Thermal-Field Structure and Characteristics of Unburned-Fuel Emission of a Turbulent Diffusion Flame under a Strong Pressure Gradient

Abstract

Spatio-temporal structures of the thermal field of a non-premixed turbulent flame formed in a curved rectangular duct and the relevant unburned-fuel emission characteristics were investigated experimentally. The combustion field was dominated by a pressure gradient in the radial direction of the duct curvature, which caused strong gradient diffusion in turbulent heat transfer on the inner-wall side of the flame and, conversely, counter-gradient heat transfer on the outer-wall side. Two-point correlation measurement of temperature fields revealed that, in the strong gradient-diffusion region, a spatial thermal pattern generated by turbulent mixing of high-and low-temperature fluid parcels was advected downstream with little diffusion. In contrast, the pattern was attenuated and rapidly diffused in the counter-gradient diffusion region. The behaviors of the unburned-fuel emission were also dominated by the pressure gradient and showed quite different characteristics between the inner- and outer-wall regions of the curved duct. In the downstream combustion field, the volume concentration of the unburned fuel remained much higher on the outer-wall side of the flame than the inner-wall side. This characteristic conforms closely to the turbulence structures of the thermal field.