Experimental and Numerical Verification of Effect of Using Curvilinear Element Blades for Low-Solidity Cascade Diffuser in Centrifugal Compressor

The effect of using curvilinear element blades (CEBs) on a low-solidity cascade diffuser (LSD) in a centrifugal compressor was investigated both experimentally and numerically. Centrifugal compressors require a higher efficiency and wider operating range to reduce the lifecycle costs of plants in which they are used and the effects their users have on the environment. A LSD has a wider operating range than a common vaned diffuser that has high solidity, but worse efficiency at the design point. For this reason, this research aims to improve the efficiency of the LSD and keep its operating range wide without changing the blade setting angle or two-dimensional geometry. As a first step, LSDs with three different curvilinear element profiles were compared with a conventional LSD, and the mechanism of the performance improvement was investigated by using a numerical simulation. These diffusers have the same two-dimensional geometry, blade setting angle, and number of blades but different blade stacking geometry in the span-wise direction. Unsteady Reynolds Averaged Navier-Stokes (RANS) simulations were conducted using a one-blade path model with an inlet, impeller, and LSD diffuser passages. The LSD using the CEBs improved the efficiency by 1.9% over that of the conventional LSD at the design point while maintaining the same operating range. The calculation results reveal that the recirculation region decreased at the suction surface of the diffuser vane in the CEB case and the diffuser efficiency improved. The concaved suction surface increased the blade loading and induced higher velocity on the suction surface. The higher velocity prevented reverse flow on the suction surface at the hub side, and the secondary flow removed the thick boundary layer at the trailing edge to the downstream of the diffuser. Experimental verification was also conducted using a single-stage compressor with an impeller, diffuser, and scroll casing to verify the prediction. The experimental results showed the LSD using the CEB has a higher pressure rise at the stator region than the conventional LSD and verified the performance improvement due to using the CEBs.

Unsteady Flow Analysis of Interaction Between Jet-Wake Flow and Blades of Low Solidity Cascade Diffuser in a Centrifugal Blower

A low solidity cascade diffuser (LSD) shows features of a wide operating range and a high pressure ratio in centrifugal compressors and blowers. According to the steady flow analysis results shown previously by the authors, the noise could be reduced effectively without deterioration of the LSD performance by means of a small blade tip-groove located at the shroud side, and a high blade loading could be achieved without stall even at the angle of attack as large as 15 deg. The high LSD performance at low flow rates was achieved by both formations of the stable and intense vortex in the shroud tip-groove and of the secondary flow moving circumferentially along the shroud wall toward the impeller exit. In the present study, the influence of the rotating jet-wake flow discharged from the impeller on the vortex in the shroud tip-groove and on the secondary flow moving circumferentially along the shroud wall were investigated by comparing the simulation results between the unsteady flow analysis and the steady flow one. It is clearly shown that the lift force of the LSD blade fluctuates periodically with the blade passing frequency, however, the flow behaviors of the vortex in the shroud tip-groove and the secondary flow on the shroud wall are little affected by the rotating jet-wake in the vaneless space.

Analysis of Secondary Flow Behavior in Low Solidity Cascade Diffuser of a Centrifugal Blower

Aiming at reducing noise without deterioration of diffuser performance in a low solidity cascade diffuser (LSD) of a centrifugal blower, the authors have proposed to locate a shallow and short groove or a slot between the diffuser wall and the LSD blade tip limiting to near the blade leading edge. The effect of the LSD blade tip-groove on the blower characteristics and the noise characteristics were investigated experimentally as well as numerically. The mechanism being able to maintain the high LSD blade loading even at small flow rates was pursued in view points of the vortex formation and the induced secondary flow. In addition, the effect of the tip-groove length on the vortex formation in the shroud tip-groove and the secondary flow behavior in the LSD were analyzed numerically and an optimum tip-groove configuration was proposed. It is concluded that formations of the stable and intense vortex in the shroud tip-groove and the recirculating secondary flow along the shroud wall toward the impeller exit are the key factors for achieving a high LSD performance and reducing noise simultaneously at small flow rates.