Abstract:Centrifugal compressors have reached advanced stages of their development, and it is only through a detailed understanding of their complex airflows that improvements in the overall performance will be achieved. This article presents a numerical investigation of unsteady flows through the different components of a radial compressor, emphasizing on the impellervaneless-diffuser-scroll interactions. A transient rotor-stator simulation model was used for the calculations carried out by means of the code CFX-TASCf… Show more
“…The computational domain as depicted by Figure 7 encompasses four sub-domains: an intake, an impeller (one passage), a pinched vaneless-diffuser and a scroll and a cavity, which connects the throat slot with the intake bell-mouth. The grids used in the basic radial compressor 11 are retained in this study. Figure 7.Computational domain of the radial compressor with CT.…”
Section: Steady Flow Simulations Of the Radial Compressor With Ctmentioning
confidence: 99%
“…This article is a continuity of a previous work carried out by Khalfallah and Ghenaiet 11 to investigate steady and unsteady aerothermodynamic aspects of a radial compressor Schwitzer used in heavy truck engines. This study concerns the flow stability and the blockage assessments for the same model of a radial compressor when involving different designs of CT.…”
Centrifugal compressors used for turbochargers are required to achieve high performance within wide flow ranges. However, their operability is limited at low mass flow rates by the flow instabilities leading to rotating stall and surge, which could be delayed by a suitable casing treatment. This article presents a numerical investigation aimed at studying the effects of a type of casing treatment on the flow structure through a radial compressor and the subsequent improvement of aerodynamic stability. The considered casing treatment consists of an oblique slot recess inserted in the shroud near the impeller throat and a large annular cavity connecting this slot to the bell-mouth. The flow computations were carried out by means of the code ANSYS-CFX, considering the shear stress transport (k–ω) turbulence model, in order to investigate the effects of the cavity dimensions on improving the stability limit of the compressor. The analysis of flow structure near the stall limit has revealed that the large vortex appearing near the blade tip is being controlled by such a type of casing treatment; hence the stall onset is delayed. As consequence, the operating range and the stall margin are improved by reducing the aerodynamic blockage.
“…The computational domain as depicted by Figure 7 encompasses four sub-domains: an intake, an impeller (one passage), a pinched vaneless-diffuser and a scroll and a cavity, which connects the throat slot with the intake bell-mouth. The grids used in the basic radial compressor 11 are retained in this study. Figure 7.Computational domain of the radial compressor with CT.…”
Section: Steady Flow Simulations Of the Radial Compressor With Ctmentioning
confidence: 99%
“…This article is a continuity of a previous work carried out by Khalfallah and Ghenaiet 11 to investigate steady and unsteady aerothermodynamic aspects of a radial compressor Schwitzer used in heavy truck engines. This study concerns the flow stability and the blockage assessments for the same model of a radial compressor when involving different designs of CT.…”
Centrifugal compressors used for turbochargers are required to achieve high performance within wide flow ranges. However, their operability is limited at low mass flow rates by the flow instabilities leading to rotating stall and surge, which could be delayed by a suitable casing treatment. This article presents a numerical investigation aimed at studying the effects of a type of casing treatment on the flow structure through a radial compressor and the subsequent improvement of aerodynamic stability. The considered casing treatment consists of an oblique slot recess inserted in the shroud near the impeller throat and a large annular cavity connecting this slot to the bell-mouth. The flow computations were carried out by means of the code ANSYS-CFX, considering the shear stress transport (k–ω) turbulence model, in order to investigate the effects of the cavity dimensions on improving the stability limit of the compressor. The analysis of flow structure near the stall limit has revealed that the large vortex appearing near the blade tip is being controlled by such a type of casing treatment; hence the stall onset is delayed. As consequence, the operating range and the stall margin are improved by reducing the aerodynamic blockage.
