Mechanism of stall and surge in a centrifugal compressor with a variable vaned diffuser

To expand the stable operating range of compressors, understanding the mechanism of flow instability at low flow rates is necessary. In this paper, the mechanism of stall and surge in a centrifugal compressor with a variable vaned diffuser is experimentally investigated, where the diffuser blade setting angle can be adjusted. Many dynamic pressure transducers are mounted on the casing surface of the compressor. From the design condition to surge, dynamic pressure data is recorded throughout the gradual process. According to the signal developing status, the typical modes of compressor instability are defined in detail, such as stall, mild surge, and deep surge. A relatively high-frequency stall wave originates in the impeller and propagates to the diffuser, and finally stimulates a deep surge in the compressor. The compressor behavior during surge differs at different diffuser vane angles. When the diffuser vane angle is adjusted, both the unstable form and the core factor affecting the overall machine stability change. A specific indicator is proposed to measure the instability of each component in a compressor, which can be used to determine the best region for stability extension technologies, such as a holed casing treatment, in different compressor applications.

HYDRODYNAMIC OPTIMIZATION DESIGN OF LOW SOLIDITY VANED DIFFUSER FOR A CENTRIFUGAL PUMP USING GENETIC ALGORITHMS

This paper presents a hydrodynamic redesign of the conventional vaneddiffuser into the low solidity varied diffuser for the maximum static pressure recovery in acentrifugal pump. A Bezier curve representation for profile description was coupled with ablade-to-blade flow calculation and a real-coded genetic algorithm. A low solidity vaned diffuser of0.89 in solidity was obtained through the present optimum design. Numerical analysis andexperimental test were made to evaluate the hydrodynamic performance of the centrifugal pump withthe designed low solidity vaned diffuser and original vaned diffuser. The obtained resultsdemonstrate that the centrifugal pump with the optimized vaned diffuser has compact size comparedwith the original one while the performance requirements have been met.

Numerical Investigation of the Centrifugal Compressor Stability Improvement by Half Vaned Low Solidity Diffusers

Centrifugal compressors for the fuel cell vehicles often operate near the surge line compared with the turbocharger compressors.Low solidity and half vaned diffusers are recognized as good ways to improve the stability of the centrifugal compressor.The presented work investigated four diffuser configurations (i.e.,the vaneless diffuser (VLD),full-height low solidity vaned diffuser (LSVD),hub-side half vaned diffuser (HVD) and shroud-side half vaned diffuser (SVD)) through steady-state and unsteady numerical simulations.The results show that the best performance is achieved by the LSVD,HVD and SVD at the design,surge and choke conditions.The flow rate at the surge operating point of the HVD has decreased by 15.53% compared with the LSVD,and 9.21% compared with the VLD.At near surge operating point,a longitudinal suction side passage vortex is formed on the hub of the LSVD and rotates as circumferential stall cells.A hairpin vortex is formed along the leading edge and is dragged by the main flow along the suction side as a local vortex shedding.The mechanism of the stability improvement by half vaned diffusers is that the tip leakage vortex migrates from the clearance side to the vane mounting side and replenishes the low-momentum zone on the mounting side.The best position where the half vaned diffuser should be mounted is based on the impeller outlet flow conditions,namely,the location of the wake region,where the meridional velocity and relative stagnation pressure is low.

Applications of Additively Manufactured Adjustable Vaned Diffusers in Centrifugal Compressor

As a variable-condition adjustment technology,the adjustable vaned diffusers(AVDs)can expand the working flow range of the compressor in the compressed air energy storage(CAES)system and improve its aerodynamic performance.In order to investigate the regulatory mechanism of AVDs and capture the details of vane loading distribution for the diffuser design optimization,additively manufactured AVDs for testing in a centrifugal compressor closed test facility are designed and implemented.Firstly,the regulation law of AVDs was summarized by numerical analysis and experimental support,and the corresponding vane loading data was extracted for the distribution law.Then,based on the distribution characteristics,3D diffuser models were designed suitably for the adjustable components.Then,the laser selective melting(SLM)technology and die steel material 1.2709 were selected for metal printing according to the actual operating environment.Finally,performance testing and accuracy detection were performed on the finished test pieces,almost all inlet hole’s deviations were within the 0.3 mm tolerance.The research results indicated that additive manufacturing can significantly improve the accessibility of the internal flow channels of the diffuser,and derive the load of the blade on the pressure surface and suction surface in detail,also provide adjustable functions for variable operating conditions.It can not only break through the traditional processing bottleneck of the complicated internal flow channels of AVDs but also improve the design matching degree with adjustable components;simultaneously,it ensures high performance with high precision and effectively shortens the long lead time.