Transonic Centrifugal Impeller Research Articles

Collaborative Robust Design Optimization of Blade Geometry and Manufacturing Tolerance for a Transonic Centrifugal Impeller

Abstract Turbomachinery components are inevitably subjected to various sources of manufacturing errors. The resultant variations in blade geometry eventually lead to performance degradation. This is especially true for transonic centrifugal compressor impellers where the complex interaction between the geometric variations and shock waves may amplify such degradation. However, relevant studies on uncertainty quantification or robust design optimization of centrifugal impellers are quite rare. The mechanisms of how the realistic manufacturing errors influence the impeller aerodynamic performance are not clear. In addition, most existing studies have considered a fixed level of manufacturing tolerance applied to the impeller blade surface, which neglects the combined effect of blade geometry and manufacturing tolerance on impeller performance. In this study, a collaborative robust design optimization was performed for a transonic centrifugal impeller considering realistic manufacturing errors. The realistic manufacturing error field was first modeled based on the measurements of 92 centrifugal impellers. With a combination of computational fluid dynamics simulation and the non-intrusive polynomial chaos method, the influence of manufacturing errors on impeller performance and flow field variations was quantified. To save computational costs for the uncertainty quantification, a dual dimensionality reduction method was proposed to reduce the dimensionality of uncertainties. Finally, blade angles and the manufacturing tolerance of the impeller were collaboratively optimized to enhance the impeller performance robustness against manufacturing errors. The results show that the impeller aerodynamic performance exhibited a downward trend in the presence of manufacturing errors. The flow mechanisms responsible for this trend were mainly associated with increased intensity of the shock waves near the inducer blade tip. Such shock waves were significantly alleviated by reducing impeller inlet blade angles without the loss of pressure rise capability. Consequently, the impeller performance robustness against manufacturing errors was enhanced with the standard deviation of polytropic efficiency being reduced by 35% at a lower anticipated manufacturing cost due to increased tolerance allowance.

Using Tip Injection to Stability Enhancement of a Transonic Centrifugal Impeller with Inlet Distortion

This paper aims to understand the effects of circumferential inlet distortion and tip injection on a transonic impeller performance and flow field. For distorted inflow, the impeller is subjected to a stationary 120-degrees circumferential total pressure distortion. Full annulus unsteady three-dimensional analysis has been used to study the inlet distortion and tip injection effects on the impeller performance, stability and flow field. The results show that the circumferential inlet distortion reduces the impeller total pressure ratio and adiabatic efficiency; however, it has no significant impact on the safe operating range. Unlike the inlet distortion, the tip injection considerably increases the operating range. According to the results, the distortion and tip injection effect on the compressor performance is mainly due to changes in tip leakage flow. The inlet distortion has unfavorable influences on the flow field, especially near the impeller tip; however, the tip injection ameliorates the flow field in this region. In both the clean and distorted inflow, the tip injection causes downstream shock transmission, weakening the shock-tip leakage interaction. Hence, stall inception is postponed, and the impeller stability is improved in the presence of the tip injection.

Lightweight design of the transonic centrifugal impeller based on fluid–structure coupling

The impeller is the core component of the centrifugal compressor. Aiming to solve problems related to a large mass, high-energy consumption, and large vibration of the existing centrifugal impeller, a semi-open supersonic centrifugal impeller provided by Krain et al. was redesigned in this paper. The redesign was based on Tsai–Wu failure theory and the two-way fluid–structure coupling method. First, the geometric and numerical models were established using the geometric data found in the literature. Then, the established geometric and numerical models were verified through experimental data. Finally, the impeller was optimized from three aspects: material lightweight, composite material layering, and tip clearance. The results have shown that following the optimization, the maximum impeller stress was reduced by 48.77% when compared to the traditional 17-4PH material impeller. The maximum vibration deformation was reduced by 60.4%, as well as the resonance probability. The pressure ratio was increased by 0.8%, and the pressure and velocity reverse gradient trend near the blade tip were significantly reduced, while the flow field was more stable. The research results presented in this paper hence provide a basis for the application of composite materials in centrifugal impellers.

