Performance Of Centrifugal Compressor Research Articles

A New Global Gas Dynamics Performance Optimization Method for Refrigeration Centrifugal Compressor Stage Based on the Immune Algorithm

Abstract This study proposes a new global gas dynamics optimization method, which was applied to a multi-objective optimization task of centrifugal compressor performance with the aim of determining the improvement probability for achieving high efficiency across a wide operating range. Initially, the original nondominated neighbor immune algorithm (NNIA) was extended to solve constrained multi-objective optimization problems for the first time, which mainly incorporated a procedure for handling inequality and equality constraints without additional parameters. Subsequently, an adaptive topological back-propagation multilayer feed-forward artificial neural network (BP-MLFANN) was trained using the modified NNIA to quickly evaluate the fitness value of the centrifugal compressor stage performance during the optimization. The feasibility of the method was validated using the first stage of a refrigeration centrifugal compressor. The results indicated a substantial enhancement in the stage efficiency of the optimized impeller at the Near-stall, Design, and Near-choke operating points, with increasement of 1.8%, 1.9%, and 4%, respectively, as compared to the baseline stage. The flow field analysis shows that the impact loss at impeller leading edge and flow separation in the passage reduced greatly, the mixing process between the leakage flow and mainstream in the channel is significantly weakened, thus the flow field becomes more uniform after optimization. The new global gas dynamics optimization method provides a reference for the development of efficient and rapid optimization techniques for centrifugal compressor.

Effects of Near-Critical Condensation and Cavitation on the Performance of S-CO2 Compressor

The supercritical carbon dioxide (S-CO2) Brayton cycle efficiency increases as the compressor inlet condition approaches the critical point. However, the thermodynamic properties of CO2 vary dramatically near the critical point, and phase change is most likely to happen. Both cavitation and condensation bring about significant adverse effects on the performance of compressors. In this paper, the quantitative effects of nonequilibrium condensation and cavitation on the performance of an S-CO2 centrifugal compressor with different inlet-relative entropy values are investigated. The properties of CO2 were provided by the real-gas property table, and the nonequilibrium phase-change model was adopted. The numerical simulation method with the nonequilibrium phase-change model was validated in the Lettieri nozzle and Sandia compressor. Furthermore, simulations were carried out in a two-stage centrifugal compressor under conditions of various inlet-relative entropy values. The type of nonequilibrium phase change can be distinguished by inlet-relative entropy. Cavitation makes the choke mass flow rate decrease due to the drop in the speed of sound. Condensation mainly occurs on the leading edge of the main blade at a large mass flow rate, but cavitation occurs on the splitter. The condensation is more evenly distributed on the main blade, but the cavitation is mainly centered on the leading edge.

Numerical research on performance of new structure centrifugal compressor for supercritical CO2 power systems

The supercritical carbon dioxide (sCO2) power cycle system is a potential thermoelectric conversion system with the advantages of high efficiency and compactness, which can be used in the fields of solar thermal power, nuclear power, and other thermal power systems. The compressor is one of the important machines in sCO2 power systems. To improve the performance and reduce the condensation risk of sCO2 centrifugal compressors, a new structure sCO2 centrifugal compressor is proposed in this paper, which has a partial-cover impeller and self-circulation-channel casing (PISC). The internal flow of the compressor with the new structure is analyzed by the CFD method. The flow and condensation in sCO2 centrifugal compressors with the PISC structure and that with the semi-open impeller and traditional casing (SITC) are compared. The results show that the PISC sCO2 compressor has an inhibiting effect on the condensation at the compressor inlet location, reducing the condensation volume to approximately 82% of that of the original structure under design conditions. The total pressure ratio and isentropic efficiency of compressors are simulated, and the result shows that the minimum stable mass flow rate is reduced by about 13% by introducing the new PISC structure into the sCO2 centrifugal compressor, and the maximum isentropic efficiency is increased from 83.5% to 84.4% compared to that of the SITC compressor. A new design idea for the impeller and sCO2 compressor structure is provided in this paper. The research results can be applied to sCO2 compressors and improve the performance and stability of sCO2 power cycle systems.

