Parametric study of slotted diffuser effects on a highly loaded centrifugal compressor

Abstract

Flow separations in the vaned diffuser play a critical role in the stall margin of centrifugal compressors. To extend the stable operating range of a highly loaded centrifugal compressor, a flow control technique of locating a slot near the vaned diffuser's leading edge is proposed. A parametric study of slot geometry variations, in terms of the slotted portion depth and length, is carried out to summarize the trend of compressor performance and to reveal the corresponding flow physics related to the slot geometries. The design methodology is concluded to provide a guideline for stall margin improvement by the slotted diffuser. Application of a well-designed slotted diffuser shows that the flow separations inside the diffuser passages are suppressed at low flow rates, and the stall margin is increased without performance deterioration. The leakage flow passing through the slotted portion intensifies the passage vortex at the hub/suction surface corner, and decreases the flow angle at the diffuser inlet, leading to the suppression of flow separations on both the suction and pressure surfaces of the diffuser, respectively. However, it is found that the compressor performance deteriorates with increasing slotted portion depth and length, even though the stall margin is increased. When the slot depth is increased by up to 12% of the diffuser vane height, the leakage flow is not able to roll-up into a vortex that has the potential to mitigate the flow separation at the hub/suction surface corner. Instead, it enhances the flow separation region near the suction surface due to the negative effect of leakage flow; eventually, the stage performance is deteriorated. In conclusion, the slot depth should not exceed 9% of the diffuser vane height, and the optimum slot length is no greater than 6%–12% of the diffuser chord length, which provides a basic guideline for slotted diffuser design.

Investigation of New Design Principles for the Centrifugal Compressor Vaned Diffusers

“…The design of compact and efficient diffuser is very challenging, and it is also a major technical obstacle for the high-pressure-ratio centrifugal compressors [6,7]. At present, there are many researches on the design of the diffusers, such as low solidity vanes, half guide vanes, pipe diffuser and leading edge slotted design, these could improve the performance of the diffusers [8][9][10][11][12].…”

Centrifugal Compressor Stall Control by the Application of Engineered Surface Roughness on Diffuser Shroud Using Numerical Simulations

“…The size of the roughness elements is increased at the same design condition from smooth surface to roughness magnitudes of (50 μm, 100 μm, 200 μm) and compared with the baseline compressor, which indicates that stall margin has been increased in each case of roughness heights by simulating it using SST turbulence model. However, there is drastic drop in total pressure ratio and isentropic efficiency with the increase in the roughness heights as compared to the slotted diffuser studied by Yingjie et al [ 13 ]. The most significant drop in pressure ratio was observed when roughness magnitude rose to 200 μm.…”

“…It was concluded that when the air flow angle of incidence increases at the inlet of diffuser, the aerodynamics load decreases and the flow structure of the diffuser vaneless region improves significantly. Yingji et al [ 13 ] studied the parametric analysis of slotted diffuser geometry variation to augment the stable operating range of the high-pressure ratio centrifugal compressor. Their analysis concluded that at reduced mass flow rate, there is flow separation at the corner of diffuser hub, which has been eliminated by the length of slot, which was 8–16% of chord length of diffuser without significant deterioration in performance.…”

Numerical investigation of the diffuser throat length effect on a transonic centrifugal compressor

“…Recently, much investigation has focused on the structure of the diffuser blade itself, including its low solidity, partial blade height, and slotted leading edge. 7–10 Ubben et al. 11 studied the effects of a diffuser on compressor performance using a variable speed control and adjustable diffuser clearance height.…”

“…Recently, much investigation has focused on the structure of the diffuser blade itself, including its low solidity, partial blade height, and slotted leading edge. [7][8][9][10] Ubben et al 11 studied the effects of a diffuser on compressor performance using a variable speed control and adjustable diffuser clearance height. Onesided diffuser clearance showed no negative impact on the operation or loss behavior of the centrifugal compressor, while improving its pressure ratio and efficiency.…”