4.3. Design of vaned diffuser

In order to obtain higher pressure ratio in a radial diffuser, the diffusion process has to be achieved across a relatively shorter radial distance. This requires the application of vanes, which provide greater guidance to flow inside diffusing passages. The vaned diffuser was designed by observing various flow parameters reflected at impeller exit after performing numerical simulations.

To circumvent flow separation, divergence of diffuser blade passages in vaned diffuser ring can be kept small by incorporating a large number of vanes. However, this can lead to higher friction losses. Thus, an optimum number of diffuser vanes must be employed and ensure flow passage divergence not to exceed 12. Thus, final diffuser design contains 30 blades. The diffuser vane leading edge was at a radius of 136 mm, and the trailing edge of the diffuser vanes was set at a radius of 166 mm. Blade inlets and outlet angle were set at 64�. The leading and trailing edge were defined by an elliptic ratio of 6 and a radius of 0.25 mm.

Another method to prevent very steep velocity gradients at diffuser entry is by providing a small vane-less space (0.05d2–0.1d2) between impeller exit and diffuser entry. Therefore, a vane-less space of 7.023 mm was allotted.

The design methodology adapted was mainly focused aiming at decrease in Mach number and flow angle at diffuser exit by satisfying a diameter constraint of 340 mm.