Effect of Different Types of External Guide Vanes on the Performance of High-Pressure Centrifugal Compressor

In order to reduce exit swirl and obtain the desired Mach number, axial exit guide vanes (EGV) are often employed in a centrifugal compressor. NASA CC3 compressor, with wedge vane diffuser and without EGV, is considered as the base model for the analysis and validation. An axial flow domain with exit guide vane is added to this base model after the diffuser outlet to study the effect on the compressor performance. The performance of exit guide vane with different profiles: flat plate, symmetric wedge, circular arc, and airfoil vane profiles by maintaining the same chord, number of vanes, and flow angle of the vanes are studied. Numerical simulations are carried out with 60 number of exit guide vanes for all four types of vanes. Among several combinations, when the centrifugal compressor is equipped with 60 circular arc vanes as EGV, the efficiency and pressure recovery values at the design point have increased by 6.5% and 8.9%, respectively.

Improving a Centrifugal Compressor's Performance at Low Mass Flow Rates by Adding an Acoustic Resonator

Abstract Acoustic resonators are widely used to dampen the sound radiation of centrifugal compressors of automobile turbochargers. Their advantages are simplicity, robustness, ease of integration, and acoustic tunability. Interestingly, some resonators mounted to the inlet side improved compressor performance at low mass flow rates significantly. In this paper, this phenomenon was investigated experimentally by an extensive study of resonator geometries on a turbocharger test rig and numerically by uRANSs of selected compressor resonator configurations near the surge limit. A detailed description is given on how the resonator can limit the disturbance of the incoming flow and improve the impeller's operating conditions by ingesting the backflow coming from the blade tips. The backflow's kinetic energy and angular momentum are greatly reduced inside the resonator before the fluid flows back into the inlet duct. Concluding, design guidelines for a resonator geometry, which improve performance as an addition to its acoustic properties, are shared.

A New Unsteady Flow Control Technology of Centrifugal Compressor Based on Negative Circulation Concept

The tip leakage vortex (TLV) induced by the tip clearance flow has a significant impact on the performance of centrifugal compressors, causing impeller flow losses and reducing the stall margin. To solve this problem, an unsteady flow control technology called the NCFC method is proposed based on the concept of negative circulation control, realized by a vortex generator placed in a tube connected with the shroud through a hole. The approach is derived from a theoretical study of the compressor TLV by introducing a two-dimensional vortex model. A numerical simulation is then performed to verify the effectiveness of the NCFC method. The result shows that the NCFC method can greatly stabilize the flow field at the blade tip and improve the stall margin and efficiency of the compressor without reducing the total pressure ratio of the compressor, which has the characteristics of both unsteadiness and negative circulation effect. In addition, a HC method with only unsteady excitation effect is also studied for comparison, which only slightly stabilizes the blade tip flow and increases the stall margin of the compressor, suggesting that the NCFC is more effective than the HC. Finally, it is highly recommended to improve the efficiency of any unsteady jet/suction and separation flow interaction.

Centrifugal compressor performance prediction and dynamic simulation of natural gas hydrogen blended

It is great significant to explore the changes of compressor performance for hydrogen transportation. We found that a Gray-RBF neural network prediction model improved prediction accuracy. The prediction error of the model is 2.34 % by data verification. It was found that the hydrogen blending ratio was increased from 0 % to 30 %, the pipeline transmission power was reduced by 9.3 %. Simultaneously injecting hydrogen to natural gas causes the performance curve of the compressor to shift downwards. The pressure ratio decreases by 18.9 % and shaft power decreases by 28.6 % when the hydrogen blending ratio increases from 0 % to 30 %. The injection of hydrogen to natural gas causes the surge range of the compressor to increase, so the stable range to narrow. In general, this research may provide reference and guidance for the transportation research of hydrogen blended natural gas and compressor operation about gas industry.

