Compressor Flow Field Research Articles

Numerical investigation of the axial compressor performance with inlet distortions

In this paper, a three-dimensional body force model for predicting compressor performance and stability is implemented in the Ansys CFX. The influence of the blade rows on the flow field is represented by the source terms of CFX-solver equation. At first, a high-speed and high-pressure-ratio transonic compressor with the clean inlet is investigated. The overall performance and the flow fields are in agreement well with those of the experimental date, so the model is reliable and correct. Then, the effects of the circumferential distortions in the inlet total pressure and the total temperature on the compressor performance and flow field are also illustrated, respectively. In summary, the proposed body force model is suitable to investigate the flow field of the compressor with the inlet distortions.

Investigation on Unsteady Flow Characteristics of a SCO2 Centrifugal Compressor

Supercritical carbon dioxide (SCO2) is a vital working fluid in the application of power units and its high density helps to achieve a compact mechanical structure. Centrifugal compressors are of vital use in various kinds of equipment. In this paper, a SCO2 centrifugal compressor of large input power and mass flow rate is designed and numerically investigated. A thorough numerical analysis of the unsteady flow field in the centrifugal compressor is performed in ANSYS-CFX. The computation adopts hexahedral mesh, finite volume method, and the RNG k-ε two-equation turbulence model. Streamlines, temperature, pressure, and Mach number distributions at different time steps in one revolution period are covered to present the unsteady effect of turbomachinery. Meanwhile, the force on a single rotor blade is monitored to investigate the frequency components of exciting force, thus providing the foundation for vibration analysis. Moreover, the torque, output power, pressure ratio, and isentropic efficiency in the steady and the unsteady time-averaged condition are calculated and compared with the design condition to measure the validity of the design. In summary, the unsteady computation result reveals that the unsteady flow characteristics are prominent in the designed compressor and the design of impeller and diffuser meet the requirement.

Effect of splitter leading edge location on performance of an automotive turbocharger compressor

Design of centrifugal compressors can be accomplished using methods provided in text books and open literature. In this general approach, after determination of dimensions in meridional plane, blade profiles are designed using aerodynamic methods. Then, some blades are trimmed at the beginning to avoid inlet blockage. But there is no clear guidance on how this trimming must be performed. With this widely implemented method, splitter blades have the same profile as main ones.In this research, blade profiles and dimensions of the compressor impeller are not changed while the effect of changes in the position and angle of splitters leading edge are investigated. An optimization scheme is performed to find the best configuration over the complete operating curve of the compressor using genetic algorithm and considering structural aspect of the problem. CFD codes with experimental support are used to predict the compressor performance and flow field specifications with the original impeller as well as the modified one. After taking into account some geometrical details, a new impeller is manufactured based on the modified design and tested in a wide range of operating conditions. Results show 2.7 points improvement in efficiency at the nominal point and 5% decrease in impeller moment inertia. No change in choke behavior, but notable improvement in surge margin of the modified compressor is observed. This is an important achievement in all applications of centrifugal compressors, especially in turbochargers.

Investigation of Performance and Rotor Tip Flow Field in a Low Speed Research Compressor with Circumferential Groove Casing Treatment at Varying Tip Clearance

Experimental investigations in a single-stage low speed axial research compressor are presented. The influence of four different rotor tip clearances on the overall compressor performance and on the rotor tip flow field is investigated in configurations with and without circumferential groove casing treatments. Piezo-resistive pressure transducers are used to capture the unsteady flow field in the rotor tip region. The investigated casing groove is effectively working at the three largest investigated tip clearance sizes. The largest achieved operating range increase by the groove is 6.9%. The groove can delay the upstream movement of the flow interface between leakage flow and main flow and thus increase the stable operating range. Rotating instabilities are shown to exist at large tip clearance sizes in throttled operating conditions. Their amplitudes can be damped by the casing groove. No modal activities could be detected in the current single-stage compressor build.

