Natural convection mass transfer at vertical electrodes under turbulent flow conditions

Abstract

Turbulent natural convection in electrolysis with vertical electrodes was studied by a dark field method (making the flow pattern visible) and by the method of the current/voltage curve (yielding the limiting current and the mass transfer rate). The range studied (2 × 1011 < Sc Gr < 1015) included the region of transition from laminar to turbulent flow. In the range 4 × 1013 < Sc Gr < 1015 the results of the limiting current measurements can be represented in dimensionless form by the relation Nu = 0·31 (Sc Gr)0·28, which differs only slightly from the relationship valid for laminar flow (Nu = Nusselt number, Sc = Schmidt number, Gr = Grashof number). With the tallest electrodes used (which were up to 1 m high) the limiting current density tends, however, to become independent of height. From the optical experiments, as well as from a comparison with heat transfer data taken from the literature, it appears that the transition to turbulent flow is not governed solely by the product Sc Gr, but depends on the value of Schmidt number too.