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)
An analysis is presented for the stability of a viscoelastic liquid film flowing down an inclined plane with respect to three-dimensional disturbances. It is shown that under certain circumstances, these disturbances are more unstable than the two-dimensional ones, contrary to Squire's theorem.
An analysis is made of the stability of a layer of an elastico-viscous liquid flowing down an inclined plane in the presence of two-dimensional disturbances. The modified Orr-Sommerfeld equation is solved by a regular perturbation technique for disturbances of large wavelengths. It is shown that in the absence of surface tension, the layer is more unstable as compared with that for an ordinary viscous liquid if Q1 > Q2, Q1 and Q2 being stress relaxation and strain retardation parameters respectively.
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