Effect of the turning angle on the flow and performance characteristics of long S-shaped circular diffusers

Abstract

The effect of the turning angle on the¯ow and performance characteristics of long Sshaped circular diffusers (length±inlet diameter ratio, L=D iˆ1 1:4) having an area ratio of 1.9 and centre-line length of 600 mm has been established. The experiments are carried out for three S-shaped circular diffusers having angles of turn of 158/158, 22.58/22.58 and 308/308. Velocity, static pressure and total pressure distributions at different planes along the length of the diffusers are measured using a ®ve-hole impact probe. The turbulence intensity distribution at the same planes is also measured using a normal hot-wire probe. The static pressure recovery coef®cients for 158/158, 22.58/22.58 and 308/308 diffusers are evaluated as 0.45, 0.40 and 0.35 respectively, whereas the ideal static pressure recovery coef®cient is 0.72. The low performance is attributed to the generation of secondary¯ows due to geometrical curvature and additional losses as a result of the high surface roughness (*0.5 mm) of the diffusers. The pressure recovery coef®cient of these circular test diffusers is comparatively lower than that of an S-shaped rectangular diffuser of nearly the same area ratio, even with a larger turning angle (908/908), i.e. 0.53. The total pressure loss coef®cient for all the diffusers is nearly the same and seems to be independent of the angle of turn. The¯ow distribution is more uniform at the exit for the higher angle of turn diffusers. NOTATIONA i cross-sectional area at the diffuser inlet (m 2 ) AR area ratioˆ…D o =D i † 2 C loss coef®cient of total pressure losŝ …P o i ¡ P oo †=P dyn…in † C p coef®cient of static pressure recoverŷ …P s o ¡ P s i †=P dyn…in † CC concave surface CR curvature ratioˆ2R c =D i CV convex surface dA elemental area to calculate the massaveraged quantities D i diameter of the test diffusers at the inlet (m) D n Dean numberˆR n =CR 1=2 D o diameter of the test diffusers at exit (m) F any¯ow parameter (static pressure, total pressure, velocity, etc.) L centre-line length (m) P dyn…in † dynamic pressure at the inlet 1 2rU 2 ave…in † …N=m 2 † P o i mass-averaged total pressure at the inlet (N/m 2 ) P oo mass-averaged total pressure at the outlet (N/m 2 ) P s i mass-averaged static pressure at the inlet (N/m 2 ) P so mass-averaged static pressure at the outlet (N/m 2 ) P wall wall static pressure (N/m 2 ) R radius of test diffuser (m) R c radius of curvature (m) R i radius at inlet (m) R n Reynolds numberˆrD i U ave…in † =m u 0 mean¯uctuating component of velocity (m/s) U local time-averaged (10 s) total velocity (m/s) U ave…in † mass-averaged velocity at the inlet (m/s) U long velocity in the axial direction (m/s) U sec cross-¯ow velocity (secondary motion) (m/s) x distance along the centre-line of the diffuser from the inlet plane (m)The MS was Downloaded from a pitch angle (deg) b turning angle of the curved diffuser (deg) y yaw angle (angle of rotation of the ®ve-hole probe) (deg) m absolute viscosity (Ns/m 2 ) r¯uid density (kg/m 3 ) f total angular position of the section from the inlet (deg)

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