Using Tip Gaps on Tandem Diffuser to Broaden the Operation Range of a Centrifugal Compressor

Gaps in compressors are generally unfavorable because they lead to energy loss. However, gaps on diffuser blades can broaden the operation range of a compressor. To broaden the operation range of a centrifugal compressor with a tandem diffuser, gaps with different sections and widths on the diffuser blades were examined to determine their influence on the compressor performance. The results show that the compressor's surge margin increased from 33.28% to 49.2% for a 2.5% tip gap ratio, while its stability margin increased from 44% to 60.5%. However, pressure ratio and efficiency losses of 1.16% and 1.14%, respectively, were incurred. Gap widths less than 5% of the height of the diffuser blade are adequate for broadening the operation range, whereas gap ratios greater than 5% cause more energy loss without broadening the operation range further. Gaps can also reduce the Mach number in the diffuser blade channels and can be influenced by the impeller-tip gap. Moreover, diffuser gaps on the shroud side yield better performance than those on the hub side. An experiment was conducted to investigate the reliability of the calculation and conclusions in this study, and the results proved that the conclusions are valid for different types of centrifugal compressors.