Simulation on performance and regulation strategy of centrifugal refrigeration compressor with gas bearings in water chiller

The centrifugal refrigeration compressor is a key equipment of chiller and using gas bearing instead of traditional bearing is an important development direction. The optimized regulation strategies based on the performance of the compressor are necessary to improve system efficiency. In this paper, the mathematical model of a two-stage compressor with gas bearings and refrigeration system is established and verified to simulate the performance of the compressor under variable operating conditions, and the regulation process by rotating speed is provided. The results show that the regulation by rotating speed changes the work process of the compressor, and the various combinations of pressure ratio and mass flow rate can be obtained. For the different refrigerating capacity at the same evaporating and condensing temperature, the rotating speed and power of the compressor and the mass flow rate of the cooling motor increase with the increase of refrigerating capacity, but the isentropic efficiency and COP increases at first and then decreases. The maximum COP is 7.11 which appears at the refrigerating capacity of 358.22 kW and rotating speed of 21000 rpm. When the same refrigerating capacity is needed, the rotating speed and power of the compressor and the mass flow rate of the cooling motor increase with the condensing temperature, and the COP gradually decreases. The compressor speed should be increased from 16000 rpm to 22330 rpm if 25%-100% of the refrigeration load is needed, but the compressor can achieve high isentropic efficiency at only 50%-75% of the refrigeration load.

Numerical investigation on the combined effects of pinching and rotation on the performance of a high-speed centrifugal compressor with a vaneless diffuser

Computational investigations are carried out on a high-speed centrifugal compressor with the vaneless diffuser to study the combined pinch and rotation effect. The axial inlet width of the rotating vaneless diffuser is reduced at diffuser inlet, either at the hub wall or at the shroud wall, or both the walls. The reduction is varied from 0% (base case) to 20% at an interval of 5%. Numerical simulations are conducted for different speed range by varying the rotating diffuser diameter ratio from 1.25–1.75. Among all these cases, the best performance of 25% increase in pressure recovery with uniform diffusion and 2.3% increase in isentropic efficiency is obtained with 15% shroud pinch at a diameter ratio of 1.375 at the design mass flow rate.

Numerical Simulation on the Influence of Inlet Flow Characteristics on the Performance of a Centrifugal Compressor

Although inlet bent pipes are usually adopted due to limited installation space, the influences of different bend pipes on the inlet flow characteristics and performance of centrifugal compressors are still unclear. The numerical simulation of a centrifugal compressor is established and validated by experimental results with the case of a straight inlet pipe. Then, the internal flow characteristics of the centrifugal compressor with a 90-degree bent pipe (p90) and Z-shaped bent pipe (pz) are simulated and discussed. The results show that the adoption of two inlet bent pipes reduces the performance of the centrifugal compressor to a certain extent, which reduces more greatly with pz, with a maximum reduction of 6.82% in pressure ratio and 14.83% in efficiency, respectively. The pressure ratio and efficiency reduction of the centrifugal compressor both increase with the increment of distortion degree, which maintains the increasing trend as the flow rate increases, and the maximum distortion degree of p90 and pz reaches 0.0351 and 0.0479, respectively. The reduction degree of the pressure ratio shows a power–law relationship with the distortion degree, while the reduction degree of efficiency shows an exponential relationship with it. The flow characteristics at the outlet section of the inlet pipe affect the flow field distribution at the inlet of the impeller, and the distortion area ranges of the total pressure and axial velocity at the inlet of the impeller are near 72°–144° in the circumferential direction for p90, while those of pz are close to 108°–180° and 288°–360°. When the flow with a high distortion degree enters the impeller, a large area with high turbulent kinetic energy is formed in the downstream flow channel, resulting in an increase in the flow loss.

