MASS-TRANSFER RATES AT ROUGH SURFACES

The effect of different grades of artificial roughness on mass-transfer rates was studied using the ferricyanide-ferrocyanide redox system on vertical nickel electrodes by observations of the limiting current density of the cathodic process for electrodes of different heights in unstirred solutions. It is shown experimentally that roughness increases the mass transfer rates. A general correlation of the data may be represented by the equations: • electrode 0 (smooth) Nu = 0·052(Sc Gr)0·324 • electrode 1 Nu = 0·053(Sc Gr)0·323 • electrode 2 Nu = 0·067(Sc Gr)0·318 • electrode 3 Nu = 0·087(Sc Gr)0·312</lt> where Nu, Sc and Gr are the mass-transfer Nusselt, Schmidt and Grashof numbers respectively, and where the peak-to-valley heights of the electrodes are of the order 3 > 2 > 1 > 0. © 1969.