Experimental investigation of flow field in a laboratory-scale compressor

The inner flow environment of turbomachinery presents strong three-dimensional, rotational, and unsteady characteristics. Consequently, a deep understanding of these flow phenomena will be the prerequisite to establish a state-of-the-art design system of turbomachinery. Currently the development of more accurate turbulence models and CFD tools is in urgent need for a high-quality database for validation, especially the advanced CFD tools, such as large eddy simulation (LES). Under this circumstance, this paper presents a detailed experimental investigation on the 3D unsteady flow field inside a laboratory-scale isolated-rotor with multiple advanced measurement techniques, including traditional aerodynamic probes, hotwire probes, unsteady endwall static pressure measurement, and stereo particle image velocimetry (SPIV). The inlet boundary layer profile is measured with both hotwire probe and aerodynamic probe. The steady and unsteady flow fields at the outlet of the rotor are measured with a mini five-hole probe and a single-slanted hotwire probe. The instantaneous flow field in the rotor tip region inside the passage is captured with SPIV, and then a statistical analysis of the spatial distribution of the instantaneous tip leakage vortex/flow is performed to understand its dynamic characteristics. Besides these, the uncertainty analysis of each measurement technique is described. This database is quite sufficient to validate the advanced numerical simulation with LES. The identification process of the tip leakage vortex core in the instantaneous frames obtained from SPIV is performed deliberately. It is concluded that the ensemble-averaged flow field could not represent the tip leakage vortex strength and the trajectory trace. The development of the tip leakage vortex could be clearly cataloged into three phases according to their statistical spatial distribution. The streamwise velocity loss induced by the tip leakage flow increases until the splitting process is weak and the turbulent mixing phase is dominant.

On the aero acoustic and internal flows structure in a centrifugal compressor with hub side cavity operating at off design condition

This paper covers the characterization of the acoustic noise and the unsteady flow field of a high speed centrifugal compressor NASA CC3. In order to accurately predict the noise, all analyses are carried out through the use of Large Eddy Simulation and Ffowcs Williams–Hawkings model for noise prediction. The relative effect of hub cavity on flow characteristics and sound levels is investigated, for a compressor stage with a total pressure ratio equal to 4, working from surge to near choke condition. In comparison with the experimental results from literature, the predicted compressor performance and flow field are predicted well, some trends seen in experiments are captured. The hub cavity flow effect on the compressor aero acoustic generated noise is shown in the paper. The unsteady static pressure and sound pressure levels are compared not only at different location but also for design and off design operating points. The internal flow results inside the hub cavity are presented at surge, design and near choke points. The conclusion is that the cavity effect of the centrifugal compressor cannot be ignored in the numerical prediction of aerodynamic generated noise. The impeller back plate of the rotor experiences a strong pressure fluctuation, which is maxima at the impeller outer radius for all operating point, but higher pressure values at the surge point.

Improvement in the stability of a turbocharger centrifugal compressor by tip leakage control

The occurrence of surge or stall in a centrifugal compressor and the role of the tip clearance flow in the instability in the centrifugal compressor are investigated in this study. A computational method is used to study the flow field in the centrifugal compressor in order to gain a better understanding of the surge or stall mechanism. It is found that, near surge or stall conditions, the tip leakage flow at the leading edge deflects more upstream; as the deflection increases, a more severe spillage occurs which finally leads to instability of the compressor. A ring air jet injection is used to eliminate the instabilities and to extend the stable flow range of the compressor. Using an air jet injection, the stable flow range of the compressor was successfully increased with minimal decrease in the efficiency of the compressor. The effects of different injection parameters such as the mass flow, the yaw angle, the injection angle, the slot width and the slot distance on the compressor flow field are studied, and an optimum design for the air jet injection is developed. Further investigation of the compressor with the optimum injection configuration shows that, near surge or stall conditions, the tip leakage at the leading edge is still under control, manifesting a much smaller spillage than does the Dresser–Rand Datum compressor without an air injection. The dominant factor for the instability of the compressor with an injection is found to be the leading-edge separation rather than the tip leakage.