Effect of flow channel structures and refrigerants on the motor cooling process of compressor with gas bearings

Energy-saving and environment-friendly water chiller is the development trend. The efficient compressors and new refrigerants are hot spots. The gas bearing can significantly improve the performance of centrifugal refrigeration compressors, and effective motor cooling is the guarantee of performance and reliability. The influence of channel structure and refrigerant on the performance of the motor cooling process and refrigeration system is analyzed. The results show that adding the bearing gas supply circuit decreases the motor temperature and the maximum variation of permanent magnet temperature is about 60 °C. The cooling channel structure in the interlayer of the motor shell will affect the cooling process. The spiral channel can reduce the stator temperature and improve the uniformity of temperature distribution. But the temperature of the motor changes slightly when the length of the channel is more than 6000 mm. The motor temperature is the highest with refrigerant R1234ze(E), followed by it R1234yf and R134a. The maximum temperature of the permanent magnet for R1234ze(E) is 20–50 °C higher than it for R1234yf and 50–60 °C higher than it for R134a under different inlet diameters. The maximum temperature of the permanent magnet for R1234ze(E) is 30 °C higher than it for R1234yf and 50 °C higher than it for R134a under different inlet temperatures. The system with a motor cooling circuit has controlled the motor temperature within the safe operation range, while the COP of systems with R134a, R1234yf, and R1234ze(E) has only decreased by 4.63 - 5.36%, 4.22 - 4.6%, and 3.78 - 4.15% respectively.

Mechanism Underlying the Effect of Self-Circulating Casings with Different Circumferential Coverage Ratios on the Aerodynamic Performance of a Transonic Centrifugal Compressor

The aim of this research was to explore the mechanisms underlying the effect of self-circulating casing treatment with different circumferential coverage ratios on the aerodynamic performance of a transonic centrifugal compressor. A three-dimensional unsteady numerical simulation was carried out on a Krain impeller. The circumferential coverage ratios of the self-circulating casings were set to 36%, 54%, 72% and 90%, respectively. The numerical results showed that the Stall Margin Improvement (SMI) increased with the increase in circumferential coverage ratios. The self-circulating casing with a 90% circumferential coverage ratio exhibited the highest SMI at 20.22%. Internal flow field analysis showed that the self-circulating casing treatment improved the compressor stability by sucking the low-speed flow in the blade tip passage and restraining the leakage vortexes breaking, which caused flow blockage. The compressor performance was improved at most of the operating points, and the improvement increased with increase in circumferential coverage ratio. The improvement in compressor performance was mainly attributed to reduction in the area of the high relative total pressure loss in the blade tip passage and significant decrease in the flow loss by the self-circulating casings.

Quantitative investigation of the turbulence model effect on high-pressure-ratio centrifugal compressor performance prediction

The accuracy of centrifugal compressors performance and flow field prediction by Computational Fluid Dynamics (CFD) is a serious problem facing in the design and analysis process due to its complex flow feature inside. Different turbulence models choosing in the Reynolds-Averaged Navier-Stokes Equations (RANS) leads to different simulation results. In this paper, a high-pressure-ratio centrifugal compressor was used as the simulation object to discuss the effect of common turbulence models, including a one-equation model (Spalart-Allmaras model), a two-equation linear model (Shear Stress Transport model) and a two-equation nonlinear model (Explicit Algebraic RSM model), on performance prediction under design speed. By introducing the calculation method of local entropy generation rate, an improved method of dividing different types of loss in centrifugal compressors was proposed. After comparing with the experiment results, the effect of turbulence models on loss prediction inside the compressor was quantitative investigated. Further, the flow mechanism underlying was discussed. The results show that the secondary flow loss have the largest impact on the components' prediction performance, also, the reasons for different magnitudes of secondary flow loss inside the impeller and the diffuser are different. These comparisons hope to help designers choose an appropriate turbulence model in further compressor design process and may benefit future turbulence modelling development.

Multi-Point Surrogate-Based Approach for Assessing Impacts of Geometric Variations on Centrifugal Compressor Performance

The increasing demand for robust and high-performance centrifugal compressor stages has led to the development of several optimization and uncertainty quantification approaches. However, in the industrial scenario, geometric variations of such pre-engineered stages can occur during customer orders or non-conformity evaluations. In this regard, a rapid low-effort quantification of the impact of these changes has become critical for manufacturers. Against this backdrop, the present study provides an approach based on the joint use of computational fluid dynamics (CFDs) and artificial neural networks to instantly assess the impact of geometric variations on the aerodynamic performance and operating range of centrifugal compressor stages. As a theoretical contribution, the research investigates the capacity of a CFD-based surrogate approach for evaluating variations of stage efficiency and work coefficient. On a practical level, a business-friendly tool for stage performance assessment is provided. As an example case study, the approach is applied to a group of stages for medium–high Mach number applications. Results show how the multi-point surrogate approach enables a rapid quantification of stage performance changes without requiring additional CFD analyses. The research lays the foundation for future studies aiming to reduce efforts when assessing geometric variation impacts on centrifugal compressor stages.

