AbstractEach of the two friction factors in an annulus is defined in terms of the average kinetic energy of the fluid in the region between the radius of maximum velocity and the appropriate wall. These friction factors thus defined can be uniquely correlated against corresponding Reynolds numbers. The correlation is excellent for the outer portion of flow for all values of the radius ratior1/r2, but not as good for the inner region of flow whenr1/r2is small. Applying the Colburn analogy, inner and outer wall j‐factors are similarly correlated with the same degree of accurac