Comparison of non-reactive flow fields in conventional swirl-air combustion and swirl distributed combustion

Abstract

Flow field distribution in traditional swirl flow and simulated distributed reaction swirl flow configuration was examined at 3 kHz frequency using high-speed Particle Image Velocimetry (PIV). Non-reactive flow field conditions in distributed reaction zones were fostered from the conventional swirl flow case using the designed inlet mixture preparation with CO2 dilution of the inlet airstream that corresponded to the combustion case at a heat release intensity of 5.72 MW/m3-atm. High-speed imaging of flow fields was performed at a framing rate of 3000 frames/s using a Neodymium-doped Yttrium Lithium Fluoride (Nd:YLF) laser beam at 527 nm wavelength to capture the Mie scattering images of the flow field. Mean flow velocity obtained from the post-processing of raw PIV images showed higher momentum flux injection in the swirl distributed flow zone case as compared to the normal swirl flow case. Enhanced velocity fluctuations were also observed in both the axial and radial direction for the distributed swirl flow case resulting in higher turbulence for the examined condition. The distributed swirl flow case showed a prominent corner recirculation zone along with higher flow entrainment within the recirculation zone. Enhanced interaction of average streamlines with vortices was found in the swirl distributed flow case. Increased turbulent fluctuation in the two different spatial directions was responsible for higher Reynolds stress distribution (u′v′) in the distributed flow case, indicating improved mixing behavior at lower global oxygen concentrations from the carbon dioxide (CO2) dilution, and showed different length scales of turbulent eddies. Furthermore, the spatial local Damkӧhler number values for distributed flow case (for its implication in reacting case) were found to be less than unity. The results showed a shorter flow timescale than the corresponding chemical timescales in distributed reaction zone. These results help support substantiating the difference of reaction regimes on the Borghi diagram (plot of velocity fluctuation/ laminar flame speed versus the integral length scale/ flame thickness) between the distributed reaction zone (that possesses thickened reaction zone) and conventional swirl reaction flow regimes having corrugated flamelet zone.