CFD analysis of performance characteristics of S-shaped diffusers with combined horizontal and vertical offsets

This paper presents the effect of combined horizontal (90°/90° turn) and vertical offsets (inlet and outlet in two different horizontal planes) on S-shaped diffusers, one with rectangular inlet (AS = 2) and rectangular outlet and other with rectangular inlet (AS = 2) and semi-circular outlet with area ratio 2 in both the cases having downstream settling length of 50 mm at Reynolds number 1.37 × 105. A computer program based on finite volume technique, using standard k–ε turbulence model, has been adopted and modified to predict the flow. The results obtained from this study indicate reduced outlet pressure recovery accompanied with increase in non-uniformity in flow at the exit contributed by the offset effect. The comparison of pressure recovery of the extreme vertical offset value i.e. 1B in case of rectangular outlet diffuser and 1D in case of semi-circular outlet diffuser with zero vertical offset indicates that there is significant drop of pressure of the order of 14% and 7.3% respectively. Non-uniformity in flow at the outlet also increases from 12% to 32% and 6.4% to 21% in case of S-shaped diffuser with rectangular outlet and S-shaped diffuser with semi-circular outlet respectively, starting from zero vertical offset to extreme vertical offsetting magnitude for the case. This paper also shows that increase in Reynolds number has marginal effect on the outlet pressure recovery for both types of diffuser with all cases of vertical offset. The S-diffuser with semi-circular outlet with 0.25D vertical offsetting only fulfills most of the requirements of the flow diffusion process efficiently and effectively. For other cases, redesigning of geometry is required.