Abstract: The effect of changing area ratio on the flow and performance characteristics
of long S-shaped circular diffusers (length/inlet diameter ratio, L/Di = 11.4) having a
center line turning angle of 30o and centre-line length of 600mm has been established.
The numerical investigation using three dimensional large eddy simulation model has
been performed for four S-shaped circular diffusers having area ratio of [1, 1.35, 1.51 and
1.9]. The simulations were performed at Re=105 based on S-duct inlet diameter, using
1.87 million grid nodes and using Smagorinsky-lilly as a subgrid turbulence model
Cs=0.1. Velocity, total pressure and turbulence intensity distributions at different planes
along the length of the diffusers are obtained .The performance parameters include static
pressure recovery coefficient, total pressure losses and distortion descriptor are calculated
at the S-duct outlet plane of each diffuser. No separation is detected for S-duct with area
ratio equal 1 and 30°/30° turning angle. The static pressure recovery is increased from -
0.19 to 0.41 for increase in area ratio from 1 to 1.9, No much enhancement in pressure
recovery by increase the area ratio from 1.51 to 1.9 but noticed increase in flow
distortion. The duct with area ratio 1.51 has the best values for performance parameters,
high pressure recovery, low total pressure losses and not much high value of distortion
descriptor compared with the other ducts.
Nomenclature
: filtered (large-scale, resolved) part for any flow variable
: sub-filter (unresolved) part for any flow variable
t : time
μ : Dynamic viscosity (N.s/m2)
: The turbulence or eddy kinematic viscosity
AR : area ratio (Do=Di)2
CC : concave surface
CV : convex surface
L : centre-line length (m)
V : Velocity vector
ρ : Density (kg/m3)