The ratio of diameters is calculated as
If
Loss coefficient of contraction at inlet:
Loss coefficient of enlargement at outlet:
otherwise,
Loss coefficient of contraction at inlet:
Loss coefficient of enlargement at outlet:
The total loss coefficient is defined by using the linear approximation for the transition between contraction and enlargement:
References
[1] : Flow of Fluids Through Valves, Fittings, and Pipes, Crane Valves North America, Technical Paper No. 410M. 1979, p A-26