Study On Separation Characteristics And Application Performance Of Zeotropic Mixture In Horizontal T-junction

For many years,there is a lack of breakthrough in the basic theory of the construction of the actual thermodynamic cycle in the field of medium and low temperature thermal energy utilization in China,resulting in a lack of strong guidance in the construction method level,which makes the efficiency of such actual thermal cycle low,and the thermodynamic perfection is very low,which restricts the effective utilization of low and medium temperature thermal energy in China.In view of the problem that the construction method of the actual thermodynamic cycle is not clear,some scholars have proposed the technical route to improve the performance of all thermal processes by changing the working fluid of the cycle,and then invented the three-dimensional construction method of the thermodynamic cycle.The working fluid used in the circulation is zeotropic organic working fluid.In the circulation process,through the mixture and separation of working fluid,the zeotropic working fluid can be adjusted among various components,so as to meet the requirements of all thermal processes for the thermal physical properties of working fluid.By improving the actual efficiency of multiple thermal processes in the circulation at the same time,the overall performance of the circulation can be improved,and the approximation of the overall circulation to the ideal circulation can be realized.T-junction can be used as the component of the mixture composition regulation,and has the potential to become an important means to realize the thermal process in the three-dimensional construction of thermal cycle,but its phase separation rule,especially when organic working fluid is used as the working fluid,is not clear.In view of this,the two-phase separation process of R22 and R134a in horizontal T-junction was simulated by using three-dimensional numerical simulation method based on Euler method and k-s turbulence model.The results of the model were compared with the existing experimental and phenomenological model data,the flow characteristics and phase separation parameters of pure two-phase flow in T-junction were obtained.The results show that the inlet quality and saturation temperature have little effect on the phase separation.However,the inlet mass flow has a significant effect on the phase separation.In the range of 100-500kg·m-2·s-1,FG increases with the increase of inlet mass flow.The effect of the R22 and R134a on the mass flow ratio can be ignored.In addition,it is found that there are two symmetrical return zones at the branch inlet.Although the application of T-junction has expanded from the field of petroleum engineering to the field of new advanced thermodynamic cycle,the research on local pressure drop of T-junction is still insufficient.Therefore,based on the Euler method,a three-dimensional numerical model of two-phase flow in horizontal T-junction was established,and the accurate pressure distribution at the cross section was obtained,especially the pressure distribution at the inlet of the branch pipe.The local pressure drop coefficient was parameterized,and two new local pressure drop coefficients were fitted considering the mass separation ratio F,the density ratio and the diameter ratio.The results show that the pressure drops sharply at the cross section of T-junction,then rises at the inlet section of branch pipe,and finally decreases along the direction of fluid flow.The prediction model could predict the local pressure loss of two-phase flow at the T-junction.For K12J,more than 93%of the data prediction error is within±15%;for K13J,more than 90%of the data prediction error is within±30%.As an important separation and mixing component in the three-dimensional construction method of thermodynamic cycle,the separation rule of zeotropic working fluid in T-junction separator is very important for the realization of component control.In order to clarify the rule of component separation in T-junction,the numerical simulation of component separation of zeotropic mixture R134a/R600a in horizontal T-junction and the discussion of inter phase mass transfer model were carried out.The flow details such as flow velocity,density and mass fraction distribution of component R134a which could not be obtained by experiments were revealed.The separation efficiency was obtained when the inlet quality was 0.1-0.4.The results show that the mass fraction of R134a varies from 0.44 to 0.70 when the inlet quality is 0.204.The mass fraction of R134a in liquid phase is less than that in gas phase.Phase separation ratio has influence on component separation.Understanding the entropy generation due to the irreversible interaction of two-phase flow in T-junction could provide support for the overall performance analysis of thermodynamic cycle.Three models of entropy generation(Bajan's method,Revellin's method and the thermodynamic force and flux method)were calculated and analyzed by Computational Fluid Dynamics.The UDF was compiled and loaded into FLUENT 18.0 software to solve the entropy generation.The results show that the maximum entropy generation occurs at the intersection and near the branch pipe inlet.For the cases of pure organic working fluid and mixture organic working fluid,the total entropy generation calculated by Bajan's method is the smallest,while that calculated by Revellin's method is the largest,namely Sm1<Sm3<Sm2.Revellin's method is more accurate in calculating entropy generation rate that other methods,while the thermodynamic force and flux method gives the expression of entropy generation rate caused by various irreversible factors,which is helpful for more detailed energy calculation and analysis of two-phase flow system.On the basis of the above research,two circulation systems were selected for case study.One is the component adjustable Organic Rankine cycle system,the other is the household refrigerator system with reverse circulation based on zeotropic refrigerants.Both systems applied T-junctions as separators.Based on the above cycles,the influence of operation parameters on the performance of the cycles was obtained by thermodynamic cycle calculation.The results show that for the component adjustable Organic Rankine Cycle system,when the T-junction phase separation ratio increases,the cycle efficiency increases,and then affects the cycle efficiency.When the component proportion of R245fa is 0.5851,the cycle work and thermal efficiency reach the maximum value.When the phase separation ratio changes from 0.1 to 0.7,the minimum thermal efficiency is 11.95%and the maximum is 13.18%.The pressure change of T-junction affects the efficiency of the component adjustable Organic Rankine Cycle,and the reasonable phase separation ratio can improve the thermodynamic cycle efficiency.The pressure drop could cause the diffusion of components,but the mass of phase transfer is very small,which has little effect on the cycle efficiency.For the household refrigerator system based on zeotropic working fluid,with the increase of the initial component ratio z,the performance coefficient COP increases first and then decreases.By selecting the best component ratio of zeotropic mixture,the maximum COP can be obtained.In addition,the performance coefficient COP is sensitive to the change of R290 component at the outlet of separator,but the influence trend of the mass separation ratio at the outlet of different separators on the performance coefficient COP is different.