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The problem of heat transfer (or mass transfer at low transfer rates) to a strip of finite length in a uniform shear flow is considered. For small values of the Péclet number (based on wall shear rate and strip length), diffusion in the flow direction cannot be neglected as in the classical Leveque solution. The mathematical problem is solved by the method of matched asymptotic expansions and expressions for the local and overall dimensionless heat-transfer rate from the strip are found. Experimental data on wall mass-transfer rates in a tube at small Péclet numbers have been obtained by the well-known limiting-current method using potassium ferrocyanide and potassium ferricyanide in sodium hydroxide solution. The Schmidt number is large, so that a uniform shear flow can be assumed near the wall. Experimental results are compared with our theoretical predictions and the work of others, and the agreement is found to be excellent.

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m = number of components N = Avogadro's number n = number of chain linkages P = total pressure g = number of principal moments of inertia R = radius of gyration of molecule T = absolute temperature u = molar volume u partial molar volume y = vapor-phase mole fraction z = compressibility factor -Greek Letters 9 = fugacity coefficient c = intermolecular potential energy u = diameter of molecule as defined by eq 9 Subscripts i, J , 1 , 2 = components mix = mixture Literature CitedA theoretical model for polarization in ion-exchange membrane electrodialysis is developed. The model incorporates "repulsion zones" in the solutions adjacent to the membrane surface where co-ion-fixed charge repulsion dominates. In these zones the recombination rate of H+ and OH-ions is therefore strongly affected by the local electric field. Experimental data are used to compute thicknesses of these zones. The results show that for cation-exchange membranes, very high concentrations of H+ ion are built up at the membrane surface during polarization, creating additional counterions (Hf) for current-carrying. The decrease in the recombination rate of H+ and OH-ions near the membrane is responsible for this.

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