Experimental research on turbulent reacting flows using gaseous and liquid fuels

Abstract

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%.