The concentration changes with time and space of methane gas were examined following its release into air at atmospheric pressure within a long smooth cylindrical tube of 2.5 in. (63.5 mm) diameter, taking into account the combined effects of molecular diffusion and natural convection. The concentration profiles as a function of diffusion time at specified regions of the tube was determined by measuring the velocity of sound in the medium along a tube diameter using an ultrasonic gas analyzer. Comparative tests involving carbon dioxide, neon and helium were also made. It was shown that the experimental concentration profile for the diffusion of methane observed may be reproduced using the simple one-dimensional classical diffusion equation when the molecular diffusivity was replaced by a very much larger effective eddy diffusivity that appeared to depend on the diffusion time and local concentration gradient.

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