Multi-field coupling dynamic characteristics based on Kriging interpolation method

Multi-field coupling problems are taken more and more attention mainly because of the higher requirement of load, efficiency, and reliability in aero-engine operation. This research takes an aero-engine compressor as the research object, 3D flow field and structural models are established. For the method of cyclic symmetric, single-sector model is selected as the calculation domain. Considering the influence of former stator wakes, compressor flow field is simulated. The article analyzes the distribution law of unsteady aerodynamic load on rotor blade. Based on Kriging model, load transfer of aerodynamic pressure and temperature is achieved from flow field to blade structure. Then the effects of centrifugal force, aerodynamic pressure and temperature load are discussed on compressor vibration characteristic and structural strength. The results show dominant fluctuation frequencies of aerodynamic load on rotor blade are manly at frequency doubling of stator–rotor interaction, especially at one time frequency (1 × f0). Magnitude and pulsation amplitude on pressure surface are far greater than that on suction surface. Load transfer with Kriging model has a higher precision, it can meet the requirement of multi-field coupling dynamic calculation. In multi-field coupling interaction, temperature load makes the natural vibration frequencies decrease obviously, centrifugal force is the main source of deformation and stress. Bending stress induced by aerodynamic pressure and temperature load can counteract part of bending stress induced by centrifugal force. However, temperature load causes the maximum displacement of blade-disk system to increase.

遷音速多段軸流圧縮機における非定常流れ場の大規模DES解析

遷音速多段軸流圧縮機における非定常流れ場の大規模DES解析

Vane Clocking Effects on Stator Suction Side Boundary Layers in a Multistage Compressor

The stator inlet flow field in a multistage compressor varies in the pitchwise direction due to upstream vane wakes and how those wakes interact with the upstream rotor tip leakage flows. If successive vane rows have the same count, then vane clocking can be used to position the downstream vane in the optimum circumferential position for minimum vane loss. This paper explores vane clocking effects on the suction side vane boundary layer development by measuring the quasi-wall shear stress on the downstream vane at three spanwise locations. Comparisons between the boundary layer transition on Stator 1 and Stator 2 are made to emphasize the impact of rotor-rotor interactions which are not present for Stator 1 and yet contribute significantly to transition on Stator 2. Vane clocking can move the boundary layer transition in the path between the wakes by up to 24% of the suction side length at midspan by altering the influence of the Rotor 1 wakes in the 3/rev modulation from rotor-rotor interactions. The boundary layer near the vane hub and tip experiences earlier transition and separation due to interactions with the secondary flows along the shrouded endwalls. Flow visualization and Stator 2 wakes support the shear stress results.

Aeroacoustic Simulation for NASA CC3 Centrifugal Compressor Operating at off Design Condition

This paper covers the characterization of the acoustic noise and the unsteady flow field of a high speed centrifugal compressor NASA CC3. In order to accurately predict the noise, all analyses are carried out through the use of Large Eddy Simulation and Ffowcs Williams–Hawkings model for noise prediction. The relative effect of hub cavity on flow characteristics and sound levels is investigated, for a compressor stage with a total pressure ratio equal to 4, working from surge to near choke condition. In comparison with the experimental results from literature, the predicted compressor performance and flow field are predicted well. The hub cavity flow effect on the compressor aeroacoustic generated noise is shown in the paper. The unsteady static pressure and sound pressure levels are compared not only at different location but also for design and off design operating points. The internal flow results inside the hub cavity are presented at surge, design and near choke points. The conclusion is that the cavity effect of the centrifugal compressor cannot be ignored in the numerical prediction of aerodynamic generated noise. The impeller back plate of the rotor experiences a strong pressure fluctuation, which is maxima at the impeller outer radius for all operating point, but higher pressure values at the surge point.

A twin-screw rotor profile design and computational fluid dynamic simulation method

There are increasing demands for more efficient screw compressor design method which could overcome the problems of rotor profile modification and compressor performance test establishment. As a result, non-uniform rational B-spline curves and computational fluid dynamics theory were introduced as a new method of twin-screw compressor rotor profile design. In this paper, unilateral asymmetric combined cycloid with pin gear arc rotor profile was put as the research object to develop a new rotor profile. Non-uniform rational B-spline curves, which are easy to be modified, are applied to the new rotor profile design. The twin-screw compressor flow field simulation models of both the new and the original one are created. The pressure and velocity distribution law of the twin-screw compressor under the operating speed are explored with the FLUENT software. The simulation results provide the basis for further optimisation of the new rotor profile design.

