A global evolution of the downward jet flame behavior: From the laminar to the turbulent

Abstract

The diffusion jet flame is a high-speed flow gas ignited at the outlet, a common combustion behavior in fundamental science of combustion, including applications in the industrial exhaust gas treatment torches and engines. This paper investigates experimentally the downward jet flame characteristics from the laminar to the turbulent, which has not been reported comprehensively yet. The overall jet flame length, jet flame downward distance, temperature and flame radiation heat flux profile are studied, as important characteristic parameters determining the flame boundary in vertical direction, the farthest distance that the flame could travel, as well as the thermal effect on the surrounding, respectively. Experiments were conducted to explore the global evolutionary process of downward jet flame with increasing initial fuel jet velocity for various circle nozzle diameters (3 mm, 4 mm and 5 mm), fuel types (pure fuel and blended fuel), and Reynolds numbers widely ranging from 65 to 97,209 involving laminar, transition and turbulent combustion regimes before reaching the flame blowout limit. The downward jet flame length and the downward distance show a non-monotonic evolution with the Reynolds number or heat release rate, i.e., first increase at the laminar combustion regime, change a little at the transition regime, and finally increase significantly after reaching fully turbulent combustion regime. The vertical temperature profile along the centerline of the downward jet flame is associated with the downward jet flame morphologic characteristic parameters, it decreases significantly with the non-dimensional height at the intermittent region compared to the upward jet flame. The flame radiation fraction of the downward jet flame based on the measured flame radiation heat fluxes first changes a little and then decreases with the Reynolds number as a power function. The flame length could be well correlated by the non-dimensional heat release rate, the flame Froude number as well as the non-dimensional volumetric flow rate based on the air entrainment dynamics. The jet flame downward distance could be well correlated by the momentum-buoyancy length scale at the turbulent combustion regime. A non-dimensional global model involving the momentum-buoyancy length, flow rate length scale and the stoichiometric air-fuel ratio is developed to describe the global jet flame downward distance evolution. This work provides essential and fundamental knowledge about the dynamic evolution of downward jet flame in designing rocket propulsion/combustor structures, combustion and evolution characteristics.