Test Diffuser Lab Report

Abstract—
I.
II. S DUCT DIFFUSER The complete geometry of the test diffuser along with the coordinate system used is shown in Fig.1. The diffuser was designed as suggested by Fox and Kline and based on linear area-ratio from inlet to exit.
The inlet area (Ai) of the test diffuser was chosenas 65 × 65 mm2. It was designed based on area ratio (Ar) of 1.923 with radius of curvature (RC) = 280 mm and turning angle (β) = 45º with a centerline length (CL) = 440 mm. A straight diffuser of centerline length of 232 mm with Ar = 1.923 and Ai = 65 × 65 mm2 has been given a circular shape of 45º turn with a radius of curvature, Rc = 280 mm. Due to reversal of the curvature, an inflexion point is produced and it results the S-shape of the centerline

Literature survey on S-shaped diffusers reveals that the flow at the exit plane of diffusers is not uniform and hence offers an uneven impact loading to the design point of view, it is undesirable. Here, an attempt is made to uniform the flow of the S-diffuser, especially at its exit by changing its corner shapes (i.e. sharp 90º, 45ºchamfered etc.) as well as using submerged vortex generators (VG). Lin(7) conducted an exploratory study of such VG devices to control turbulent flow separation. Such VG produce vortices, which transport high momentum fluid into the boundary layer, making it thinner and more resistant to the adverse pressure gradients with respect to separation. Further study by Lin (8) found that if the height of the vortex generators is shorter than the boundary layer height they can have a larger affect ton the downstream flow, since the velocity gradient is quite high and hence are named as ‘submerged’ vortex generators. The boundary layer from either side of the walls at inlet grows around 11mm, thereby for a span of 65 mm width (or height) at the inlet; a total of 22 mm is covered with boundary layer.
The maximum height of the ‘fishtail’ shaped submerged vortex generator is kept as 2.6 mm, and hence it is completely submerged within the boundary layer. The following sections describe the effects of corner shapes as well as submerged VG on

The SVG were fabricated from 0.8 mm thick aluminum sheet. While the wishbone type vortex generators create their own losses, the SVG being very thin in cross section generate very less losses. The different combination of vortex generators is shown in Fig. 3.The average inlet flow velocity of the S-duct diffuser was maintained at 15.79 m/s for all the cases. The maximum height (h2) of the SVG was chosen as 5 mm. The boundary layer thickness developed at the inflexion plane (plane-3) is measured as 12 mm. Hence the SVG are fully submerged in the boundary layer and injects momentum in it by producing small vortices