A Numerical Study on the Flow Mechanism of Performance Improvement of a Wide-Angle Diffuser by Inserting a Short Splitter Vane

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.

Methods for increasing the energy efficiency of fan systems using wide-angle plane diffusers with splitter vanes

“…In the present section, a numerical study of the flow in plane diffusers having no splitter vanes is performed for three different wide-angle flat diffusers. Analyzing the reviewed literature sources makes it clear that different approaches are used to model turbulent separating internal flows: Direct Numerical Simulation (DNS) in [15], Large-Eddy Simulation in [14] и [16], Shear Stress Transport (SST) model in [19], Baseline Reynolds Stress Model (BSL RSM) in [11]. It is known that DNS and LES are modeling methods that require significant time and computational resources, and for thios reason twoequation models based on turbulent viscosity are most often applied to solve engineering problems -…”

“…The experiments are accomplished using water at Reynolds numbers Re=1000, 6000 and 12000. A similar study, but numerical, is presented in [19]. The studied diffuser has an expansion angle of 28 degrees, expansion rate 4.9, working fluid -dry air, ideal gas with a temperature of 20°C and a regime of movement, where Re=240000.…”