Thermodynamic Cycle Analysis Based On Physical Properties Of Working Fluid And Study On Separation Characteristic Of T-Junction

With the growing concerns on the environmental pollution,it has been widely recognized to develop clean and renewable energy resources vigorously.Nowadays,thermodynamic cycles such as organic Rankine cycle(ORC)and vapor compression cycle have become the main technologies to utilize and convert the renewable energy.However,compared with the high-grade fossil fuels,the grade of renewable energy is generally lower.Therefore,how to increase the efficiency of energy transfer and conversion in the cycle is the main challenge for the efficient utilization of renewable energy.In this work,based on the property prediction,working fluids are designed for ORC;phase equilibrium and temperature-entropy saturation boundary,which closely relate with the cycle processes,are analyzed;performance comparison is conducted between the mixtures and corresponding pure fluids;the limiting efficiency of ORC is derived.Furthermore,in order to improve the cycle efficiency further,T-junction is introduced as a phase separator to construct advanced cycles.The performance of horizontal branch T-junction is experimentally investigated for the vapor-liquid separation of pure fluids and the constituent separation of mixtures.In thermodynamic cycles,working fluid is an indispensable carrier to achieve energy conversion.The physical properties of working fluid directly determine the design of system components,the cycle performance,the cycle stability and safety.Therefore,How to achieve efficient selection of working fluids for thermodynamic cycles is an urgent problem to be solved.In this study,a molecular design method is employed to screen the working fluids with the required properties,based on the relationship between the molecular structures and properties.In order to establish the structure property relationship of working fluids,group contribution methods(GCMs)are developed to predict the boiling temperature and the critical temperature.For simplicity of molecular group division,16 molecular groups are divided in accordance with the functional groups,based on the results of refrigerant design and existing refrigerants.Furthermore,in order to distinguish a large number of structural isomers existing in pure fluids,a topological index EATII is firstly introduced into the GCM.Thereafter,an artificial neural network-genetic algorithm(ANN-GA)is employed to estalish the boiling temperature and the critical temperature of working fluid as functions of molecular groups and EATII.The absolute average relative deviations(AARDs)are 1.87% and 1.27%,respectively.On this basis,four basic processes of the ORC,including compression,evaporation,expansion and condensation are modeled from the estimated properties of GCMs.The proposed model can get ORC performance only based on the molecular structure of working fluid.Compared with the REFPROP,the relative errors of thermodynamic properties and cycle parameters are less than 10% for most of working fluids.After that,a systematic model is developed for the efficient design of working fluids and the optimization of cycle parameters at the molecular scale,so that optimal working fluids can be identified by simultaneously considering cycle parameters,environmental and safety properties.Under the given operating conditions,the optimal candidates,namely R254 eb,R254cb,are firstly reported in ORC application.For the evaporation and condensation of mixed working fluids,accurate vaporliquid equilibrium(VLE)data are essential for designing and modeling these cycle processes.Thus,in this work,the typical and recent models for the VLE calculation of mixed working fluids are summarized.Based on the group contribution methodUNIFAC,three predictive models are established by combining Peng-Robinson(PR)equation and three different mixing rules,so that VLE properties at different temperatures can be predicted.Compared with the experimental data,no model can get VLE properties accurately for any type of mixtures.Therefore,a generalized model for mixtures should be derived from the molecular theory or group contribution methods.The group contribution values can be obtained from the large VLE data fitting.As for the expansion and compression of woking fluids,the thermodynamic perfomance strongly depends on the temperature-entropy saturation boundary slope.Therefore,an analytical expression on the slope of saturated curve is obtained based on the highly accurate Helmholtz energy equation.According to the calculated results,it can be found that with an increase of the number of molecular groups,the positive liquid slope of pure fluids increases and the positive slope of saturated vapor usually appears in the vicinity of reduced temperature 0.8.Furthermroe,the range of reduced temperature for the positive