The geometry of a curved diffuser often inviting the flow to separate. The flow separation phenomenon plays a vital role in the recovery of pressure inside the diffuser. Besides, the geometrical and operating aspects of a curved diffuser have always been taken into consideration in most relatable researches. Pressure recovery and flow uniformity of a steady, developed entering turbulent flow of a circularsectioned 180° curved diffuser has been investigated numerically by varying geometrical and operating parameters. The curved diffuser with area ratio (2, 3 and 4), radius of inlet and curvature of respectively 50 mm and 180 mm was considered. Different values of Reynolds number 6 × 10 4 , 12 × 10 4 , 18 × 10 4 , 24 × 10 4 , and 30 × 10 4 were tested in every area ratio set. ANSYS code Fluent was used to run the simulation by considering different turbulence models, i.e. the standard kturbulence model (std k-), Spalart-Allmaras turbulence model (SA), k-kl-transition turbulence model and transition SST model. The present work proposed the curved diffuser with AR=2 operated at Re=6x10 4 as the optimum set of parameters, with minimal flow separation occurring in the system.

Diffuser is a fluid mechanical device which allows the recovery of pressure energy from the fluid flow on the outlay of kinetic energy. Performance of turning diffuser is evaluated primarily by pressure recovery coefficient, and flow uniformity,. The varying of geometrical and operating parameters could alter the performance of the diffuser. Hence, this study aims to investigate the effect of angle of turn, on 2-D curved diffuser performance and to develop performance correlations. The angle of turn was varied from 30°to 180°. The evaluation for performance of the curved diffusers was done by using ANSYS Fluent with the most appropriate turbulence model of Reynolds Stress Model (RSM) added with enhanced wall treatment of + =1.0. Performance correlations in terms of pressure recovery and flow uniformity were developed by implementing Asymptotic Computational Fluid Dynamics (ACFD) technique. Pressure recovery of the curved diffusers showed a declining pattern by approximately 79.5% while flow uniformity shows higher distortion up to 52.6% with the increase of angle of turn from 30°to 180°. Hence, it has been proven that the increase of angle of turn has affected the performance of curved diffuser significantly. The ACFD performance correlations has provided acceptable deviations to both CFD and experimental results.

Curve diffuser is often used in HVAC and wind tunnel systems to provide pressure recovery and avoid excessive energy loss to the surrounding environment. Performance of curve diffuser is disturbed mainly by the presences of flow separation and secondary flow vortices occurred due to the effect of turning angle, in which scarce literature found. In this study, the effect of turning angle from 30° to 180° configured with an area ratio of 1.60 to 4.00 and inflow Reynolds number of 5.934x104 – 1.783x105 on loss characteristics and flow rectification of curve diffuser is investigated with optimum configuration is proposed. Performance of curve diffuser is evaluated in terms of pressure recovery and flow uniformity using ANSYS CFD equipped with validated Standard k-ɛ model (ske) and enhanced wall treatment of y+ = 1.2 - 1.7. Results show that performance of pressure recovery and flow uniformity decreases respectively by 85.71% and 45.84% as the angle of turn increases from 30° to 180°. Curve diffuser with minimum angle of turn 30o, optimum area ratio 2.16 and intermediate Rein 8.163x104 turns out to be the best configuration to provide pressure recovery of 0.399 and flow uniformity of 3.630 m/s.

Curve diffuser is frequently used in applications such as HVAC, wind- tunnel, gas turbine cycle, aircraft engine etc. as an adapter to join the conduits of different cross-sectional areas or an ejector to decelerate the flow and raise the static pressure before discharging to the atmosphere. The performance of the curve diffuser is greatly affected by the abrupt expansion and inflection introduced, particularly when a sharp 90o curve diffuser is configured with a high area ratio (AR). Therefore, the paper aims to numerically investigate the effect of the expansion direction of AR=1.2 to 4.0 curve diffuser on loss characteristic and flow rectification. 90o curve diffuser operated at inflow Reynolds Number, Rein=5.934 × 104 to 1.783 × 105 was considered. Results show that pressure recovery improves when the area ratio increases from 1.2 to 2.16 for both 2D expansion (z- direction) and 3D expansion (x- and z- direction). On the other hand, the increase of inflow Reynolds number causes the flow uniformity to drop regardless of the expansion directions. 3D expansion (x- and z- direction) curve diffuser with AR=2.16, operated at Rein=8.163 × 104, is opted as the most optimum, producing the best pressure recovery up to 0.380. Meanwhile, 2D expansion (z-direction) curve diffuser of AR=2.16, , operated at Rein= 5.934 × 104, is chosen to provide the best flow uniformity of 2.330 m/s. 2D expansion (x- direction) should be as best avoided as it provides the worst overall performance of 90o curve diffuser.