Investigation of the unsteady flow of a transonic compressor using the nonlinear harmonic method

In this paper, the nonlinear harmonic method implemented in Fine/Turbo CFD code has been used to analyze the unsteady flow field in a transonic compressor. The numerical accuracy and computational efficiency of this method have been demonstrated by comparing its predictions with experimental data and conventional time-domain unsteady simulation for the test case. The rotor–stator interactions are studied in detail to improve the understanding of the flow physics involved. The deterministic stresses in the nonlinear harmonic method have also been compared to the time-domain unsteady postprocessing results to analyze distinct sources of unsteadiness. For the present case, the nonlinear harmonic method can not only capture the unsteady results satisfactorily, but also gain a significant computational advantage over conventional unsteady method.

Numerical investigation on the effect of a plenum chamber with slot-type casing treatment on the performance of an axial transonic compressor

Casing treatment is an effective approach for improving compressor stall margin in axial compressors. A plenum chamber, which connects slots in the circumferential direction, can occasionally be found in slot-type casing treatments. This study investigates the effect of a plenum chamber on the compressor overall performance and stall margin via time-accurate three-dimensional numerical simulations in a transonic compressor NASA Rotor 37 with a slot-type casing treatment. Four configurations with variable height of plenum chamber are numerically investigated. The unsteady influence of the casing treatment on compressor flow fields is studied using fast Fourier transforms to gain a deeper insight into the function of the plenum chamber. The results indicate that the plenum chamber can reduce the penalty on the compressor’s total pressure ratio and adiabatic efficiency induced by the casing treatment effect at the design operating condition. A detuning effect of the plenum chamber is found and proven to be responsible for the decreased compressor performance penalty through a detailed analysis in both the blade passage and casing treatments. The detuning effect dampens the unsteady influence of the casing treatment on the compressor passage flow in terms of shock oscillations and mass flow fluctuations. Thus, the radial scope and degree of the influence induced by the casing treatment is distinctly restrained by the plenum chamber. Slot circulation, which is composed of a bleed flow from the passage into the slots and injection flow from the slots into the passage, is closely related to the stall margin improvement as the casing treatment is implemented. The plenum chamber has no significant impact on the slot circulation and thus on compressor stall margin. However, shallow slots decrease the slot circulation efficiency, which leads to a much lower stall margin enhancement.

Experimental investigation of effects of suction-side squealer tip geometry on the flow field in a large-scale axial compressor using SPIV

It is well known that the non-uniform tip geometry is a promising passive flow control technique in turbomachinery. However, detailed investigation of its effects on the unsteady flow field of turbomachinery is rare in the existing literatures. This paper presents an experimental investigation of effects of suction side squealer tip configuration on both the steady and unsteady flow field of an isolated compressor rotor. The flow field at 10% chord downstream from the trailing edge was measured using a mini five-hole probe. Meanwhile, the unsteady flow field inside the passage was investigated using stereo particle image velocimetry (SPIV). The steady results show that the SSQ tip configuration exerts positive effect on the static pressure rise performance of this compressor, and the radial equilibrium at the rotor outlet is obviously rearranged. The SSQ tip configuration would create a stronger tip leakage vortex at the formation phase, and it experiences a faster dissipation process around the rear chord. Also, the splitting process of the tip leakage vortex is severer, which is the main cause of the relatively higher probability of the presence of the streamwise reverse flow. The quantitatively analysis of the tip leakage vortex indicates that the velocity loss inside the blockage region is direct response of the evolutionary procedure of the tip leakage vortex. It keeps increasing until the end of the splitting process. Although the blockage coefficient grows sustainably, the velocity loss will reduce once the turbulent mixing procedure is dominant.

Non-axisymmetric flow characteristics in centrifugal compressor

The flow field distribution in centrifugal compressor is significantly affected by the non-axisymmetric geometry structure of the volute. The experimental and numerical simulation methods were adopted in this work to study the compressor flow field distribution with different flow conditions. The results show that the pressure distribution in volute is characterized by the circumferential non-uniform phenomenon and the pressure fluctuation on the high static pressure zone propagates reversely to upstream, which results in the non-axisymmetric flow inside the compressor. The non-uniform level of pressure distribution in large flow condition is higher than that in small flow condition, its effect on the upstream flow field is also stronger. Additionally, the non-uniform circumferential pressure distribution in volute brings the non-axisymmetric flow at impeller outlet. In different flow conditions, the circumferential variation of the absolute flow angle at impeller outlet is also different. Meanwhile, the non-axisymmetric flow characteristics in internal impeller can be also reflected by the distribution of the mass flow. The high static pressure region of the volute corresponds to the decrease of mass flow in upstream blade channel, while the low static pressure zone of the volute corresponds to the increase of the mass flow. In small flow condition, the mass flow difference in the blade channel is bigger than that in the large flow condition.