Assessment of a novel k–ω turbulence model for transonic centrifugal impeller simulations

Numerical simulation of high pressure ratio transonic centrifugal compressors is challenging for the existing turbulence models. A lagged k–ω model proposed by Olsen and Coakley for nonequilibrium effects was first applied to simulate the transonic centrifugal impeller SRV2-O. As comparative case studies, four other turbulence models ( k–ω model, RNG k–ε model, SST-CC model, and EARSM model) were also computed. The comparison showed that ( i) the selection of the turbulence model had a great influence on SRV2-O impeller simulations; ( ii) the lagged k–ω model had an advantage over other models in terms of overall pressure ratio and internal flow characteristics; and ( iii) the lagged model predicted a smaller blockage area caused by leakage vortex breakdown than other models, closer to the experimental result. The detailed parameter examination indicated that the nonequilibrium parameter a0 in the lagged model had little influence on the Mach number distribution and choking mass flow rate but a significant influence on the static pressure on the shroud casing. For a higher Mach number compressor, a smaller a0 is recommended for bettering the simulation accuracy.

Tip injection effects on a transonic centrifugal impeller with various tip clearances in the presence of inlet distortion

Tip injection effects on a transonic centrifugal impeller with various tip clearances in the presence of inlet distortion

Influences of the Flow Cut and Axial Lift of the Impeller on the Aerodynamic Performance of a Transonic Centrifugal Compressor

In this study, the influences of the flow cut and axial lift of the impeller on the aerodynamic performance of a transonic centrifugal compressor were analyzed. The flow cut is a method to reduce the flow rate by decreasing the impeller passage height. The axial lift is a method of increasing the impeller passage height in the axial direction, which increases the impeller exit width (B2) and increases the total pressure. A NASA CC3 transonic centrifugal compressor with a backswept angle was used as a base compressor. After applying the flow cut, the total pressure at the target flow rate was lower than the total pressure at the design point due to the increase in the relative velocity at the impeller exit. After applying the axial lift, the total pressure at the design flow rate was increased, which was caused by the reduction in the relative velocity as the passage area at the impeller exit was increased. By applying the flow cut and axial lift methods, it was shown that the variation in relative velocity at the impeller exit has a significant effect on the variation in total pressure. In addition, it was found that the relative velocity at the impeller exit of the target flow rate is maintained similar to the base impeller when the flow cut and the axial lift are combined. Therefore, by combining the flow cut and the axial lift, three transonic centrifugal impellers with flow fractions of 0.7, 0.8, and 0.9 compared to the design flow rate were newly designed.

Effects of Compound Sweep and Lean on the Aerodynamic Performances of Transonic Centrifugal Impellers

The blade lean and sweep has a significant effect on the centrifugal compressors performance. This paper modifies the blade leading edges of two transonic centrifugal impellers with two compound sweep and lean (CSL) methods: the blade tip forward sweep with a negative lean (SFB), the blade tip backward sweep with a positive lean (SBB), and the CSL effects are numerically studied. For the unshrouded impeller, the SFB improves the highest isentropic efficiency and total pressure ratio. The SBB decreases the total pressure ratio and almost has no effect on the isentropic efficiency. The SFB expands the stall margin. Conversely, the SBB decreases the stall margin. The CSL effects on the shrouded impeller are less obvious compared with the unshrouded impeller. By analyzing the flow field, it is found that, the CSL has important effects on the shock, the accumulation of low momentum fluid and the blade loading near the blade leading edge for unshrouded impeller and shrouded impeller. Since the CSL effects on the stall margin of the unshrouded impeller are associated with the blade tip loading and tip clearance leakage flow, the CSL does not influence the stall margin of the shrouded impeller without a tip clearance.

