Investigation on Broadening Compressor Surge Margin by Using Active Magnetic Bearing

Abstract

The impeller design usually expects to achieve high efficiency and a wide working range while they are influenced by tip clearance in different ways. In this study, a blower with a controllable magnetic suspension bearing was studied. The magnetic bearing enables the impeller to adjust the tip clearance during operation to optimize the efficiency of the blower at the design point and significantly expand the surge margin when it deviates from the design point. The clearance ratio (CR) is defined as the ratio of tip clearance over impeller outlet width. The computational fluid dynamics (CFD) method is used to study the performance of a centrifugal impeller under CR = 2.61%∼7.83% and verified by experiments. The results show that the pressure ratio and efficiency of the impeller at the design point decreased linearly with the increase of the gap. For each 1% increment in CR, the pressure ratio reduces by 0.02 and the polytropic compression efficiency declines by 0.5%. Conversely, the surge margin increases with the expansion of CR. For every 1% rise in CR, the surge margin would improve by 7.8%. A control method of changing the reference suspension position according to the optimal efficiency fitting model is proposed. It realizes the real-time control of the axial suspension position of the impeller according to the changes of pressure and flow rate signals to obtain the optimal work efficiency with a broad surge margin. Experimental results show that using an impeller equipped with a controllable magnetic suspension bearing could minimize the gap stably in the nonsurge area to obtain high efficiency, and when necessary, it could enlarge the gap to meet the low flow rate condition.