Research On The Performance And Design Method Of Helium Compressor

The helium compressor is one of the main components of a closed Brayton cycle power plant with helium as the working fluid.Its performance has significant effects on the power output and cycle efficiency.A major challenge in helium-based systems is the compressibility of helium as it is difficult to compress due to its thermodynamic properties,such as specific heat and specific heat ratio.A helium compressor requires additional stages in order to compress helium to a certain pressure ratio in comparison with air compressor.This makes the helium turbomachinery relatively large in size and weight.The slender rotors not only cause the problem of the rotor dynamics which is difficult to deal with,but also lower the power density and overall energy conversion efficiency of the closed Brayton cycle system.In view of this,this thesis systematically analyzes the influence of physical properties on the aerodynamic performance of the helium compressor and explores the highly loaded design method of the helium compressor based on the characteristics of the helium.It discusses the influence of the highly loaded design method on the aerodynamic performance of the helium compressor.At last,the characteristics of highly loaded cascade losses were analyzed by using cascade testing.Physical properties of helium are the fundamental cause of variance in performance of helium compressor and air compressor.In numerous studies,scholars have assumed that the gas constant Rg and adiabatic index ? are included in the dimensionless velocity and mass flow.However,the similarity principle indicates that the isentropic index y itself is a similarity criterion.The air compressor performance map is applicable to air compressors under similar working conditions because the working fluid in both compressors is same.However,it is not applicable to compressors with different working fluids,such as helium.Therefore,the performance map should be revised considering isentropic indices of working fluids,even if the other similarity criteria are met.In this paper,the effects of the specific heat ratio on the subsonic and transonic axial compressors rotors performances are investigated by using theoretical analysis and numerical validation.Special attentions are paid to the relationships between the total pressure ratio,adiabatic compression efficiency and surge margin of the transonic axial compressor rotors with the specific heat ratios of the working fluids.Based on the detailed understanding of the influence of helium properties on the performance of helium compressor,the highly loaded design method of helium compressor is analyzed.This thesis discusses the selection criteria of design parameters when helium compressor adopts the highly loaded design method,and constructs the Smith chart applicable to the highly loaded design method of helium compressor.Applying the said principles,the aerodynamic design of the four-stage highly loaded helium compressor was numerically verified by numerical simulation.The numerical results show that the highly loaded design method can increase the single-stage pressure ratio of the helium compressor close to air compressor with the adiabatic compression efficiency same as that of the prototype.The low density level of supercritical helium,the characteristics of the small flow channel in the turbomachine and the variable working condition method of the system determine that the compressor may run at low Reynolds numbers.In this thesis,the influence of Reynolds number on supercritical helium compressor rotor is investigated under different conditions with numerical simulation program.Numerical results show that the highly loaded supercritical helium compressor has good performance at low Reynolds number and the sensitivity of the Reynolds number increases with the increase in specific heat ratio.The highly loaded design method of helium compressor can effectively solve the compression problem of helium in a closed Brayton cycle power plant.However,the reduction of blade height and the increase of transverse pressure gradient lead to a more leakage flow problem at the tip clearance.This paper develops a deeper understanding of the tip leakage flow physics and its influence on the loss mechanism.The tip leakage loss characteristics and loss mechanisms of highly loaded helium compressor are analyzed with numerical simulation in order to provide necessary knowledge for optimizing the blade tip aerodynamic design.Based on the results and analysis,a new loss model is developed,which can be fully utilized to predictthe tip clearance loss of highly loaded helium compressor.The cascade test of highly loaded helium compressors has always been a gray area,and this thesis has partially explored this area too.Helium is a rare gas with relatively high price.It is very uneconomical to use helium to conduct large-scale open wind tunnel cascade measurement.Therefore,the experimental research carried out in this paper is mainly based on air,supplemented by helium.In the air test,the static pressure distribution on the surface of the compressor cascade,the distribution of the total pressure loss at the outlet of the cascade,the limit flow distribution of the surface and end wall of the blade were measured.In the helium test,the static pressure distribution and wake distribution of the compressor cascade surface were also measured.The experimental study on the cascade proves the feasibility of highly loaded design method in helium compressor.