Effect of Impeller Geometry and Tongue Shape on the Flow Field of Cross Flow Fans

Experiments were conducted to investigate the effect of impeller geometry and tongue shape on the flow field of cross flow fans.Three impellers(Ⅰ，Ⅱ，Ⅲ）having same outer diameter,but different radius ratio and blade angles were employed for the investigation.Each impeller was tested with two tongue shapes.Flow survey was carried out for each impeller and tongue shape at two flow coefficients.and for each flow coefficient at different circumferential positions.The flow is two-dimensional along the blade span except near the shrouds.The total pressure developed by the impellers in each case is found to be maximum at a circumferential position of around 270°.The total and static pressures at the inlet of impellers are more or less same regardless of impeller and tongue geometry,but they vary considerably at exit of the impellers.Impeller Ⅲ with tongue T2 develops higher total pressure and efficiency where as impeller Ⅱ with tongue T2 develops minimum total pressure.Higher diffusion and smaller vortex size are the reasons for better performance of impeller Ⅲwith tongue T2。

Effect of Streamwise Fences on Secondary Flows and Losses in a Twodimensional Turbine Rotor Cascade

To control secondary flows, streamwise fences were attached to end wall of a linear turbine rotor cascade. The cascade had 8 blades of 400 mm long and 175 mm chord. The blades deflected the flow by 120°. The fences were made out of 0.7 mm thick brass sheet and the heights of the fences were 14 mm, 18 mm respectively. The curvature of the fences was the same as that of the blade camber line. The fences were fixed normal to the end wall and at half pitch away from the blades. The experimental program consists of total pressure, static pressure measurements at the inlet and outlet of the cascade, by using five-hole probe. In addition, static pressure on the blade suction surface and pressure surface was also obtained. Fences are effective in preventing the movement of the pressure side leg of the horseshoe vortex. Consequently the accumulation of low energy fluid on the suction surface is minimised. End wall losses are reduced by the fences due to weakening of the end wall cross flow.

Investigations on Low Speed Axial Compressor with Forward and Backward Sweep

In the recent past, experimental studies have shown some advantages of blade lean and sweep in axial compressors. As most of the experimental results are combined with other features, it is difficult to determine the effect of individual parameters on the performance of the compressor. The present numerical studies are aimed at understanding the performance and three-dimensional flow pattern at the exit of swept and unswept rotors. Three rotors, namely; unswept, 200 forward swept and 200 backward swept rotors are analysed with a specific intention of understanding the pattern of the blade boundary layer flow. The analysis was done using a fully three-dimensional viscous CFD code CFX-5. Results indicated reduction in pressure rise with sweep. Backward sweep is detrimental as far as the performance near endwalls is considered. On the other hand total pressure loss in the wake in mid span region is less with backward sweep, which favours its application here. However, backward sweep adversely affects the stall margin. The ability of the forward sweep to deflect the streamlines towards hub gets diminished at low flow rates. Forward sweep changes the streamline pattern in such a way that the suction surface streamlines are deflected towards the hub and the pressure surface streamlines are deflected towards the easing. An opposite behaviour is observed in backward swept rotors.

Experimental Investigations of the Three-DimensionalFlow in a Large Deflection Turbine Cascade with Tip Clearance

Results obtained from an experbontal study of the threedimensional flow survey within and exit of a large defiection linear turbine cascade are presented for a tip clearance levels of 0.08, 1.5, 3.0 percent of chord and compared with the help of boundary layer probes and that within and exit of a blade passage was done with a miniaturised five hole probe. End wall and blade tip surface static pressures were also obtained, in addition to flow visualisation studies. A strong horse-shoe vortex forms in front of the leading edge for zero clearance whereas this vortex does not appear for 3 percent clearance indicating that for large clearance the pressure forces have dominating infiuence than the viscous forces. In addition to normally known clearance vortex, a small tip separation vortex was noticed on the blade tip surface inside the tip gap. Due to the area contraction caused by the tip separation vortex, the fluid movign towards the tip gap from the pressure side is accelerated. Downstream of the vortex, the endwall pressure increases due to flow mixing. Both vortices increase in size and strength along the chord. The miring is incomplete in the aft portion of the blade. The tip gap velocity profiles exhibit wak like characteristics especially at axial positions where the mixing is incomplete. The passage vortex in the present investigations did not diminish with increase in clearance. The discharge coefhcient and the total pressure loss coefficient within the tip gap show similar tendency with lower values near the leading and trailing edge regions.