Towards colorless distributed combustion regime

•Investigated the flowfield under swirl and distributed combustion conditions.•PIV diagnostics assisted between air dilution and modeled lower O2% entrainment.•Integral length scale was calculated at flame boundary using OH-PLIF.•Calculated Reynolds and Damkohler numbers identified the CDC combustion regime.•CDC major controllers include flame thickness, velocity and controlled low O2 conc.

Colorless distributed combustion (CDC) is a novel method for efficient and environmentally benign cleaner energy conversion of fossil and biofuels. CDC has been investigated in different configurations and geometries, with support to seek near zero emissions, uniform thermal field, energy savings, low pressure drop, and reduced combustion noise. In this paper, distributed combustion is investigated with focus on the flame-flowfield interaction and the different quantities that affect distributed combustion. The velocity field was obtained using particle image velocimetry (PIV) with focus on mean and fluctuating quantities. The flowfield information helped differentiate between the impact of increasing Reynolds number (through air dilution) and the impact of lowering oxygen concentration (through modeled entrainment). The flowfield information was further processed to give the integral length scale at the flame boundaries. The integral length scale along with the fluctuating velocity is critical to determine turbulence Reynolds number and Damköhler number. Together these numbers identify the combustion regime at which the combustor is operating. This information clearly distinguishes between traditional swirl flames and distributed combustion and helps explain the significant benefits of distributed combustion as it operates in a well-stirred reactor regime. The results revealed that major controllers of the reaction regime are flame thickness and laminar flame speed; both are significantly impacted by lowering oxygen concentration through entrainment of hot reactive species from within the combustor, which is important in distributed combustion. Understanding the controlling factors of CDC is critical for the development and deployment of this novel method for near zero emissions from high intensity combustors and energy savings using fossil and of biofuels for sustainable energy conversion.