Pure systems internal circulation

The flow and shape transitions for small and intermediate size bubbles and drops are summarized in Fig. 7.13. In pure systems, bubbles and drops circulate freely, with internal velocity decreasing with increasing k. With increasing size they deform to ellipsoids, finally oscillating in shape when Re exceeds a value of order 10. In contaminated systems spherical and nonoscillating ellipsoidal... 

These flow transitions lead to a complex dependence of transfer rate on Re and system purity. Deliberate addition of surface-active material to a system with low to moderate k causes several different transitions. If Re < 200, addition of surfactant slows internal circulation and reduces transfer rates to those for rigid particles, generally a reduction by a factor of 2-4 (S6). If Re > 200 and the drop is not oscillating, addition of surfactant to a pure system decreases internal circulation and reduces transfer rates. Further additions reduce circulation to such an extent that shape oscillations occur and transfer rates are increased. Addition of yet more surfactant may reduce the amplitude of the oscillation and reduce the transfer rates again. Although these transitions have been observed (G7, S6, T5), additional data on the effect of surface active materials are needed. 

Hence, unlike mobile electrolyte AFC, the electrolyte does not need to be pumped out. Moreover, there is no problem of an internal short-circuit, which makes it more favourable for space applications. The KOH electrolyte in an asbestos matrix possesses excellent porosity, corrosion resistance and strength. Due to the stationary nature of the electrolyte, it becomes mandatory that pure oxygen should be supplied at the cathode. In case CO2 is present in the air, carbonate formation takes place and the complete fuel cell needs to be rebuilt. Hydrogen is pumped at the anode and circulated to remove water. The cooling system comprises fluorinated hydrocarbon dielectric liquid. The product water is used for humidification of the cabin, cooking and drinking in space orbiter systems. As the water is used at the cathode, its level must be kept sufficiently high at the anode side. 

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