The present paper is part of the second 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 data for diabatic flow in an annulus are summarized and compared with the results of previous studies. The data are examined in terms of the types of flow processes occurring in an annulus, and it is found that the results for diabatic flow clearly confirm those obtained for adiabatic flow in showing the existence of three, and possibly four, modes of flow in this annulus. These modes are: (1) Laminar flow; (2) laminar-plus-Taylor-vortexes flow; (3) turbulent flow; (4) turbulent-plus-vortexes flow. The heat-transfer data were subdivided into the following two limiting cases and one general case: Case A. Axial flow with zero rotation. Case B. Rotation of inner cylinder with zero axial flow. Case C. General case of combined axial flow and rotation. The heat-transfer data from this study and of previous investigations were correlated in terms of Reynolds number and Taylor number over a wide range of these variables in terms of fairly simple equations. Radial temperature profiles in the annular gap were measured for the diabatic flow and aided in the understanding of the different flow phenomena in the annulus with an inner rotating cylinder.