Experimental investigation of CH4-air-O2 turbulent swirling flames by Stereo-PIV

• Published in: Experimental thermal and fluid science 2019-09, Vol.106, p.87-99
• Main Authors: Boushaki, T., Merlo, N., de Persis, S., Chauveau, C., Gökalp, I.
• Format: Article
• Language: English
• Subjects: Aerodynamics; Bifurcations; CH4-air; Combustion chambers; Dynamic characteristics; Energy levels; Engineering Sciences; Entrainment; Equivalence ratio; Flame propagation; Fluid dynamics; Gases; Kinetic energy; Methane; Nitrogen oxides; Oxygen enrichment; Parallelepipeds; Radial flow; Reactive fluid environment; Stereo-PIV; Swirling; Swirling flame; Three dimensional flow; Topology; Turbulence; Turbulent flame; Velocity; Velocity distribution; Water circulation
• ISSN: 0894-1777; 1879-2286
• DOI: 10.1016/j.expthermflusci.2019.04.026

•Turbulent non-premixed swirling flames with oxygen enrichment.•Stereo-PIV measurements for different planes.•Velocity fields, turbulence intensity, swirl number and entrainment rates for reacting and non reacting flows.•Effects of swirl number and global equivalence ration.•Evolution of NOx emissions throughout gas entrainment rate measurements. This paper presents an investigation of the dynamic characteristics of non-premixed turbulent swirling flames using the stereo-PIV technique. To finely characterize the flow field, experiments are performed in the longitudinal and transverse planes in non-reacting and reacting conditions. The burner configuration studied in this work is a coaxial swirl stabilized burner used in the turbulent regime. The swirler is placed in an annular part supplying the oxidant flow and the central pipe delivers radially a fuel through eight holes symmetrically distributed on the periphery of the tube. The experiments are conducted using a 25 kW parallelepiped combustion chamber, 1 m high and 0.5 m wide, using circulating water as coolant. In the reacting case, the CH4-air flame is oxygen enriched from 21% to 30% in volume. Effects of the swirl number (0.8–1.4) and the global equivalence ratio (0.8–1) on the flows topology and structure are analyzed. Results of the present work concern the 3D mean velocity fields, the turbulence intensities, the swirl number measurements and the entrainment rate along the flow. SPIV results show the three-dimensional aspect of the flow including the recirculation zone and the annular zone with swirling jet effects. The comparison between reacting and non-reacting flows shows that the presence of the flame induces a larger radial flow expansion, higher mean velocities and higher turbulence intensities. The results show a bifurcation of the tangential velocity towards the jet central axis by crossing the flame front. It is also shown that the fluctuations of the tangential velocity are mainly responsible for the highest turbulence kinetic energy levels at the top of the central recirculation zone. The entrainment rate of the surrounding gases is measured by stereo-PIV as a function of the swirl number and equivalence ratio. The results explain one possible mechanism for the decrease of the NOx emissions when the global equivalence ratio increases via an increase of the entrainment rate at the flame base.