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Vaned diffusers are widely used in centrifugal compressors due to their higher pressure-recovery coefficients compared to vaneless diffusers. In this study, the effects of the diffuser vanes' wrap angle and number of vanes on the aerodynamic performance of an industrial Methane centrifugal compressor with a pressure-ratio of 1.288 are studied using high fidelity steady-RANS numerical simulations. Three wrap angles (WA = 19.3o, 22.3o and 25.3°) and three number of vanes (NDiff. = 16, 20 and 24) are examined, while all the other geometrical and operational parameters are kept constant. Results showed that decreasing the wrap angle can enhance the choke flow rate of the compressor, with slight reduction in pressure ratio at low flow rates. However, increasing the diffuser wrap angle, intensifies the flow separation over the diffuser vanes. On average, the best aerodynamic performance of the compressor occurred at WA=22.3°. Results also showed that reducing the number of diffuser vanes enlarges the operating range of the compressor, however, the pressure ratio will be lower at the flow rates less than the design point. Conversely, higher pressure ratios will be achieved at the flow rates greater than the design flow rate. The optimal aerodynamic efficiency of the diffuser, considering both the pressure ratio and the total-to-total efficiency, was achieved when the NDiff value was set to 20. [ABSTRACT FROM AUTHOR]

In this study, the effects of solidity and rotor speed ratio on the performance of a contra-rotating fan are investigated numerically. Simulation results have been validated by experimental tests. The test stand construction and the measurements have been performed according to ISO-5801 standard. In order to investigate the effect of blade solidity, simulations have been conducted with constant blade count of the front rotor and four configurations of the rear rotor (having 6, 8, 12 and 16 blades), and also constant blade count of the rear rotor and three configurations of the front rotor (having 8, 12, and 16 blades). Results suggest that there is an optimum solidity for the rear rotor, which gives maximum pressure rise and efficiency. Furthermore, the effects of the rotor speed ratios have been investigated in this study. It is shown that the speed of the front rotor is more effective on the fan performance, as compared to the rear rotor. Results reveal that the performance drop caused by a 25% reduction in the rear rotor blade count, can be compensated by 5% increase of the front rotor rotational speed. Therefore, a suitable choice of the speed ratio can result in light-weight and high-performance contra-rotating stages. [ABSTRACT FROM AUTHOR]

Variable inlet guide vanes (VIGVs) are widely used for flow throttling and also extending the operating range of centrifugal compressors. Although there are several studies on the effects of adding IGVs on the performance curve of the compressors, none of them have focused on the number of vanes. In the current study, high-fidelity three-dimensional numerical simulations are carried out to analyze the effects of adding VIGVs with different number of vanes on the aerodynamic performance of a single-stage centrifugal compressor. The selected compressor prototype is a high flowrate single-stage compressor equipped with a vaned diffuser, designed and fabricated by Siemens. Computational fluid dynamic simulations are performed for three different number of guide vanes at three IGV inclination angles of 0, −30 and +45 degrees. The numerical results are validated by comparing the pressure-rise curves with the available experimental data of the compressor data sheet, where a good agreement was achieved. Results show that at the fully-open condition, the number of vanes does not have considerable effect on the performance curve of the compressor. However, as the IGV inclination angle increases, the number of inlet vanes plays a considerable role in the compressor efficiency. For example, at IGV inclination angle of +45 degree, increasing the number of vanes from 7 to 11 can increase the compressor maximum efficiency up to 5 points. Numerical results showed that increasing the number of inlet guide vanes imposes a higher pressure drop in the inlet passage of the compressor while generating a more uniform velocity distribution at the suction surface of the impeller. Due to the existence of several counteracting effects, an optimum number of inlet guide vanes can be found. [ABSTRACT FROM AUTHOR]

The University of Ferrara YAP-(S)PET scanner is currently being employed in small animal SPECT studies, with [sup.99 m]Tc labeled radiotracers, with the goal of obtaining quantitative activity measurements from region of interest (ROI) analysis of reconstructed images of rats. The measurements will be compared directly with traditional ex vivo measurement of activity with gamma counters. To achieve this goal, the scanner was calibrated relative to a standard Isodose dose calibrator which was calibrated with various certified activity sources. The calibration factor K was defined as the ratio of the activity measured with the Isodose (MBq) and the image count rate (cps). We find K = 0.1346 [+ or -] 0.0008 MBq/cps, with good linearity over a wide range of activities (9-88 MBq). With the calibrated scanner we compared results of activity measurements from images of whole-rat heart acquisitions versus excised hearts. We found good agreement between the two measurements. Index Terms--Calibration, cardiac radiotracers, image processing, quantitation, single photon emission computed tomography (SPECT), small animal imaging, tomography.