vapor slope becomes larger,when the molecule contains more carbon and halogen atoms.As for the binary mixture,the liquid slope is generally located between the corresponding pure fluids',while the vapor slope can be infinity by mixing dry and wet fluids ingeniously.On this basis,value of saturated vapor slope is represented by the slope angle.An ANN-GCM is firstly developed to derive the slope angle as a function of molecular groups,EATII,the reduced temperature and the molecular weight.Compared with the calculated angles from Helmholtz equation,the AARD of the established ANN-GCM is 0.67%.In order to clarify the advantages of zeotropic mixtures in the thermodynamic cycles,a comprehensive performance comparison between zeotropic mixtures and pure fluids is conducted via cycle simulation for the basic ORC and recuperative ORC driven by open heat source.In the simulation,a certain range of mass flow rate of cooling water is considered as the condition of heat sink.Performances of mixtures are optimized and compared with those of their constituents from the points of first and second laws.The results demonstrate that although zeotropic mixture generally has lower temperature differences in the evaporator and condenser,the exergy losses of these heat exchangers are not certain to be reduced.However,due to the limited number of pure fluids and the concerns on the environmental issues,mixture can still be considered as alternative working fluid.Furthermore,in order to provide guidance for the selection of working fluids and the construction of ORC,a limiting efficiency is proposed for subcritical ORC under the constraint of working fluids.For the calculation of limiting efficiency,a limiting factor is defined on the basis of the saturated slope of liquid at the reduced temperature 0.9.On this basis,a new expression is defined for thermodynamic perfectness.The calculated results indicate that the pure fluid with a higher critical temperature generally possesses higher limiting efficiency and cycle perfectness.For mixtures,the limiting efficiency generally locates between those of pure fluids,while the thermodynamic perfectness varies greatly with the composition.As a phase separator,T-junction has been employed to control and distribute the two-phase flow of working fluids in the thermodynamic cycles.In order to reveal the dividing mechanism of two-phase flow in a T-junction,an experimental set up was established for the performance test of horizontal branching T-junction.During the experiments,three refrigerants,namely R134 a,R600a and R245 fa were considered,and effects of vapor quality,mass flux,mass flow ratio and refrigerant on the phase separation were clarified.In addition,the flow patterns at the T-junction inlet were recorded by a high speed camera.Based on the experimental results,it can be concluded that the vapor prefers to flow into the branch.As the inlet quality increases,the fraction of vapor extracted into the branch decreases.Furthermore,a higher vapor fraction can be obtained by a larger mass flow ratio.While under the same flow pattern,the mass flux has little influence on the phase redistribution of refrigerant.As for the geometric effects,compared with the equal diameters,more vapors usually prefer to flow into the branch with the diameter ratio 0.75.However,compared with the influence of diameter ratio,the effect of horizontal branch angle on the phase separation is less.For the effect of refrigerant on the phase separation,the vapor faction of the branch satisfies the following order: R245fa>R600a>R134a.Accordingly,the product of momentum ratio and viscosity ratio between vapor and liquid is defined to characterize the phase separation degree of refrigerant qualitatively.What's more,in order to test the applicability of the published phase separation models in the literature,the present data of refrigerants are compared with the predicted values.Based on the same experiments,the constituent separation of binary zeotropic mixtures,R134a/R600 a,at horizontal branch T-junctions was investigated.Based on the experimental data,separation efficiency is defined to present the constituent distribution at T-junction.The effects of flow condition,mixture composition and T-junction geometry on the constituent separation performance are revealed.The results indicate that as the vapor quality increases,the efficiency gradually decreases from the positive to the negative.It means that more R600 a is extracted into the branch.For the considered mixtures,R134a/R600a(0.3030/0.6970,mass)shows the highest capacity of constituent separation.As for the effect of T-junction geometry,when the inlet quality is less than 0.4,the diameter ratio 1.0 has a higher capacity of the constituent separation than the ratio 0.75.Compared with the angles 45°and 135°,the largest separation capacity is obtained by the angle 90o.