Multi-objective parameter optimization of centrifugal compressor impeller with Kriging model

Abstract: The high-speed centrifugal compressor used in the air supply system is the major noise source of the fuel cell vehicle. Therefore, it is important for the compressor to achieve low noise level as well as high compression ratio and efficiency. This paper presents an optimal design method for the centrifugal compressors using numerical simulation, Kriging model and genetic algorithm at the operating point. The rotational speed at the operation point is 80 000 r/min, the mass flow rate is 0.08 kg/s, and the compression ratio is 1.65. The steady RANS simulations are preliminarily used to provide the performance maps as well as the consistent initial conditions for the subsequent unsteady simulations. Performance maps are compared between numerical and experimental results at 40 000 and 50 000 r/min, which show a good agreement. Next, the unsteady simulations are performed to calculate the sound power level of the compressor. In order to analyze the influences of the blade inlet angle, blade outlet angle, trailing edge angle, tip clearance and blade thickness on the compression ratio, isentropic efficiency and sound power level, the optimal Latin square design is adopted to create the sample space. Each one of the sample points is simulated with the presented numerical method. The results show that the tip clearance and blade thickness are 2 primary factors. The compression ratio and efficiency decline when the tip clearance and blade thickness decrease, while the sound power level rises. The front incline is found to be better than the back incline. The Kriging model is built to reflect the functional relationship between the impeller design parameters and the performance parameters. Then, the multi-objective optimization is conducted with the genetic algorithm based on the Kriging model instead of the numerical model. The errors of the compression ratio, isentropic efficiency and sound power level between the Kriging model and the numerical model at optimized point are 0.11%, 0.46% and 0.01%, respectively. The blade inlet angle, blade outlet angle, trailing edge inclined angle, tip clearance and blade thickness of the baseline design are 37°, 45°, 26.7°, 0.3 mm and 1.2 mm, and the optimized design are 35.226°, 50.863°, -2.465°, 0.365 mm and 0.611 mm, respectively. Compared with the initial design, the compression ratio and isentropic efficiency of the optimal design are increased by 3.56% and 1.02%, respectively and the sound power level is decreased by 3.79 dB. The sound pressure spectrums show that the noise at blade passing frequency decreases by 16 dB. The rotational frequency and its noise at the harmonic frequency as well as broadband noise at the 0-1 800 Hz and 10 000-16 000 Hz also decrease. The compression ratio and isentropic efficiency of the centrifugal compressor are also improved at the off-design points. Internal flow fields are analyzed to find out the mechanism of the improvements. The results show that the velocity distribution is more uniform and the secondary flows in the blade flow channel significantly decrease after optimization, which means that the mixing loss at the impeller outlet decreases. This research provides a reference for optimizing the acoustic behavior as well as the performance parameters of centrifugal compressors at the early design stage.