Abstract
Usage of a wide-angle diffuser may result in unfavorable separated flow and a significant diffuser loss. To improve the performance of the diffusers, inserting short splitter vanes is known as a useful method that has been demonstrated experimentally. Regarding the role of the vane in the diffuser flow, Senoo & Nishi (1977) qualitatively explained that the lift force acting on the vane should be a key factor. However, its quantitative verification remains since then. To challenge this issue, numerical simulations of incompressible flow in a wide angle of 28° two-dimensional diffuser with and without a short splitter vane were conducted in the present study. An improvement of pressure-recovery by the vane and oscillatory flows in the diffuser are reasonably reproduced from comparison with the experimental results made by Cochran & Kline (1958). It is also found that the lift force acting on the vane varies periodically in an opposite phase with the detachment point moved back and forth on a diverging wall, since one vane is not sufficient to fully suppress the flow separation that occurred on the wall and the incoming main-flow shifts toward the other diverging wall in the diffuser. Thus, as a role of splitter vane in the diffuser, “the lift force of the vane is a key factor” may be quantitatively verified from the present numerical simulation. Further, it is confirmed by the local loss analysis that the turbulent kinetic energy production observed in mixing layers contributes most of the loss in the diffuser. Consequently, the present numerical technique may be usable to investigate the flow character in a diffuser with splitter vanes at a design stage.
Highlights
Diffusers are widely used in fluid machines and model test facilities as a component to convert the dynamic pressure to the static pressure by decelerating the main flow velocity
Nishi [26] qualitatively explained that the lift force acting on the vane should be a key factor, its quantitative verification still remains to be done
The numerical results show that an increase of about 43% in CPR is achieved after inserting a short splitter vane in the diffuser
Summary
Diffusers are widely used in fluid machines and model test facilities (e.g., draft tubes of turbines [1,2,3], subsonic wind tunnels [4,5], etc. [6]) as a component to convert the dynamic pressure to the static pressure by decelerating the main flow velocity. Diffusers are widely used in fluid machines and model test facilities [6]) as a component to convert the dynamic pressure to the static pressure by decelerating the main flow velocity. If the diffuser loss ∆pLoss is required to design the above-mentioned systems, the following relationships derived from the one-dimensional theory for the incompressible steady flow have been used [7,8,9]: ! = Cpi − Cp = Cpi 1 − = Cpi (1 − ηd ) q1 Cpi Cpi = 1 −.
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