Effect of bend-induced inlet distortion on a centrifugal compressor performance and stall inception

Bend pipes are usually installed upstream of a centrifugal compressor because of spatial constraints. With the interaction between radial pressure gradient and centrifugal force in bend pipe, a pair of counter-rotating vortices (Dean vortices) is formed after the fluids passed through the bend, and the shape of Dean vortices keeps changing in the straight pipe between the bend and compressor. The effects of the changed Dean vortices on the aerodynamic performance and stall inception of a centrifugal compressor are investigated with an experimentally validated numerical method based on four models, in which the distance of the straight pipe located between the bend and the compressor varies from 0D to 1D through 1/4D and 1/2D. The results show that as the straight pipe length increases from 0D to 1D, both the total pressure deficit at compressor inlet and the flow separations at inlet pipe symmetry plane are enlarged, and the mass flow rate distribution of full-annulus become more fluctuant. As a consequence, the compressor aerodynamic performance degrades and its maximum flow capacity decreases. More importantly, bend-induced inlet distortion has the effect of inducing/suppressing impeller stall inception, it depends on the location and intensity of swirl distortion and total pressure distortion. The low total pressure region located at impeller tip leads to reversed flow and stall inception, while the positive pre-swirl component in swirl distortion decreases the incidence and suppresses stall inception. These results provide a reference for the design of upstream inlet curved pipes of centrifugal compressors.

Mechanism Affecting the Performance and Stability of a Centrifugal Impeller by Changing Bleeding Positions of Self-Recirculating Casing Treatment

This study aimed to investigate the influence of the bleeding position of a self-circulating casing on the aerodynamic performance of a transonic centrifugal compressor. Three types of self-circulating structures with the bleeding positions of 11% Ca (the axial chord length of the blade tip), 14% Ca and 20% Ca from the leading edge of the blade were studied by using the numerical simulation method, with the Krain impeller taken as the research object. It was found that all three types of self-recirculating casing treatments can expand the stable operating range of the impeller, and that at medium and small flow rates, the total pressure ratio and efficiency of the impeller increase gradually with the backward movement of the bleeding position. The self-circulating casing treatment can restrain the development of tip leakage vortex, reduce the blockage area, and improve the stability of the impeller by sucking low-energy fluid. The farther back the bleeding position is, the greater the bleeding mass flow rate of the self-circulating casing for the low-energy fluid in the blade-tip passage becomes. Additionally, a greater inhibition effect on the tip leakage vortex, and a better effect of improving the performance and stability of the impeller, can be obtained. The best air bleeding position is 20% Ca, but it is not directly above the blade-tip blockage center of the solid wall casing passage. Instead, it is downstream of the blockage area.

Influence of Diffuser Vane Thickness on Centrifugal Compressor Performance

The structure of diffuser vanes has a significant influence on the performance of centrifugal compressors. To research the influence of diffuser vane thickness on the overall performance and internal flow condition of the centrifugal compressor. This paper takes a centrifugal compressor as the research object. By changing the diffuser vane thickness, and using three-dimensional fluid simulation technology to carry out simulation research. The results show that the thinner diffuser vanes can greatly improve the efficiency, flow rate and outlet temperature of the centrifugal compressor. For internal local fluid characteristics, thin diffuser blades have lower internal heat load. However, the local flow of fluid is not consistent with the change of vane thickness.

