The present paper is part of the third phase of an investigation of the phenomena and variables which control the rate of heat transfer in the air gap of a rotating electrical machine. Experimental studies were made in the developing adiabatic flow of air in an annulus having a rotating inner cylinder and stationary outer cylinder. Experiments were made to determine when Taylor vortices, and/or turbulence occurred as a function of Reynolds number of the flow; rotational speed of the inner cylinder; and distance from the inlet. Lines of demarcation of the several modes of flow were established for positions along the axis of the annulus. Two length effects were present: the distance to the point where vortices occurred and the distance required for the vortices to develop. A criterion is presented for determining the point in a developing flow where Taylor vortices originate. The value is given in terms of the dimensionless parameter,  
ωr1δb*νδθ*r1>24.
A discussion of the occurrence of vortices in well-developed flows is presented.
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