Measurement of local and overall mass-transfer coefficients to a sphere ina quiescent liquid using a limiting current technique

The mass-transfer rate to a sphere in a quiescent ferri-ferrocyanide soluti on was determined from measurement of the limiting current through the sphe rical cathode. Mass-transfer rates to local areas on the sphere, without ma ss transfer to the rest of the sphere, were also measured. Convective movem ent of the electrolyte was observed visually although the experimental appa ratus was jacketed for maintaining a constant temperature. Under this condi tion, the experimental Sherwood number for mass transfer to the whole spher e was 110 rather than 2 as for pure diffusion. The local Sherwood number fo r a 2 mm spot on the sphere was 275, which was about 1.7 times that of a 8 mm spot on the sphere. The local mass-transfer rate, in the absence of mass transfer to the rest of the sphere surface used in the present study, did not change significantly with the angular position to the upward vertical a s it does when mass transfer occurs over the whole sphere simultaneously. T he transient transfer approached steady state at the dimensionless time (tu /d) of about 20. The mass-transfer rate did not vary significantly with the distance between the two electrodes varied from 2.5 to 11 radii of the sph erical cathode. (C) 2001 Elsevier Science B.V. All rights reserved.