Two-wavelength laser transmissometer for measurements of the mean size and concentration of coal ash droplets in combustion flows

Abstract

A two-wavelength transmissometer employing a He-Cd laser (λ₁ = 0.325 μm) and a He-Ne laser (λ₂ = 3.39 μm) has been developed for measuring the Sauter mean diameter of mineral ash droplets in high-temperature high-velocity coal-fired combustion flows. From transmission measurements at the two wavelengths, it is shown that mean diameters in the 0.3–3.5-μm range may be inferred with a weak sensitivity to particle refractive index and size distribution shape. The volume concentration or loading of the aerosol may then be determined from the measured transmission at either wavelength. The instrument has been used to measure the mean size and loading of ash droplets in a pulverized coal-fire channel flow at temperatures to ~2900 K and velocities of up to 400 m-sec⁻¹ for combustion MHD power generation applications.