Research On Flow Characteristics And Similarity Method Of Noble Gas Turbomachinery

The closed Brayton cycle s is a potential solution to meet the requirements of long endurance,high power density for underwater and space applications.Due to the advantages in chemical stability and heat transfer,the selection of noble gas as working fluid,including helium,helium-xenon and argon,benefits the miniaturization and high efficiency of closed Brayton cycle power system.Turbomachinery is the core component of the power unit.Due to the obvious difference between the physical properties of noble gas and air,the design practice of noble gas turbomachinery cannot completely inherit the experience of the air.In addition,due to the high experimental cost of noble gas,it is impossible to conduct systemic experiments on the aerodynamic performance of turbomachinery.In view of these,it is of great benefit to the construction of the design system of noble gas turbomachinery to study the aerodynamic performance of noble gas turbomachinery and model its efficiency and expansion ratio characteristics based on the similarity theory.Meanwhile,the research on the fundamental flow characteristics of noble gases and the analysis of the similarities and differences between rare gases and air from the perspective of flow similarity,can lay a solid foundation for the in-depth understanding of the aerodynamic law of noble gas turbomachinery.In this paper,based on the actual flow conditions of turbomachinery,the continuum properties and the constitutive relationship between stress and strain rate of the noble gas were demonstrated,and the ideal gas hypothesis were analyzed.The feasibility of using the existing fluid dynamics theory and software to describe the noble gas flow was thus made clear.The physical properties of noble gases and mixture were predicted by Chapman&Enskog molecular dynamics theory and the corresponding state principle of two equations.The boundary layer flow model was constructed based on the T3A non-pressure gradient plate.The typical parameter distributions of helium boundary layer under different flow states were analyzed,and the change laws of transitional boundary layer with Re and free stream turbulence conditions were explored.The boundary layer flow characteristics of different fluids were compared,the effects of Re,Ma and Eu on flow similarity were explored,and the Eu-Re criterion combination was determined to realize the similarity of boundary layer flows among different fluids.Based on the analysis of boundary layer loss,the internal relationship between flow loss,flow parameters and viscosity of fluid was revealed and the equivalent modeling methods were proposed for viscous flow losses aiming different fluids under the condition of strict and approximate flow similarity.The cascade experiments of helium,argon and air were carried out,and the suction surface topology and surface pressure coefficient distribution of each fluid under different separation degrees were obtained.The flow loss of cascades of different fluids under different working conditions was obtained by measuring the parameters behind the cascades.Through the combination of experiment and numerical study,the flow structure and the variation trend of parameter distribution of different fluids cascade with Re and Eu were investigated,and the combination of Eu-Re criterion to achieve similar flow structure among different fluids cascade was determined.For cascade flows,where pressure differential resistance loss is dominant,when cascade flows of different fluids are similar,the flow losses are equivalent.Based on the analysis of the flow field of different fluids radial turbines,the influences of Eu and Re on turbine efficiency were explored,and the effect of speed conditions on turbine efficiency equivalence under off-design conditions was emphasized.The Eu-Re-u conditions combination to achieve the efficiency characteristics equivalence among different fluids radial turbines was determined,and the applicable range of the method was defined,by which the similarity deviation of the parameters at inlet and outlet was 1%and of efficiency was 0.2%.The internal relation between the expansion ratio of the radial turbine and the flow parameters,efficiency and the specific heat ratio of the fluids was derived,and the complete equivalence in efficiency and expansion ratio characteristics of the radial turbine between helium-xenon and argon was achieved.For the characteristic equivalence between helium-xenon and air centripetal turbines,the similarity method worked at the low working conditions and the factors causing the difference in critical flow rate of two were discussed.A new criterion number combinationγ^0.497Ma²-Re-u was established to make the equivalence in efficiency characteristics of the two.The transformation method of the expansion ratio between helium-xenon and air radial turbines was derived.the complete equivalence in efficiency and expansion ratio characteristics of the two turbines was achieved under the premise of approximate flow similarity.