In a high-specific-speed Francis runner the diameter at the entrance varies from crown to band and the nominal diameter D is taken at the mid-height of the guide vanes.
For our purpose much use will be made of this dimension.
For the designer, however, an important dimension is the throat diameter D_t, which is less than D for low-specific-speed Francis runner and greater than D for high-specific-speed Francis runners.
The diameter D_d of the upper end of the draft tube is slightly less than D_t for low-specific-speed Francis runners and slightly greater than D_t for high-specific-speed Francis runners.
For axial-flow runners D = D_d= D_t (Fig. 16.2c)