85
2.4 Diffusers
For a general diffuser the same formula is used with the hydraulic radius and
the term equivalent opening angle is employed. From the performance diagrams
2.27–2.28 follows that optimum operation approximately corresponds to an opening angle 2a = 18° to 8°, for the length ratio varying from small to large. If the diffuser length that is available for the desired velocity reduction, i.e. the area ratio, is
too small, either a diffuser followed by a sudden expansion (Fig. 2.23) or guiding
surfaces (Fig. 2.25) should be applied. The number of guiding surfaces should be
determined so that the equivalent opening angle criterion is met for each partial
diffuser.
2.4.7 Diffusion in a Bend
All phenomena occurring in bends and straight diffusers occur in a still more extreme form in a bent diffuser. By adding a bend immediately downstream of a
diffuser, the adverse pressure gradient at the outward part of the bend entrance
increases (see Fig. 2.18) and there is high risk for separation. Addition of a diffuser downstream of a bend steepens the pressure gradient at the inward part of the
exit of the bend (see Fig. 2.18) and makes separation very likely. From the above
Fig. 2.29 Performance of conical diffusers incorporated in ducts; adapted from Miller [6]
2.4 Diffusers
For a general diffuser the same formula is used with the hydraulic radius and
the term equivalent opening angle is employed. From the performance diagrams
2.27–2.28 follows that optimum operation approximately corresponds to an opening angle 2a = 18° to 8°, for the length ratio varying from small to large. If the diffuser length that is available for the desired velocity reduction, i.e. the area ratio, is
too small, either a diffuser followed by a sudden expansion (Fig. 2.23) or guiding
surfaces (Fig. 2.25) should be applied. The number of guiding surfaces should be
determined so that the equivalent opening angle criterion is met for each partial
diffuser.
2.4.7 Diffusion in a Bend
All phenomena occurring in bends and straight diffusers occur in a still more extreme form in a bent diffuser. By adding a bend immediately downstream of a
diffuser, the adverse pressure gradient at the outward part of the bend entrance
increases (see Fig. 2.18) and there is high risk for separation. Addition of a diffuser downstream of a bend steepens the pressure gradient at the inward part of the
exit of the bend (see Fig. 2.18) and makes separation very likely. From the above
Fig. 2.29 Performance of conical diffusers incorporated in ducts; adapted from Miller [6]