Theoretical And Experimental Study On Adaptive Hold Casing Treatment For Centrifugal Compressor

Centrifugal compressor is used in many areas, such as pertroleum industry, chemical industry, metallurgic industry, steel industry, food industry and textile industry. Stall and surge are the two kinds of unstable flow which appears when the compressor works in low flowrate and influence the stable working range of the compressor. The important work for the compressor researchers is to delay the appearance of stall, avoid surge and expand the stable working range of the compressor.The flow field in an unshrouded centrifugal impeller was studied numerically. It's found that there is a large vortex region near the tip of the impeller. Based on that, a new kind of self-adaptive casing treatment for unshrouded centrifugal compressor was proposed in this thesis and was named as holed casing treatment. The configuration of the flow channel, the flow field in the channel and the effect of the recirculated flow in the channel are different from the traditional casing treatment methods. The holed casing treatment is a self-adaptive technique. The mass flowrate in the hole can change with the flowrate condition of the compressor automatically to meet the stability expansion requirement. When the flowrate condition is near design point, there is little mass flowrate and little flow resistence in the hole. The pressure ratio and efficiency of the compressor at design point remains. In small flowrate condition, the pressure ratio of the impeller increases. The flow direction in the hole is opposite with that in the impeller. The stall or surge can be put off. In large flowrate condition, the pressure ratio of the impeller decreases. The flow direction in the hole is the same with that in the impeller. The choked margin can be increased. Detailed studies were carried out here. The main contents and conclusions of the research are described in the following:1) Detailed theoretical and experimental studies on holed casing treatment were carried out. It's found in experiments that the stall margin of the impeller was expanded by 20%, the choked margin was also increased and there was no significant reduction in efficiency. The results of numerical simulations agreed well with experimental results.2) The testing system for unsteady flow in the 22790r/min high speed compressor was set up. It was found the first time in the experiments that there were two kinds of flow in the holes. The mechanism of self-adaptation of the holed casing treatment was demonstrated. The results of numerical simulations also found the two kinds of flow in the holes.3) Both steady and unsteady numerical simulations were carried out to study the influence of holed casing treatment on the impeller performance and flow fields in the impeller. It's found that the holed casing treatment improved mainly the flow field in the tip region of the impeller, especially the tip region near the leading edge which was caused by the unsteady reinjected flow in the holes. This is the main reason why the stall margin of the compressor can be expanded by 20% while there was only 5% mass flowrate in the holes. It's indicated that the flow field in the tip region can influence the stable working range of the compressor. 4) The formulas which can be used to determine the radial position of the holes were established and the important parameters which can influence the effect of holed casing treatment were studied. It's found that the diameter of the holes has a significant influence on the effect of holed casing treatment and there is an optimum diameter for the impeller. It's also found that the hole number has little influence on holed casing treatment. The studies provide important technological conditions for the application of holed casing treatment.