Dazhong Lao

Effects of Blade Counts and Clocking on the Unsteady Profile Pressure Distribution in an Axial-Radial Combined Compressor

A new hypothesis is presented for the superimposed effects of the blade pressure distribution in a multistage compressor. The effects of the unsteady pressure fluctuations on the blade surface are separated into three groups. The influences of the upstream or downstream rotors can be obtained by numerical simulation for the R/S or S/R configuration; the data produced by all the influences can be obtained from the R/S/R configuration. The effects of the blade counts and clocking on the superimposed effects, acting on the profile pressure distribution, are studied using a special data analysis method that had been previously developed by the authors. The results indicate that the blade counts of the upstream and downstream rotors determine the periods of the unsteady pressure fluctuations on the stator surface. The clocking moving blade rows modulate the relative superimposed phases and interactions between two rotors such that the unsteady pressure fluctuates with different amplitudes on the surface of the stator blade.Copyright

A detailed investigation of a variable nozzle turbine with novel forepart rotation guide vane

One of the disadvantages of a variable nozzle turbine in practical application is the stage performance degradation due to nozzle endwall leakage flow at small nozzle openings. Aiming at restricting the nozzle leakage flow rate to improve turbine stage performance, a novel forepart rotation guide vane has been proposed and numerically studied in present work. First, the numerical results of baseline turbine were validated by experimental data to ensure the accuracy of numerical methods. Then steady and unsteady simulations were performed on both baseline and forepart rotation guide vane turbines to demonstrate the effectiveness of the novel vane and to study the characteristics of nozzle leakage flow, respectively. Results indicate that there is up to 13.5% peak efficiency improvement that has been achieved at 10% nozzle opening with the forepart rotation guide vane design; besides, rotor–stator interaction for forepart rotation guide vane is also mitigated due to the reduced nozzle leakage flow rate, thus the intensity of loading fluctuation on rotor blades is weakened significantly, which is beneficial to improve rotor blade forced response.

Casing wall static pressure distribution behavior in a centrifugal compressor with asymmetric inlet/outlet structures

Asymmetric structures of the bent inlet pipes and outlet volute are typically adopted in centrifugal compressors. By using asymmetric inlet/outlet structures, the uniformity of the compressor’s internal flow field in the circumferential direction will be changed. The static pressure distribution behavior around the casing wall is significantly influenced by the coupling effect of the bent inlet pipe and outlet volute. In the present work, three compressors were numerically and experimentally investigated. One compressor had a straight inlet pipe, and the other two had bent inlet pipes. Seventy-two static pressure sensors were mounted around the casing wall to obtain the static pressure distribution at different flow rates for three rotational speeds. The results show that at high rotational speeds with large flow rate conditions, when the static pressure waves induced by the bent pipe and volute act on the same circumferential position, the casing wall static pressure will be increased at the corresponding position. Furthermore, this high static pressure will further influence the static pressure values at other circumferential positions and leads to a more nonuniform circumferential static pressure distribution. Near the design flow rate, when the high static pressure strips, which are induced by both the bent pipe and volute impact different circumferential positions, the high static pressure strip induced by the volute will be weakened. As a result, the high static pressure strip induced by the volute cannot propagate upstream into the impeller. At small flow rate under designed rotational speed, the influence of the volute tongue on the casing pressure distribution will be enhanced. At small flow rate under low rotational speed, the casing pressure distributions of the three models were almost the same because the secondary flow effect of the bent pipe diminishes as the flow rate reduces.

Investigation of Detailed Flow in a Variable Turbine Nozzle

Abstract The detailed flow behavior of the nozzle channel of a variable turbine is presented in this paper. The numerical model of a variable nozzle turbine was developed by using computational fluid dynamics method, and validated by the measured performance data of the turbine. Two opening positions of the nozzle vane, as well as two inlet conditions of the nozzle representing different vane loadings, were investigated to evaluate the clearance flow behaviors. It is shown that the channel shock waves are produced at proper conditions, such as small opening and large inlet pressure, which has significant impact on the end wall clearance leakage flow. When the leakage flows through the end wall clearance from the pressure side to the suction side encountering by the main stream, and the leakage vortex is formed. It is found that this leakage vortex gradually enhanced from the trailing edge to the middle edge.

