Spectroscopic study on local radiative properties of luminous flame, i.e. emission from soot could, is performed by utilizing counterflow burner. By means of propane-air counterflow diffusion flames, measurements of one-dimensional (perpendicular to the flame surface) distributions of temperature and radiative quantities in the stationary flame are accomplished. Examined spectral range is in the visible and near-infrared regime (0.6 μm-1.0 μm). It turns out that bulk emissivity from the luminous flame, εLF, shows apparent wavelength dependency in the observed spectral range (εLF=εLF(λ-α)), and its power-law constant, α, varies along the perpendicular to the flame surface. By taking the longer observed wavelength in the visible regime (eg., 0.9 μm), α closes to the constant irrespective of the observed location. By taking the shorter observed wavelength (eg., 0.7 μm), on the other hand, α tends to be monotonically decreased to the high temperature regime (α has inverse correlation to the flame temperature). This trend is somewhat similar to the particle diameter or volume fraction of the soot cloud according to the previous literatures. It is suggested that non-gray body feature of the luminous flame (i.e. wavelength dependency on bulk emissivity) is pronounced when the large soot fragments are coarsely distributed. Adopting the shorter wavelength may work for better diagnostics on local soot status.
Spectroscopic study on local radiative properties of luminous flame, i.e. emission from soot could, with various hydrocarbon fuels (methane, ethane, propane, butane) are performed by utilizing 1-D counterflow burner. Emission from soot from counterflow diffusion flames is analyzed and bulk emissivities of soot cloud by different kind of fuels are obtained. Model parameter of emissivity, α(εLF=εLF(λ-α)): wavelength dependency to the bulk emissivity, is calculated in different locations. Various fuel types and imposed flow velocities are considered as experimental parameters in the present study. As the carbon number of the fuel is increased, the produced soot cloud tends to be optically thick and the wavelength dependency parameter, α, becomes smaller. This trend suggests that the luminous flames provided by high-carbon contained fuel is close to gray-body emitter. An engineering model parameter, ξ, is introduced for precise prediction of the universal local soot condition and it works fairly well in the wide range of the fuel (C1∼C4) under the conditions considered in the present study.