Research On Stability Improvement Of Centrifugal Compressor By Asymmetric Case Treatment

The sustainable development of today’s society is beset by environmental and energy problems,so energy-saving and emission reduction has become the theme of the times.As an important part of the turbocharging system,the centrifugal compressor improves the fuel economy and dynamic performance of the internal combustion engine used in vehicle to reduce the pollution discharge,besides it also overcomes the power problems of the modern UAV flied in high altitude.The centrifugal compressor plays an irreplaceable role in the national life and industrial production.The wide application of supercharging technology in different fields needs higher requirements for the technical performance of modern centrifugal compressor.Under the premise of pursuing high pressure ratio and high efficiency,the wide working range is pursued in modern high speed and high pressure ratio centrifugal compressor.Therefore,the internal flow field and the instability mechanism of the centrifugal compressor are widely studied.The selfcirculating casing treatment,as a common method of stability control,also raises new concerns of the researchers.And the research of Non-Axisymmetric self recirculation casing treatment becomes the new trend of a centrifugal compressor’s stability enhancement.In order to achieve the efficient stability of the centrifugal compressor,it is necessary to grasp the internal flow field characteristics and instability mechanism of the compressor.Based on the study of the internal flow field characteristics of a high pressure ratio and high speed centrifugal compressor,the selection strategy of the numerical calculation method for the centrifugal compressor is determined.The single passage calculation method is used to describe the complex three-dimensional flow field in compressor.It’s specified that the interaction between the tip flow and the other secondary flows is the main cause of centrifugal compressor losses.Then,the same calculation method is used to study the instability mechanism in low flow condition of centrifugal compressor.It’s found that the low-energy fluid mass produced by the interaction of the leakage from the blade tip,shock wave and boundary fluid near the main blade suction surface is an important incentive for system instability.Especially in the near-instability condition,low-energy fluid will span the entire flow channel near the shroud,and plug the deterioration of the flow field,finally induce impeller flow instability.And then the asymmetry of the flow field which leads the earlier instability of the flow channel,was studied by using the full passage numerical simulation method.It’s found that the uniform distribution of the internal flow field in the off-design condition is caused by he circumferential distortion of volute.The results show that the number of blades in the impeller,the rotation speed of the impeller and the characteristics of the different flow conditions determine the strength and the circumferential distribution of distortions by studying the propagation of the circumferential static pressure distortion.Circumferential distortion in impeller flow field induced flow earlier instability,so it should be emphasized in stability enhancement.The use of self-circulation casing treatment is the common Centrifugal compressor stability control method.This article further clarifies two stability enhancement strategies of the self-circulation casing device in high and low massflow condition.The symmetry hypothesis in design of the casing device is inconsistent with the true flow of the circumferential.So the circumferential instability characteristics of the full passage and the full passage stability enhancement mechanism of symmetric self-circulation casing treatment were studied.By using flow field stability parameters to measure the stability of the circumferential flow passages,it’s found that when the flow rate decreases,the existence of circumferential distortion destroys the flow field stability of some flow passages.And when the number of circumferential instability flow passages reaches the critical passage numbers,the system is unstable.Stability enhancement is mainly realized by differential pressure between inlet and outlet of casing treatment.The distortion of the circumferential static pressure causes Non-Axisymmetric flow field,but also provides the power of self-adaption in casing treatment.The inhibition of the symmetrical casing device on the flow circumferential distortion is limited,so stability enhancement can’t reach the maximum effect when the flow field distortion still exists.Therefore,it is necessary to seek the optimal form of circumferential design for casing treatment.Considering the instability mechanism of the compressor and the stability enhancement mechanism of the casing treatment,the factors that need to be considered in the design of the asymmetric casing treatment are discussed.The applicability of the asymmetrical casing in different speed conditions,the effect of the asymmetric casing device on near stall condition and the impeller rotation effect on the stability enhancement of the asymmetric casing devices are analyzed.Based on the realization of the stability control of the casing treatment and the design idea of the existing asymmetric casing treatment,the new criteria for the design of the asymmetric casing treatment is proposed and verified.Finally,a new asymmetric casing device was desigened by using the new asymmetric design criteria,and the stable working range of centrifugal compressor,comparing to the traditional symmetrical casing device and Zheng et al’s asymmetric casing device,is highest increased by 9.5% and 7.2% individually.