Influence of tip clearance on flow behavior and noise generation of centrifugal compressors in near-surge conditions

CFD has become an essential tool for researchers to analyze centrifugal compressors. Tip leakage flow is usually considered one of the main mechanisms that dictate compressor flow field and stability. However, it is a common practice to rely on CAD tip clearance, even though the gap between blades and shroud changes when compressor is running. In this paper, sensitivity of centrifugal compressor flow field and noise prediction to tip clearance ratio is investigated. 3D CFD simulations are performed with three different tip clearance ratios in accordance to expected operating values, extracted from shaft motion measurements and FEM predictions of temperature and rotational deformation. Near-surge operating conditions are simulated with URANS and DES. DES shows superior performance for acoustic predictions. Cases with reduced tip clearance present higher pressure ratio and isentropic efficiency, but no significant changes in compressor acoustic signature are found when varying clearance. In this working point, tip clearance is immersed in a region of strongly swirling backflow. Therefore, tip leakage cannot establish any coherent noise source mechanism.

Computer Simulation for Transient Flow in Oil-free Scroll Compressor

In this paper, the numerical simulation of the two-dimensional and three-dimensional transient flow field in an oil-free scroll compressor is performed by the CFD dynamic mesh technique. During the procedure of simulation, the air in the scroll compressor is treated as ideal gas, and fit for the flow governing equations as well as gas state equation. In our approach, RNG k-e model is used for turbulent model. The simulation results show that the flow field in the scroll compressor is time-periodic, in addition, the generation, motion and size change of the vortices inside the scroll compressor can be visualized. By contrasting the two-dimensional and three-dimensional flow field, the advantages and disadvantages are clearly found in simulation. The simulation not only extremely vividly reveals the scroll compressor inherent laws but also provides theoretical references for the optimal design of scroll compressors.

Quasi-Steady Prediction of Coupled Bending-Torsion Flutter Under Classic Surge

In this paper, a quasi-steady method is developed for predicting the coupled bending-torsion flutter in a compressor cascade during classic surge. The classic surge is one of the major compressor flow field instabilities involving pulsation of the main flow through the compressor. The primary reason for the occurrence of the classic surge is the stalling of the blade rows and if the conditions are favorable this can trigger flutter, which is a self-excited aero elastic instability. The classic surge flow is modeled by using the well-established model of Moore and Greitzer and the obtained flow condition is used to determine the aerodynamic loads of the cascade using the linearized Whitehead's theory. The cascade stability is then examined by solving the two dimensional structural model by treating it as a complex eigenvalue problem. The structural stability is analyzed for a range of values of the frequency ratio and primary emphasis is given for the frequency ratio value of 0.9 as many interesting features could be revealed. The cascade shows a bifurcation from bending flutter to the torsional one signifying that only one of the flutter modes are favored at any instant in time. The torsional flutter is found to be the dominant flutter mode for a range of frequency ratios during classic surge whereas the bending flutter is found to occur only for some values of frequency ratio very close to unity as the torsional loads acting on the blades are found to be orders of magnitude higher than the bending loads. A rapid initiation of torsional flutter is seen to occur during classic surge for frequency ratio values very close to unity and it is perceived that during blade design, frequency ratios should be kept below 0.9 to prevent the flutter possibilities. An estimate of structural energy variation with time indicates that even if the total structural energy is negative one of the modes can go unstable during classic surge.

Computer Simulation of Oil-free Scroll Compressor Compression Process

In this paper, the numerical simulation of the three-dimensional transient flow field in a scroll compressor is performed by the CFD dynamic mesh technique. During the procedure of the simulation, the air in the scroll compressor is treated as ideal gas, and meets the flow governing equations and gas state equation. In our approach, RNG k-e model is used for turbulent model. The simulation results show that the flow field in the scroll compressor is time-periodic, in addition, the generation, motion and size change of the vortices inside the scroll compressor can be visualized. The opening and closing of the discharge port have significant influence on the flow loss. The simulation not only extremely vividly reveals the scroll compressor inherent laws but also provides theoretical references for the optimal design of scroll compressors.