| Turbocharging technology is one of the important means to achieve low emission and high efficiency of internal combustion engines.Centrifugal compressor is the core component of supercharger,and its efficient flow stabilization method has become the main bottleneck restricting the development of advanced turbocharging technology for internal combustion engines.Self-recirculating casing treatment is the main strategy to achieve flow stabilization of centrifugal compressor,but usually introduces additional aerodynamic losses.How to achieve flow stability expansion for casing treatment without reducing aerodynamic performance is a difficult problem in the design of highpressure ratio centrifugal compressors.To this end,this study developed a coupling optimization method suitable for the stability expansion design of high-pressure ratio centrifugal compressors,which achieved the dual goals of stability expansion and efficiency enhancement for the self-recirculating casing treatment.Taking a transonic centrifugal compressor as the research object,the mechanisms of stability expansion and aerodynamic loss of the self-recirculating casing treatment are analyzed.The casing treatment removes the low-energy fluid near the tip of the compressor passage,reduces the relative Mach number of the suction side of the blade and the blade loading near the impeller leading edge,which are the main reasons for the realization of flow stabilization.The mixing of the recirculation flow and the main flow of the self-recirculating casing treatment is the main factor to reduce the efficiency,and the positive pre-swirl effect at the impeller inlet caused by the recirculation flow is the reason for the pressure ratio drop.In order to eliminate the aerodynamic loss introduced by the casing treatment,a global multi-objective optimization is implemented for the baseline casing treatment based on optimal Latin hypercube sampling,polynomial surrogate model,and gradient mutation hybrid optimization algorithm.The results show that the efficiencies at the design point and near the stall point are increased by 1.45% and 1.32% respectively,and the stable operating range is increased by 0.91 percentage points,but the efficiency at the design point is still lower than that of the solid casing compressor.Therefore,individual casing treatment optimization cannot compensate for the efficiency loss caused by the self-recirculating casing treatment.In order to achieve the dual goals of stabilization and efficiency enhancement of the self-recirculating casing treatment,the compressor design space was expanded to include the design parameters of both the casing treatment and the impeller/diffuser.In terms of the problem that high-precision surrogate models cannot be constructed due to the high dimension and strong nonlinearity of the extended design space,this study introduces the Fourier amplitude sensitivity testing algorithm to analyze the global sensitivity of the design space,and obtains the key design parameters.Dimensionality reduction is achieved under the premise of preserving the main features of the design space.The key design parameters of the compressor include: the incidence angle of the return groove,the width of the suction groove,the axial position of the suction groove of the casing treatment,the blade sweep angle of the impeller leading edge,the blade installation angle of the impeller trailing edge,and the trailing edge radius of the diffuser.The mechanisms of their influences on the aerodynamic performances of the compressor are as follows: the incidence angle of the return groove of the casing treatment can improve the mixing loss and radial distortion of the compressor inlet;the width and the axial position of the suction groove jointly affect the recirculation flow of the casing treatment;the blade sweep angle of the impeller leading edge can change the impact loading of blade and the throat area of impeller,and then will affect the stall margin and choke flow;the blade installation angle of the impeller trailing edge affects the impeller’s work coefficient and the degree of diffusion;the trailing edge radius of the diffuser determines the flow trajectory of the working fluid,which in turn affects the friction loss.The above key design parameters constitute the refined design space for the compressor coupling optimization.On the basis of the refined design space,the isentropic efficiencies of the compressor at the design point and the near-stall point are taken as the optimization objectives,and the total pressure ratio at the design point and the near-stall point and the choke flow rate are selected as the constraints.Then,the coupling optimization of the baseline compressor is carried out.The results show that the aerodynamic performances of the compressor are improved comprehensively after the coupling optimization,and the isentropic efficiencies at the design point and near the stall point are increased by 2.79% and 1.82%,respectively.Compared with the solid casing compressor,the stable range is increased by 9.59 percentage points,and the peak efficiency is increased by 0.11%,which means that the dual goals of stability expansion and efficiency enhancement are achieved.This is infeasible for the single optimization of casing treatment or the single optimization of impeller/diffuser.Compared with the baseline compressor,the main reasons for the improvements of stability and efficiency of the coupling optimization include: the recirculation flow is increased,and more lowenergy fluid is removed;the impeller blade incidence angle is improved,and the risk of flow separation is reduced;the leading-edge shock moves downstream,and its interference effect with the tip leakage vortex is suppressed,the low-energy fluid area downstream of the impeller is reduced,and the circumferential uniformity of the impeller’s adverse pressure flow is enhanced;the incidence angle of the return groove of the casing treatment increases,the mixing process of the recirculation and the main flow is smoother,and the radial distortion of the impeller inlet decreases.Compared with the solid casing,the isentropic efficiency loss caused by the self-recirculating casing treatment is mainly compensated by the following two aspects: firstly,the increase of the blade installation angle of the impeller trailing edge weakens the diffusion degree and the jet-wake structure near the impeller outlet,which makes the flow at the outlet of the impeller and inlet of the diffuser more uniform;secondly,the trailing edge radius of the diffuser is increased,the flow trajectory of the working fluid is shortened,and the friction loss is reduced.Finally,the improvement of the choke flow is mainly attributed to the reduction of the blade sweep angle of the impeller leading edge,which effectively increases the area of the impeller aerodynamic throat,thereby increasing the choke flow of the compressor.In this study,a coupling optimization method suitable for the stability expansion design of high-pressure ratio centrifugal compressors was constructed,which achieved the dual objectives of stability expansion and efficiency enhancement of the selfrecirculating casing treatment.It provides a feasible strategy for the stability expansion design of high-performance turbocharger compressors.It also provides a reference for the stability expansion design of compressors of aero-engines and gas turbines. |
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