Free-Form Versus Ruled Inducer Design in a Transonic Centrifugal Impeller

The detailed design of the inducer of a high pressure ratio transonic radial compressor impeller with a design inlet tip relative Mach number of 1.4 is considered. Numerical analysis has been used to compare a datum impeller with ruled inducer design with a number of different free-form design concepts, generated following the same aerodynamic design philosophy. The datum stage and one with a free-form inducer, referred to as “barrelled forward swept,” with forward swept leading edge near the tip and increased chord at midspan, have been manufactured and tested. The tests were performed with the same stationary components, including the casing, vaned diffuser, and the volute. The design with a barrelled forward sweep of the inducer allows the designer more control of the strength and position of the passage shock at the inlet while meeting mechanical constraints. Interestingly, the performance is also enhanced at off-design points at lower tip-speeds. The measurements show that the stage tested with the swept impeller achieves higher efficiency of between 0.5% and 1.6% compared to the datum design, depending on the operating speed. The computational fluid dynamics (CFD) simulations are used to further study the flow at part speeds, in order to explain the causes of the observed performance differences at off design conditions.

Analysis and Optimization of Transonic Centrifugal Compressor Impellers Using the Design of Experiments Technique

The turbomachine industry is increasingly interested in developing automated design procedures that are able to summarize current design experience, to take into account manufacturing limitations and to define new rules for improving machine performance. In this paper, a strategy for the parametric analysis and optimization of transonic centrifugal impellers was developed, using the technique of the design of experiments coupled with a three dimensional fluid-dynamic solver. The geometrical parameterization was conducted using Bezier curves and a few geometrical parameters, which were chosen after a screening analysis in order to determine the most significant ones. The range of variation of the parameters was defined taking into account the manufacturing requirements. The analysis of the influence of such parameters on the main impeller performance was subdivided into two steps: first, the effect of the parameters acting on the blade shape was investigated and an optimum configuration was chosen, then the influence of three functional parameters was analyzed, fixing the already optimized variables. The whole strategy aimed at an industrial design approach, and attention was focused on the time required in the design process. From the present analysis it was possible not only to define an optimum geometry, but also to understand the influence of the input parameters on the main machine performance.

Flow measurement in a transonic centrifugal impeller Using a PIV

Laser velocimetries, such as LDV or laser-2-focus (L2F) velocimetry, have been widely used for a flow measurement in a high-speed rotating impeller. A particle image velocimetry (PIV) is one of the popular velocity measurement techniques for the ability to measure a velocity field. And a PIV offers an extensive velocity field in an extremely shorter measurement time than the laser velocimetries. In the present experiment, a PIV was applied to a flow measurement in a transonic centrifugal impeller. A phase locked measurement technique every 20% blade pitch was performed to obtain a velocity field over one blade pitch of the inducer. The measured velocity field at the inducer of impeller clearly showed a shock wave generated on the suction surface of a blade. The validity of the present technique was also discussed.

Effects of Camber of Inducer on Performance of a Transonic Centrifugal Compressor.

Two types of transonic centrifugal impellers with a slight variation in the inducer blade camber were tested to clarify their effects on the stall and the choke. Regarding choke, the inducer-throat blockage decreased and the choke flow rate increased in spite of a smaller throat area for the lower cambered inducer blade. Regarding inducer stall, the inducer blade camber affected the critical incidence angle against the inducer tip especially at low rotor speeds. Two impellers were also tested in combination with a low-solidity cascade diffuser, and the lower cambered blade impeller realized a wide compressor operating range especially at high rotor speeds.

Detailed measurements of the flow field in vaneless and vaned diffusers of centrifugal compressors

The flow field development in vaneless and vaned diffuser of a transonic centrifugal compressors has been investigated using miniature probes. We give a detailed experimental analysis of the flow structure at the outlet of two backswept transonic centrifugal impellers, one with splitter blades. These two impellers are equipped with a large radius ratio vaneless diffuser. Using a relative frame linked to the rotor, time-dependent measurements close to the rotor provide the flow field velocity at the outlets of the impeller channels.