Turbulent mass transfer rates to a wall for large Schmidt numbers

New measurements are presented on the influence of Schmidt number on the rate of mass transfer between a turbulent fluid and a pipe wall. It is found that for large Schmidt numbers the fully developed mass transfer coefficient is related to the friction velocity and the Schmidt number by the equation equation image The experiments are accurate enough to rule out the Sc−2/3 or the Sc−2/3 relations commonly used, deduced from plausible limiting expressions for the eddy diffusivity close to a wall. It is argued that these expressions arevalid only over a vanishingly small portion of the concentration field as Sc → ∞.