Aerodynamic Excitation and Forced Response of Centrifugal Compressor Impeller in Inlet Distortion

The forced response of the turbomachinery blade originates from unsteady fluid structure interactions due to the fluctuating aerodynamic load. As one of the primary unsteady issues, inlet distortion flow breaks the uniformity of airflow through the compressor channel which deteriorates the aerodynamic performance of the compressor and intensifies the pressure fluctuation on the blade surface. The work presented here arms to investigate the forced response of a centrifugal impeller induced by compressor inlet distortion. For this purpose, the unsteady flow computation was carried out to provide the temporal evolution of the distorted flow through the compressor and also to quantify the aerodynamic loads acting on the compressor blade due to inlet distortion flow compared with uniform inlet. Meanwhile, the experimental measurement was performed to obtain the transient pressure fluctuation on the blade surface and to validate the accuracy of numeric calculation. The forced response of the compressor impeller based on unsteady excitation was simulated in a finite element (FE) method to gain insight into vibration characteristic of each blade. Time-resolved blade pressure showed the drastic load fluctuation caused by distorted flow mainly located in the blade leading edge region due to the inlet airflow variation. Towards the impeller forced vibration, each blade shows individual response amplitude due to the phase angle difference among the blades. The effect of inlet distortion on the forced response of impeller increases significantly compared with that of the uniform inlet flow.Copyright

Investigation of Inlet Bent-Pipe’s Effect on a Turbocharger Compressor: Unsteadiness of Mass Glow and Location of Main Flow Loss

As inlet bent-pipes are often used in vehicle turbocharger compressors, understanding of the inlet bent-pipe’s effect on centrifugal compressor and the mechanism of flow loss is important to improve the turbocharger compressor performance. Experiment and numerical simulation were carried out on a centrifugal compressor with straight pipe and bent-pipe. Numerical simulated compressor performance was compared with test data obtained from compressor flow bench test and to validate the numerical method. The analysis is mainly based on numerical results and it indicated that the inlet bent pipe induces a serious distortion at impeller inlet and increases the risk of blade high-cycle fatigue. The level of distortion changes to be more serious as the operation point of compressor varies from design point to high load. When an impeller channel enters distortion, the larger pressure difference between inlet and outlet makes this channel get clogged and the flow rate through this channel is reduced. At design point the bent pipe mainly influences the flow loss in the inlet pipe and impeller while at high load more serious flow distortion induced by bent-pipe is able to extend its stronger effect to the downstream diffuser and volute.Copyright

Effect of Different Geometrical Inlet Pipes on a High Speed Centrifugal Compressor

Two different inlet configurations, including a straight pipe and a bent pipe, were experimentally tested and numerically simulated using a high-speed, low-mass flow centrifugal compressor. The pressure ratios of the compressor with the two inlet configurations were tested and then compared to illustrate the effect of the bent inlet pipe on the compressor. Furthermore, different circumferential positions of the bent inlet pipe relative to the volute are discussed for two purposes. One purpose is to describe the changes in the compressor performance that result from altering the circumferential position of the bent inlet pipe relative to the volute. This change in performance may be the so-called clocking effect, and its mechanism is different from the one in multistage turbomachinery. The other purpose is to investigate the unsteady flow for different matching states of the bent inlet pipe and volute. Thus, the frequency spectrum of unsteady pressure fluctuation was applied to analyze the aerodynamic response. Compared with the straight inlet pipe, the experimental results show that the pressure ratio is modulated and that the choke point is shifted in the bent inlet pipe. Similarly, the pressure ratio can be influenced by altering the circumferential position of the bent inlet pipe relative to the volute, which may have an effect on the unsteady pressure in the rotor section. Therefore, the magnitude of interest spectral frequency is significantly changed by clocking the bent inlet pipe.Copyright

Stator Wake Research in Axial-Radial Combined Compressor With Inlet Distortion

A combined compressor consisting of an axial rotor, a stator and a centrifugal rotor was used in a vehicle engine turbocharger because of its better performance compared with a single stage compressor under space restrictions. There have been many studies on the inlet distortion effect on multistage axial compressors; however, few studies have been performed for axial-radial combined compressors. To investigate stator wake characteristics, an axial-radial compressor was unsteadily simulated with three-dimensional Reynolds averaged Navier–Stokes equations by “domain-scaling” the rotor/stator interface method under uniform, circumferential distorted and combined distorted total pressure inlet conditions. In addition to the axial-radial configuration, the unsteady method of analysis was applied to a stand-alone axial stage by the same unsteady method. A comparison shows that at the vane stator outlet, a radial distortion is formed and different total pressure loss occurs at different spans. The circumferential interfaces between the distorted and non-distorted regions generate different flow characteristics due to different pitchwise pressure gradient directions and the absence of a downstream rotor. The stator wake causes more flow loss in the distorted case but less flow angle fluctuation compared to the uniform inlet condition. The downstream potential effect and inlet distortion creates strong pressure pulsation at the stator blade surface. A stator wake oscillation characteristics study shows disturbing effects on the stator wake from the upstream axial rotor, and the downstream radial rotor acts differently both spanwise and pitchwise.Copyright