In this paper the problem of convective heat transfer between concentric rotating cylinders is studied. Experimental heat-transfer data are presented for four different values of clearance between the cylinders and for several combinations of outer to inner cylinder speed. The heat-transfer performance indicates three regimes of flow; the first at low peripheral velocities, in which laminar flow and heat transfer by conduction prevail, the second at cylinder speeds above a theoretically predictable value, in which vortex flow occurs and is the controlling mechanism, and a third at still higher speeds, in which a distorted type of vortex motion may be present. The data for the case of the inner cylinder only rotating can be correlated by the equation
$NNu/NNucond=0.175NTa12$
for the Taylor number range 90 to 2000. A heat-and-momentum-transfer-analogy solution for this case follows the trend of the data, but gives results which are somewhat high. The combined effects of rotation of both cylinders may be correlated by the empirical equation
$NNuNNucond=1.1NTa-NTacrη-NTacr01-3.5d/R141.1+NTacrη-NTacr03.5d/R112$
for values of the abscissa from about 2 to 50.
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