Research On Centrifugal Compressor Technology In Microminiature High Efficiency Brayton Cycle

The micro centrifugal compressor is one of the key components of micro Brayton cycle thermal power conversion system.Its efficiency and stable working range are directly related to the performance of the entire system.Because the internal flow of the centrifugal compressor is complicated,it is often accompanied by strong unsteady effect,which will reduce the performance of the compressor.The study of the internal flow of micro centrifugal compressor and the use of unsteady flow control technology can improve the flow of the compressor near tip area and improve the overall performance.Besides,adopting new medium can improve the overall performance to a certain extent.This paper uses the numerical simulation combined with theoretical analysis method to research the centrifugal compressor of a micro Brayton cycle thermal power conversion system.The main tasks are as follows:1.Dynamic modal decomposition（DMD）method is introduced to study the complex internal flow.By DMD analysis of two-dimensional cascade flow and three-dimensional compressor flow,results show that the DMD method has global and efficient advantage in dealing with complex flow problems.It can quickly capture all the coherent structures and the corresponding frequency information.By capturing the dominant coherent of flow field,complex flow research can be analyzed by DMD method with only several modes.The unsteady research shows that the dominant frequency of tip dynamic flow structure under different speed is about half of the blade passing frequency,and this proportion basically does not change with the speed.By using the DMD method,the reasons for the dominant frequency of the half blade passing frequency is explained.This phenomenon is related to the relationship between tangential velocity of dominant coherent structure and the main flow tip velocity.2.With the dominant frequency of tip dynamic structure,the unsteady flow control technology can be effectively applied to the compressor.By introducing of a virtual mirror vortex on the casing outside,a leakage vortex frequency prediction model of dominant frequency can be set up with no need of CFD calculation or experimental results.According to the principle of leakage vortex instability,by introducing a virtual mirror tip leakage vortex,the wingtip vortex instability theory can be applied.The relationship between the maximum unstable disturbance wavelength of the vortex frequency and the radius of the vortex and the distance between the vortices is derived.And the model is validated with the calculation and the domestic and foreign typical compressor experiments.3.Based on the mechanism of unsteady flow control,a passive unsteady flow control method for micro centrifugal compressor is proposed.By using this method the predominant structure of the blade tip of the compressor can be weakened,thus improving the stability margin of the compressor.Effective excitation can significantly reduce the fluctuation caused by the leakage vortex with only a small amount of excitation flow（0.12%of the mainstream）,which can greatly improve the stability margin（increase 19.3%）nearly without affecting the compressor efficiency.4.The SCO₂ compressor is the key component to improve the efficiency of the Brayton cycle with SCO₂ as the working medium,and is the key factor for the efficiency of the whole system to be higher than 50%.In order to analyze the internal flow characteristics of the SCO₂ compressor,a typical micro centrifugal compressor is studied based on the previous research.By comparing the structures of the flow fields when the medium is SCO₂ and air,the differences between SCO₂ and air compressors as well as their corresponding reasons are basically masterd.It provides a basis for the design and research of the SCO₂ compressor.