“…Notably, the time-averaged value and the amplitude of the pressure fluctuation differ among the diffuser vanes. 7,14 Thus, the time-averaged pressure and the fluctuating amplitude of unsteady pressure on all the diffuser vanes are investigated to evaluate the circumferential asymmetric flow in the diffuser caused by the influence of the volute.…”
Section: Resultsmentioning
confidence: 99%
“…Therefore, the harmonics with 0.5 f b and f b as the dominant harmonics can show the main unsteady characteristic. 7 Their amplitude in the circumferential direction is analyzed to investigate the volute influence on the unsteady loading and pressure on the vanes. Meanwhile, the transient vorticity fields of the diffuser under near-stall condition are also applied to deliver more insights into the unsteady characteristics and to evaluate the performance of flow passages.…”
Section: Resultsmentioning
confidence: 99%
“…The range of distorted static pressure in the circumferential direction in a vaneless diffuser varied slightly with rotational speed, and the range of distorted static pressure in an impeller was a function of the rotational speed. Khalfallah and Ghenaiet 7 found that the unsteady numerical simulations of a centrifugal compressor provided good insight into the interaction among an impeller (six main and six splitter blades), a vaneless diffuser, and a volute. The pressure in different regions, such as in the impeller/vaneless diffuser interface, the vaneless diffuser/volute interface, and the area near the tongue, was described via time and spatial Fourier decomposition under three different mass flow conditions.…”
A volute is the only circumferential asymmetric component in a centrifugal compressor, and thus, it should account for the circumferential asymmetry of the flow in a vane diffuser. This study performs a transient numerical analysis to investigate the effect of a volute on the flow in the vane diffuser of a centrifugal compressor under three operating conditions (near-stall, middle, and high mass flow). We compare numerical and experimental performance of the compressor, including polytropic efficiency, total pressure ratio, and unsteady pressure on a diffuser vane. The numerical scheme is proven valid owing to the fact that the numerical and experimental results considerably agree well with each other. Under middle and high mass flow conditions, the time-averaged static pressure recovery and the total pressure loss coefficients for all the diffuser passages indicate that the performance of the passages near and upstream of the volute tongue is affected negatively by the volute, whereas that of the passages downstream of the volute tongue is less affected. Under near-stall condition, the performance of all the passages is disturbed, and the diffuser passage marked as DP 3 demonstrates the worst performance. Investigation on the time-averaged aerodynamic forces, loading, and pressure on the vanes yields results that are consistent with those of the investigation on the performance of the passages. The harmonics with 0.5f b and f b , which are included in the unsteady loading and pressure on the pressure and suction sides of the vanes, are dominant, where f b is the impeller main and splitter blades passing frequency. Their amplitude values increase as mass flow deviates from the middle mass flow condition. Under middle and high mass flow conditions, the harmonic with 0.5f b is affected more negatively because of the larger amplitude on the vanes near and upstream of the volute tongue than those downstream, whereas the harmonic with f b is less affected by the volute. Under the nearstall condition, the transient vorticity fields along with the harmonics of 0.5f b and f b are investigated to evaluate the performance of the diffuser passages. DP 3, which is located at approximately 90 downstream of the volute tongue, suffers the strongest flow deterioration and is inferred to stall first. Further researches for designing more matching diffuser/volute combination will be performed by referring this study.
The nonuniformity of the flow field induced by a nonaxisymmetric volute significantly degrades the stability of a turbocharger centrifugal compressor. In this paper, a nonaxisymmetric vaned diffuser is investigated as a nonaxisymmetric flow control method using both three-dimensional computational fluid dynamics (CFD) and experiment. The numerical study first focuses on the relationship between the flow field and the static pressure distortion, and the steady CFD results indicate that the positive static pressure gradient in the rotating direction facilitates flow separation in the vaned diffuser and induces a nonuniform flow field. A nonaxisymmetric flow control method with variable stagger and solidity of the vaned diffuser is developed to suppress the flow separation, and the guideline of the method suggests narrowing flow passages where the flow separates or closing diffuser vanes upstream of flow separations. Steady CFD also presents the flow field of the investigated turbocharger centrifugal compressor with volute, and flow separation is found in the flow passages near the volute tongue. Under the guidance of the nonaxisymmetric flow control method, several nonaxisymmetric vaned diffusers are designed to make the flow field uniform, which are believed to be beneficial for compressor stability. Finally, an experiment is carried out to validate the positive effects of the nonaxisymmetric vaned diffuser for stability improvement. The test data show that Non-AxisVD (with a nonaxisymmetric vaned diffuser) extends the stable flow range (SFR) of the compressor by 26% compared with the AxisVD (with an axisymmetric vaned diffuser), at the cost of acceptable decreases in the maximum total pressure ratio and peak efficiency.
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