Very high flow coefficient centrifugal compressor stages are mostly applied as the first stages of multistage compressors.The performance of this type of stages is critical to the entire centrifugal compressors,but surprisingly little related information is available in the open literature.A centrifugal compressor with high inlet flow coefficient of 0.2,presenting a narrow operating range and unstable running situation even at design speed during the test,is investigated here.To reveal flow details in this centrifugal compressor,numerical simulations have been carried out and indicate that excessive impeller flow diffusion results in the poor performance of this centrifugal compressor.With the same inlet flow coefficient,six redesign cases coming from an in-house one-dimensional analysis program are proposed together with impeller trimming and equal flow area design method for corresponding vaneless diffuser.Performance comparison among these redesign centrifugal compressors is presented and the most suitable one is recommended for test in the future.In addition,three redesign cases with lower inlet flow coefficient developed by means of flow trimming are shown in the end to satisfy potential application areas.Finally,the results in this study can provide valuable reference information for multistage centrifugal compressor design.
WANG YongShengWANG KaiLIN FengNIE ChaoQunENGEDA Abraham
A stall inception model for transonic fan/compressors is presented in this paper. It can be shown that under some assumptions the solution of unsteady flow field consists of pressure wave which propagates upstream or downstream, vortex wave and entropy wave convected with the mean flow speed. By further using the mode-matching technique and applying the conservation law and conditions reflecting the loss characteristics of a compressor in the inlet and outlet of the rotor or stator blade rows, a group of homogeneous equations can be obtained from which the stability equation can be derived. Based on the analysis of the unsteady phenomenon caused by casing treatments, the function of casing treatments has been modeled by a wall impedance condition which has been included in the stability model through the eigenvalues and the corresponding eigenfunctions of the system. Besides, the effect of shock waves in cascade channel on the stability prediction is also considered in the stall inception model. Finally, some numerical analysis and experimental investigation are also conducted with emphasis on the mutual comparison.
According to the requirements of a solar dish-Brayton system,a centrifugal compressor stage with a minimum total pressure ratio of 5,an adiabatic efficiency above 75% and a surge margin more than 12% needs to be designed.A single stage,which consists of impeller,radial vaned diffuser,90° crossover and two rows of axial stators,was chosen to satisfy this system.To achieve the stage performance,an impeller with a 6:1 total pressure ratio and an adiabatic efficiency of 90% was designed and its preliminary geometry came from an in-house one-dimensional program.Radial vaned diffuser was applied downstream of the impeller.Two rows of axial stators after 90° crossover were added to guide the flow into axial direction.Since jet-wake flow,shockwave and boundary layer separation coexisted in the impeller-diffuser region,optimization on the radius ratio of radial diffuser vane inlet to impeller exit,diffuser vane inlet blade angle and number of diffuser vanes was carried out at design point.Finally,an optimized centrifugal compressor stage fulfilled the high expectations and presented proper performance.Numerical simulation showed that at design point the stage adiabatic efficiency was 79.93% and the total pressure ratio was 5.6.The surge margin was 15%.The performance map including 80%,90% and 100% design speed was also presented.
