Experiments were carried out on the diffusion of nitr ous oxide from a line source in a turbulent flow in a parallel-wall duct over a range of Reynolds number 104to 105. Eddy diffusivity was derived from the concentration profiles in th e lateral direction, with and without obstructions of various shapes in the centre of the duct. Without obstructions the velocity profiles and friction factors agreed well with previous measurements and, for the central portion where the turbulence appears homogeneous, the turbulent Schmidt number was found to be 0·64. Turbulence enhancement caused by obstructions was mainly dependent on the blockage ratio and the shape of the trailing edge.
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