An investigation into turbulent reacting
flows in an opposed jet geometry and a sudden
expansion duct has been performed. For the opposed jet geometry, measurements of
the velocity and reaction progress variable were obtained in lean premixed
flames. Both
velocity and scalar measurements were taken using PIV (Particle Image Velocimetry).
Three gaseous fuels (methane, propane and ethylene) and three liquid fuels (JP-10,
cyclopentane and cyclopentene) were considered for a range of equivalence ratios. The
broad range of fuels enabled an investigation of the effect of different fuel reactivities on
the velocity field and
flame location and also allowed the effect of the Lewis number on flame extinction to be investigated.
Preliminary work included isothermal measurements of the flow between and inside the
nozzles. The use of fractal grids inside the nozzle increased turbulence intensities at the
nozzle exit by 100% and turbulent Reynolds numbers between 50 - 220 were achieved.
Velocity and normal stress components were measured with attention focused on the
inlet boundary, along the burner centreline and the stagnation plane.
A circular duct, incorporating a sudden expansion step, was also used to investigate the
effect of swirl on pressure oscillations within the duct, the lean
flammability limits and
the NOx emissions. Measurements were performed for stratified
flow conditions using
methane as a fuel. The results show that excessive swirl leads to an increase in local
strain in the vicinity of the expansion step and makes the flame more prone to local
extinction. Moderate swirl was found to lower the amplitudes of the pressure oscillations
close to global extinction and also to decrease the lean extinction limit of the stratified flow conditions. However, it did not decrease the overall equivalence ratio of flows with
a richer core and a leaner annulus. Flows with only air in the core flow led to an overall
equivalence ratio as lean as 0.3 for methane compared with 0.6 for the uniform flow.
Stratification with a fuel rich core
flow and a leaner annular
flow led to an increase in
NOx emissions due to locally increased temperatures. The addition of moderate swirl
enhanced mixing of the annular and the core
flows, which resulted in a more uniform
fuel distribution close to the step and a reduction in NOx-levels up to 50%.