86
2 Basic Components
examples it is clear that diffusion within a bend is delicate. The phenomena discussed before, such as secondary flow and segregation of turbulent eddies, strongly
intervene. However, a design in which bend phenomena and diffusion are well
matched allows a pressure recovery that is far superior to that of a serially connected bend and a straight diffuser. The crucial aspect is useful application of secondary
flow in order to bring high-energy fluid to the places with imminent separation. The
optimum geometry strongly depends on the inlet conditions. The consequence is
that there is no universal optimum geometry. For instance, bent diffusers are optimised for vertical shaft hydraulic turbines. Figure 2.30 shows an example (Francis
turbine). A pressure recovery up to 75 % is attained. The optimisation requires computational techniques and turbulence models. At present, optimisation of stationary
diffusers is very well possible. It is more difficult however for rotating diffusing
channels (radial pumps and compressors), as complex secondary flow patterns intervene. Notice that a large pressure recovery requires a sufficient covered length
(Fig. 2.30). Most radial pumps, fans and compressors have limited rotor channel
lengths. Hence, the earlier mentioned limit of velocity deceleration ratio w 2 / w 1 of
about 0.8–0.9 in order to avoid separation. In some radial compressors, rotor channels with greater lengths are applied, by mounting an inducer (see Chap. 14). A
velocity ratio w 2 / w 1 as low as 0.6 may then be realised, but the pressure recovery
is not very good, however. Figure 2.24 is intended as a practical suggestion to combine a bend with a diffuser without applying means of optimisation. It is advisable
to mount the bend first, and then a duct segment with constant section (length about
one diameter) in which mixing occurs by secondary flow, followed by an asymmetrically mounted diffuser.
Fig. 2.30 Optimised bent diffuser of a Francis-turbine (courtesy ANDRITZ HYDRO)
2 Basic Components
examples it is clear that diffusion within a bend is delicate. The phenomena discussed before, such as secondary flow and segregation of turbulent eddies, strongly
intervene. However, a design in which bend phenomena and diffusion are well
matched allows a pressure recovery that is far superior to that of a serially connected bend and a straight diffuser. The crucial aspect is useful application of secondary
flow in order to bring high-energy fluid to the places with imminent separation. The
optimum geometry strongly depends on the inlet conditions. The consequence is
that there is no universal optimum geometry. For instance, bent diffusers are optimised for vertical shaft hydraulic turbines. Figure 2.30 shows an example (Francis
turbine). A pressure recovery up to 75 % is attained. The optimisation requires computational techniques and turbulence models. At present, optimisation of stationary
diffusers is very well possible. It is more difficult however for rotating diffusing
channels (radial pumps and compressors), as complex secondary flow patterns intervene. Notice that a large pressure recovery requires a sufficient covered length
(Fig. 2.30). Most radial pumps, fans and compressors have limited rotor channel
lengths. Hence, the earlier mentioned limit of velocity deceleration ratio w 2 / w 1 of
about 0.8–0.9 in order to avoid separation. In some radial compressors, rotor channels with greater lengths are applied, by mounting an inducer (see Chap. 14). A
velocity ratio w 2 / w 1 as low as 0.6 may then be realised, but the pressure recovery
is not very good, however. Figure 2.24 is intended as a practical suggestion to combine a bend with a diffuser without applying means of optimisation. It is advisable
to mount the bend first, and then a duct segment with constant section (length about
one diameter) in which mixing occurs by secondary flow, followed by an asymmetrically mounted diffuser.
Fig. 2.30 Optimised bent diffuser of a Francis-turbine (courtesy ANDRITZ HYDRO)