Wide angle diffusers with passive boundary-layer control

The conical diffuser is one of the oldest and most versatile hydraulic devices, however its performance is often less than optimal. This is because boundary layer separation and instabilities in th...

Effects of boundary layer suction control on flow through an axisymmetric diverging channel

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.

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

This article explores a non-conventional method for improvement of performance of a wide-angle two-dimensional rectangular diffuser. Computational fluid dynamics investigation has been carried out to analyse the effect of injecting momentum through a moving surface to control the boundary layer separation close to the wall of a two-dimensional rectangular diffuser. A cylinder was placed at the diffuser inlet and rotated at various speeds. It is seen that injection of momentum through the moving surface delays flow separation, thereby increasing the pressure recovery coefficient. An increase in the rotational speed of the cylinder from 1500 to 5500 rad/s improves the pressure recovery coefficient marginally. Placing another cylinder at a preidentified location and rotating both cylinders at the same speed further improves the pressure recovery. Overall improvement in pressure recovery is ∼28 per cent from 0.63 to 0.81. Besides improvement of pressure recovery, the flow distribution in the core regi...

Performance optimization for two-dimensional rectangular diffuser by momentum injection using computational fluid dynamics:

Experimental investigation were conducted into the pressure recovery of a two-dimensional short subsonic diffuser with passive boundary-layer control, to determine the effectiveness of such a control at high inlet turbulence. The passive control configuration consisted of a porous surface made of normal or inclined slots (located on the diverging walls of the diffuser) which were connected by a breather passage. The results showed that passive control improves pressure recovery and postpones flow separation (stall) in the diffusers and the effectiveness of the passive control was enhanced by the inlet turbulence. Nomenclature AR aspect ratio, L/W; B, inlet blockage, 28*AV; cp pressure coefficient, (p-pj)/qi cPR pressure recovery, (Pe-ft)^2 L length of the diverging walls, m p static pressure, N/m2

Short diffusers with passive boundary layer control

Experimental studies were carried out on the caracteristics of diffusers of rectangular cross section which have an angle of expansion 2θ=30°, 60°, 90°, and 180° and an area ratio of 4. Remarkabel improvement of the pressure coefficient and energy loss coefficient is achieved by means of suction at the entrance. It is found that the most effective position of suction is at the point right down stream of the corner. The diffuser efficiency increases with increase of aspect ratio. The efficiency, however, tends to be constant as the aspect ratio exceeds four. When the angle of expansion is 20°, 30°, 60°, and 90°, the rate of optimum suction is 3 to 4 per cent of the total flow rate. With this optimum suction, the efficiency of these wide angle diffusers approaches that of the diffuser of 2θ=10°.

Ichiro Nishiura

Papers

Performance of the Two-Dimensional Diffusers with Suction at the Entrance