Effect of Interstage Pipeline on the Performance of Two-Stage Centrifugal Compressors for Automotive Hydrogen Fuel Cells

At present, two-stage centrifugal compressors based on air bearing and high-speed motor technology are widely used in automotive hydrogen fuel cells. The low-pressure stage and high-pressure stage of the compressor are directly connected through an interstage pipeline, thus the structure of the interstage pipeline has an important influence on the aerodynamic performance of the compressor. In this work, a two-stage compressor with three different interstage pipelines were investigated experimentally and numerically. Results show that affected by the interstage pipeline bend section, the flow distortion will be induced at the impeller inlet of the high-pressure stage, and the distortion intensity changes with the pipeline structure. Among the three models, the EPC (elbow pipe change) model induces the most intense total pressure distortion at the condition of 80 kr/min and 130% mass flow rate, resulting in an efficiency reduction of 2 and 1.5 percentage points compared with the SPC (straight pipe change) and the TPC (total pipe change) model, respectively. Further research indicates that the upstream distortion has obvious influence on the downstream rotor blades. As the blade height decreases, the load becomes more uniform on the main blades, while the load extremum migrates to the trailing edge on the splitter blades. Finally, three models are tested, and their performance is compared at three typical rotational speeds., It is recommended that interstage pipelines similar to SPC models should be chosen to improve the two-stage compressor efficiency in the design.

On Choosing the Optimal Impeller Exit Velocity Triangles in Preliminary Design

Abstract Impeller discharge flow plays an important role in centrifugal compressor performance and operability for two reasons. First, it determines the work factor and relative diffusion for the impeller. Second, it sets the flow into the downstream stationary diffusion system. The choice made in the preliminary design phase for the impeller exit velocity triangle is crucial for a successful design. The state-of-the-art design approach for determining the impeller exit velocity triangle in the preliminary design phase relies on several empirical guidelines, i.e., maximum work factor and diffusion ratio for an impeller, the optimal range of absolute flow angle, etc. However, as modern compressors continue pushing toward higher efficiency and higher work factor, this design approach falls short in providing exact guidance for choosing an optimal impeller exit velocity triangles due to its empirical nature as well as the competing mechanism of the two trends. In light of this challenge, this paper introduces a reduced-dimension, deterministic approach for the design of the impeller exit velocity triangle. The method gauges the design of the impeller exit velocity triangle from a different design philosophy using a relative diffusion effectiveness parameter and is validated using six impeller designs, representative of applications in both turbochargers and aero engines. Furthermore, with the deterministic method in place, optimal impeller exit velocity triangles are explored over a broad design space, and a one-to-one mapping from a selection of impeller total-to-total pressure ratios and backsweep angles to a unique optimal impeller exit velocity triangle is provided. This new approach is demonstrated, and discussions regarding the influences of impeller total-to-total pressure ratio, isentropic efficiency, and backsweep angle on the optimal impeller exit velocity triangle are presented.

Effects of leading edge profiles on flow behavior and performance of supercritical CO2 centrifugal compressor

The supercritical carbon dioxide (sCO2) Brayton cycle is considered one of the promising power cycles. Centrifugal compressors are the core equipment of sCO2 systems. The inlet condition of compressors is near the critical point of CO2, which makes the flow in compressors become more complex and has the risk of condensation. To improve the flow behavior at the leading edge (LE) of the blade, a method of correcting the LE profiles (long-short axis ratio) of the blade was proposed. The cause of condensation at the LE of the blade is analyzed by numerical simulation and the effects of the LE profiles on the flow behavior in the impeller passage and the performance of the compressor. The results show that the suction peak on the LE suction side is changed under different working conditions. With the increase of the long-short axis ratio of the LE of blades, the suction peak at the LE is weakened. Under the design condition, the volume of the two-phase flow in the impeller passage with elliptical LE is 20% lower than that with arc LE. When the long-short axis ratio of the LE is 3∼6, the volume of the low-dryness flow decreases with the increase of the long-short axis ratio under the condition of the small flow rate. Under the design condition, the isentropic efficiency of the compressor with the largest long-short axis ratio of the LE increases by 1.411%, and the pressure ratio increases by 0.0353.