An Experimental Investigation of the Operating Characteristics of a Flameless Combustor

Abstract

This thesis covers the experimental characterisation of a multi-nozzle research combustor capable of achieving flameless combustion (FC). The combustor is operated with methane as fuel and air as the oxidiser, under an overall lean equivalence ratio ranging from 0.6-0.9. The work focused on designing and applying different experimental techniques under varying operating conditions. Particle Image Velocimetry (PIV) is used to characterise the flow field for reacting and non-reacting conditions. Local gas temperature and gas composition measurements are performed to characterise the formation of NOx in the combustor. Emission data and local gas temperatures are further reported as a function of the equivalence ratio, thermal power input, and oxidiser's oxygen concentration. PIV revealed the flow field structures to be very similar between non-reacting and reacting flow conditions; the location of the recirculation zones remained unaffected. Significant differences exist in the axial flow velocities and turbulence levels in the central recirculation zone (CRZ). Radial temperature measurements near the reaction zone revealed the temperature profiles to flatten for a decrease in equivalence ratio, indicative of distributed combustion. Gas composition measurements are performed near the exhaust of the combustor. The measurements showed the combustor to perform very well from an emission standpoint. Measurements showed the NOx on the centreline to be ultra-low (<10ppm) for all cases investigated. The N2O and NNH mechanisms are identified as the major contributors to the production of NOx and not the thermal pathway, as the gas temperature is consistently below the thermal NOx threshold of 1800K. A reduction in the equivalence ratio and a reduction in the oxygen concentration both reduce NOx emissions. The measured levels of CO and CH4 were negligible near the exhaust for all operating conditions investigated, confirming that combustion is fully completed at this location. Measurements of CO and CH4 near the reaction zone indicate distributed combustion, where a reduction in the equivalence ratio below 0.7 resulted in elongation and widening of the reaction zone. Oxygen measurements near the exhaust and close to the reaction zone indicate the entrainment of cooling air into the combustion chamber, which would locally influence the equivalence ratio. It is recommended to redesign the exhaust duct connection to eliminate the entrainment of cooling air.