quantity is regarded as one of the most important parameters that affect the combustion process. The scalar dissipation rate was computed from the measured mixture fraction, which was measured by laser-induced fluorescence of acetone. The measurements have been made in a turbulent non-reactive swirling coaxial jet discharged from a swirl-stabilized burner along the jet centerline at different downstream distances from the burner exit. They are reported up to 7 fuel nozzle diameters downstream of the burner exit at a Reynolds number of 29000 for three swirl numbers, namely 0.3, 0.58 and 1.07. The influence of the swirl number on the scalar mixing, unconditional and conditional scalar dissipation statistics was investigated. The purpose of obtaining these measurements is to assess experimentally the validity of the scalar variance models and the associated so-called 'filter density functions' that are being developed in the context of Large Eddy Simulation (LES).
Instantaneous (as well as mean) temperature measurements were also obtained in the same swirl-stabilized burner under the same operating conditions with reaction. Temperature was measured by Rayleigh thermometry. Careful selection of the fuel allowed nearly constant Rayleigh scattering cross section across the flame and led to temperature measurements with typical accuracy of around 5%. The temperature measurements quantify the effect of different degrees of swirl on flame stability and on the mechanism of flame stabilization. The temperature statistics, temperature power spectra and thermal dissipation rates are presented.
Finally, flow velocity measurements were obtained in the non-reacting and reacting cases by employing particle image velocimetry (PIV) technique. The purpose was to review the occurrence of the processing vortex core (PVC) and its role on flame stabilization. The effect of swirl number on a processing vortex core and a recirculation zone is shown and discussed in association with observations for the scalar measurements.